io-scheduler tuning for better read/write ratio

io-scheduler tuning for better read/write ratio

[Ralf Gross]

Hi,

I'm trying to tune the kernel/io-scheduler for better read/write
ratio on a Areca RAID0 device (4 disks, kernel 2.6.26, xfs fs).

I can get 200 MB/s seq. writes and about the same for seq. reads. My
problem is that if there are reads _and_ writes on this device, the
write throughput is much higher than the read throughput (40 MB/s
read, 90 MB/s write).

The deadline scheduler sounded like the way to go for getting better
read results, but reagardless which parameter I change, the ratio keeps
the same.

cfq, noop.. different paramter settings, but alway the same result.

short: is there a way to tune the kernel/scheduler settings in a way
to get a higher read throughput? Writes are not that important,
basicially there are only two 30 GB files on the device/filesystem
that are used to spool data for two LTO-4 tape drives. So I need a
certain read speed to keep both drives streaming. While the data gets
written to one file, I need at least 50 MB/s for reading from the
other file and sent to the tape drive.

Thanks, Ralf

Re: io-scheduler tuning for better read/write ratio

[David Newall]

Ralf Gross wrote:
> write throughput is much higher than the read throughput (40 MB/s
> read, 90 MB/s write).

Perhaps I've misunderstood, but isn't that common?  Reads have to come
from disk, whereas writes get cached by the drive.

Re: io-scheduler tuning for better read/write ratio

[Ralf Gross]

David Newall schrieb:
> Ralf Gross wrote:
> > write throughput is much higher than the read throughput (40 MB/s
> > read, 90 MB/s write).

Hm, but I get higher read throughput (160-200 MB/s) if I don't write
to the device at the same time.

Ralf

Re: io-scheduler tuning for better read/write ratio

[Casey Dahlin]

On 06/16/2009 02:40 PM, Ralf Gross wrote:
> David Newall schrieb:
>> Ralf Gross wrote:
>>> write throughput is much higher than the read throughput (40 MB/s
>>> read, 90 MB/s write).
> 
> Hm, but I get higher read throughput (160-200 MB/s) if I don't write
> to the device at the same time.
> 
> Ralf

How specifically are you testing? It could depend a lot on the particular access patterns you're using to test.

--CJD

Re: io-scheduler tuning for better read/write ratio

[Ralf Gross]

Casey Dahlin schrieb:
> On 06/16/2009 02:40 PM, Ralf Gross wrote:
> > David Newall schrieb:
> >> Ralf Gross wrote:
> >>> write throughput is much higher than the read throughput (40 MB/s
> >>> read, 90 MB/s write).
> > 
> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
> > to the device at the same time.
> > 
> > Ralf
> 
> How specifically are you testing? It could depend a lot on the
> particular access patterns you're using to test.

I did the basic tests with tiobench. The real test is a test backup
(bacula) with 2 jobs that create 2 30 GB spool files on that device.
The jobs partially write to the device in parallel. Depending which
spool file reaches the 30 GB first, one starts reading from that file
and writing to tape, while to other is still spooling.

Ralf

Re: io-scheduler tuning for better read/write ratio

[Jeff Moyer]

Ralf Gross <rg@stz-softwaretechnik.com> writes:

> Casey Dahlin schrieb:
>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
>> > David Newall schrieb:
>> >> Ralf Gross wrote:
>> >>> write throughput is much higher than the read throughput (40 MB/s
>> >>> read, 90 MB/s write).
>> > 
>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
>> > to the device at the same time.
>> > 
>> > Ralf
>> 
>> How specifically are you testing? It could depend a lot on the
>> particular access patterns you're using to test.
>
> I did the basic tests with tiobench. The real test is a test backup
> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
> The jobs partially write to the device in parallel. Depending which
> spool file reaches the 30 GB first, one starts reading from that file
> and writing to tape, while to other is still spooling.

We are missing a lot of details, here.  I guess the first thing I'd try
would be bumping up the max_readahead_kb parameter, since I'm guessing
that your backup application isn't driving very deep queue depths.  If
that doesn't work, then please provide exact invocations of tiobench
that reprduce the problem or some blktrace output for your real test.

Cheers,
Jeff

Re: io-scheduler tuning for better read/write ratio

[Jeff Moyer]

Jeff Moyer <jmoyer@redhat.com> writes:

> Ralf Gross <rg@stz-softwaretechnik.com> writes:
>
>> Casey Dahlin schrieb:
>>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
>>> > David Newall schrieb:
>>> >> Ralf Gross wrote:
>>> >>> write throughput is much higher than the read throughput (40 MB/s
>>> >>> read, 90 MB/s write).
>>> > 
>>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
>>> > to the device at the same time.
>>> > 
>>> > Ralf
>>> 
>>> How specifically are you testing? It could depend a lot on the
>>> particular access patterns you're using to test.
>>
>> I did the basic tests with tiobench. The real test is a test backup
>> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
>> The jobs partially write to the device in parallel. Depending which
>> spool file reaches the 30 GB first, one starts reading from that file
>> and writing to tape, while to other is still spooling.
>
> We are missing a lot of details, here.  I guess the first thing I'd try
> would be bumping up the max_readahead_kb parameter, since I'm guessing
> that your backup application isn't driving very deep queue depths.  If
> that doesn't work, then please provide exact invocations of tiobench
> that reprduce the problem or some blktrace output for your real test.

Any news, Ralf?

Cheers,
Jeff

Re: io-scheduler tuning for better read/write ratio

[Ralf Gross]

Jeff Moyer schrieb:
> Jeff Moyer <jmoyer@redhat.com> writes:
> 
> > Ralf Gross <rg@stz-softwaretechnik.com> writes:
> >
> >> Casey Dahlin schrieb:
> >>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
> >>> > David Newall schrieb:
> >>> >> Ralf Gross wrote:
> >>> >>> write throughput is much higher than the read throughput (40 MB/s
> >>> >>> read, 90 MB/s write).
> >>> > 
> >>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
> >>> > to the device at the same time.
> >>> > 
> >>> > Ralf
> >>> 
> >>> How specifically are you testing? It could depend a lot on the
> >>> particular access patterns you're using to test.
> >>
> >> I did the basic tests with tiobench. The real test is a test backup
> >> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
> >> The jobs partially write to the device in parallel. Depending which
> >> spool file reaches the 30 GB first, one starts reading from that file
> >> and writing to tape, while to other is still spooling.
> >
> > We are missing a lot of details, here.  I guess the first thing I'd try
> > would be bumping up the max_readahead_kb parameter, since I'm guessing
> > that your backup application isn't driving very deep queue depths.  If
> > that doesn't work, then please provide exact invocations of tiobench
> > that reprduce the problem or some blktrace output for your real test.
> 
> Any news, Ralf?

sorry for the delay. atm there are large backups running and using the
raid device for spooling. So I can't do any tests.

Re. read ahead: I tested different settings from 8Kb to 65Kb, this
didn't help.

I'll do some more tests when the backups are done (3-4 more days).

Thanks, Ralf

Re: io-scheduler tuning for better read/write ratio

[Jeff Moyer]

Ralf Gross <rg@STZ-Softwaretechnik.com> writes:

> Jeff Moyer schrieb:
>> Jeff Moyer <jmoyer@redhat.com> writes:
>> 
>> > Ralf Gross <rg@stz-softwaretechnik.com> writes:
>> >
>> >> Casey Dahlin schrieb:
>> >>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
>> >>> > David Newall schrieb:
>> >>> >> Ralf Gross wrote:
>> >>> >>> write throughput is much higher than the read throughput (40 MB/s
>> >>> >>> read, 90 MB/s write).
>> >>> > 
>> >>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
>> >>> > to the device at the same time.
>> >>> > 
>> >>> > Ralf
>> >>> 
>> >>> How specifically are you testing? It could depend a lot on the
>> >>> particular access patterns you're using to test.
>> >>
>> >> I did the basic tests with tiobench. The real test is a test backup
>> >> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
>> >> The jobs partially write to the device in parallel. Depending which
>> >> spool file reaches the 30 GB first, one starts reading from that file
>> >> and writing to tape, while to other is still spooling.
>> >
>> > We are missing a lot of details, here.  I guess the first thing I'd try
>> > would be bumping up the max_readahead_kb parameter, since I'm guessing
>> > that your backup application isn't driving very deep queue depths.  If
>> > that doesn't work, then please provide exact invocations of tiobench
>> > that reprduce the problem or some blktrace output for your real test.
>> 
>> Any news, Ralf?
>
> sorry for the delay. atm there are large backups running and using the
> raid device for spooling. So I can't do any tests.
>
> Re. read ahead: I tested different settings from 8Kb to 65Kb, this
> didn't help.
>
> I'll do some more tests when the backups are done (3-4 more days).

The default is 128KB, I believe, so it's strange that you would test
smaller values.  ;)  I would try something along the lines of 1 or 2 MB.

I'm CCing Fengguang in case he has any suggestions.

Cheers,
Jeff

p.s. Fengguang, the thread starts here:
     http://lkml.org/lkml/2009/6/16/390

Re: io-scheduler tuning for better read/write ratio

[Ralf Gross]

Jeff Moyer schrieb:
> Ralf Gross <rg@STZ-Softwaretechnik.com> writes:
> 
> > Jeff Moyer schrieb:
> >> Jeff Moyer <jmoyer@redhat.com> writes:
> >> 
> >> > Ralf Gross <rg@stz-softwaretechnik.com> writes:
> >> >
> >> >> Casey Dahlin schrieb:
> >> >>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
> >> >>> > David Newall schrieb:
> >> >>> >> Ralf Gross wrote:
> >> >>> >>> write throughput is much higher than the read throughput (40 MB/s
> >> >>> >>> read, 90 MB/s write).
> >> >>> > 
> >> >>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
> >> >>> > to the device at the same time.
> >> >>> > 
> >> >>> > Ralf
> >> >>> 
> >> >>> How specifically are you testing? It could depend a lot on the
> >> >>> particular access patterns you're using to test.
> >> >>
> >> >> I did the basic tests with tiobench. The real test is a test backup
> >> >> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
> >> >> The jobs partially write to the device in parallel. Depending which
> >> >> spool file reaches the 30 GB first, one starts reading from that file
> >> >> and writing to tape, while to other is still spooling.
> >> >
> >> > We are missing a lot of details, here.  I guess the first thing I'd try
> >> > would be bumping up the max_readahead_kb parameter, since I'm guessing
> >> > that your backup application isn't driving very deep queue depths.  If
> >> > that doesn't work, then please provide exact invocations of tiobench
> >> > that reprduce the problem or some blktrace output for your real test.
> >> 
> >> Any news, Ralf?
> >
> > sorry for the delay. atm there are large backups running and using the
> > raid device for spooling. So I can't do any tests.
> >
> > Re. read ahead: I tested different settings from 8Kb to 65Kb, this
> > didn't help.
> >
> > I'll do some more tests when the backups are done (3-4 more days).
> 
> The default is 128KB, I believe, so it's strange that you would test
> smaller values.  ;)  I would try something along the lines of 1 or 2 MB.

Err, yes this should have been MB not KB.


$cat /sys/block/sdc/queue/read_ahead_kb 
16384
$cat /sys/block/sdd/queue/read_ahead_kb 
16384

I also tried different values for max_sectors_kb, nr_requests. But the
trend that writes were much faster than reads while there was read and
write load on the device didn't change.

Changing the deadline parameter writes_starved, write_expire,
read_expire, front_merges or fifo_batch didn't change this behavoir.

Ralf

Re: io-scheduler tuning for better read/write ratio

[Jeff Moyer]

Ralf Gross <Ralf-Lists@ralfgross.de> writes:

> Jeff Moyer schrieb:
>> Ralf Gross <rg@STZ-Softwaretechnik.com> writes:
>> 
>> > Jeff Moyer schrieb:
>> >> Jeff Moyer <jmoyer@redhat.com> writes:
>> >> 
>> >> > Ralf Gross <rg@stz-softwaretechnik.com> writes:
>> >> >
>> >> >> Casey Dahlin schrieb:
>> >> >>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
>> >> >>> > David Newall schrieb:
>> >> >>> >> Ralf Gross wrote:
>> >> >>> >>> write throughput is much higher than the read throughput (40 MB/s
>> >> >>> >>> read, 90 MB/s write).
>> >> >>> > 
>> >> >>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
>> >> >>> > to the device at the same time.
>> >> >>> > 
>> >> >>> > Ralf
>> >> >>> 
>> >> >>> How specifically are you testing? It could depend a lot on the
>> >> >>> particular access patterns you're using to test.
>> >> >>
>> >> >> I did the basic tests with tiobench. The real test is a test backup
>> >> >> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
>> >> >> The jobs partially write to the device in parallel. Depending which
>> >> >> spool file reaches the 30 GB first, one starts reading from that file
>> >> >> and writing to tape, while to other is still spooling.
>> >> >
>> >> > We are missing a lot of details, here.  I guess the first thing I'd try
>> >> > would be bumping up the max_readahead_kb parameter, since I'm guessing
>> >> > that your backup application isn't driving very deep queue depths.  If
>> >> > that doesn't work, then please provide exact invocations of tiobench
>> >> > that reprduce the problem or some blktrace output for your real test.
>> >> 
>> >> Any news, Ralf?
>> >
>> > sorry for the delay. atm there are large backups running and using the
>> > raid device for spooling. So I can't do any tests.
>> >
>> > Re. read ahead: I tested different settings from 8Kb to 65Kb, this
>> > didn't help.
>> >
>> > I'll do some more tests when the backups are done (3-4 more days).
>> 
>> The default is 128KB, I believe, so it's strange that you would test
>> smaller values.  ;)  I would try something along the lines of 1 or 2 MB.
>
> Err, yes this should have been MB not KB.
>
>
> $cat /sys/block/sdc/queue/read_ahead_kb 
> 16384
> $cat /sys/block/sdd/queue/read_ahead_kb 
> 16384
>
> I also tried different values for max_sectors_kb, nr_requests. But the
> trend that writes were much faster than reads while there was read and
> write load on the device didn't change.
>
> Changing the deadline parameter writes_starved, write_expire,
> read_expire, front_merges or fifo_batch didn't change this behavoir.

OK, bumping up readahead and changing the deadline parameters listed
should have give some better results, I would think.  Can you give the
invocation of tiobench you used so I can try to reproduce this?

Thanks!
Jeff

Re: io-scheduler tuning for better read/write ratio

[Ralf Gross]

Jeff Moyer schrieb:
> Ralf Gross <Ralf-Lists@ralfgross.de> writes:
> 
> > Jeff Moyer schrieb:
> >> Ralf Gross <rg@STZ-Softwaretechnik.com> writes:
> >> 
> >> > Jeff Moyer schrieb:
> >> >> Jeff Moyer <jmoyer@redhat.com> writes:
> >> >> 
> >> >> > Ralf Gross <rg@stz-softwaretechnik.com> writes:
> >> >> >
> >> >> >> Casey Dahlin schrieb:
> >> >> >>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
> >> >> >>> > David Newall schrieb:
> >> >> >>> >> Ralf Gross wrote:
> >> >> >>> >>> write throughput is much higher than the read throughput (40 MB/s
> >> >> >>> >>> read, 90 MB/s write).
> >> >> >>> > 
> >> >> >>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
> >> >> >>> > to the device at the same time.
> >> >> >>> > 
> >> >> >>> > Ralf
> >> >> >>> 
> >> >> >>> How specifically are you testing? It could depend a lot on the
> >> >> >>> particular access patterns you're using to test.
> >> >> >>
> >> >> >> I did the basic tests with tiobench. The real test is a test backup
> >> >> >> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
> >> >> >> The jobs partially write to the device in parallel. Depending which
> >> >> >> spool file reaches the 30 GB first, one starts reading from that file
> >> >> >> and writing to tape, while to other is still spooling.
> >> >> >
> >> >> > We are missing a lot of details, here.  I guess the first thing I'd try
> >> >> > would be bumping up the max_readahead_kb parameter, since I'm guessing
> >> >> > that your backup application isn't driving very deep queue depths.  If
> >> >> > that doesn't work, then please provide exact invocations of tiobench
> >> >> > that reprduce the problem or some blktrace output for your real test.
> >> >> 
> >> >> Any news, Ralf?
> >> >
> >> > sorry for the delay. atm there are large backups running and using the
> >> > raid device for spooling. So I can't do any tests.
> >> >
> >> > Re. read ahead: I tested different settings from 8Kb to 65Kb, this
> >> > didn't help.
> >> >
> >> > I'll do some more tests when the backups are done (3-4 more days).
> >> 
> >> The default is 128KB, I believe, so it's strange that you would test
> >> smaller values.  ;)  I would try something along the lines of 1 or 2 MB.
> >
> > Err, yes this should have been MB not KB.
> >
> >
> > $cat /sys/block/sdc/queue/read_ahead_kb 
> > 16384
> > $cat /sys/block/sdd/queue/read_ahead_kb 
> > 16384
> >
> > I also tried different values for max_sectors_kb, nr_requests. But the
> > trend that writes were much faster than reads while there was read and
> > write load on the device didn't change.
> >
> > Changing the deadline parameter writes_starved, write_expire,
> > read_expire, front_merges or fifo_batch didn't change this behavoir.
> 
> OK, bumping up readahead and changing the deadline parameters listed
> should have give some better results, I would think.  Can you give the
> invocation of tiobench you used so I can try to reproduce this?

The main problem is with bacula. It reads/writes from/to two
spoolfiles on the same device.

I get the same behavior with 2 dd processes, one reading from disk, one writing
to it.

Here's the output from dstat (5 sec intervall).

--dsk/md1--
_read _writ
  26M   95M
  31M   96M
  20M   85M
  31M  108M
  28M   89M
  24M   95M
  26M   79M
  32M  115M
  50M   74M
 129M   15k
 147M 1638B
 147M    0 
 147M    0 
 113M    0


At the end I stopped the dd process that is writing to the device, so you can
see that the md device is capable of reading with >120 MB/s.

I did this with these two commands.

dd if=/dev/zero of=test bs=1MB
dd if=/dev/md1 of=/dev/null bs=1M


Maybe this is too simple, but with a real world application I see the same
behavior. md1 is a md raid 0 device with 2 disks.


md1 : active raid0 sdc[0] sdd[1]
      781422592 blocks 64k chunks

sdc:

/sys/block/sdc/queue/hw_sector_size
512
/sys/block/sdc/queue/max_hw_sectors_kb
32767
/sys/block/sdc/queue/max_sectors_kb
512
/sys/block/sdc/queue/nomerges
0
/sys/block/sdc/queue/nr_requests
128
/sys/block/sdc/queue/read_ahead_kb
16384
/sys/block/sdc/queue/scheduler
noop anticipatory [deadline] cfq

/sys/block/sdc/queue/iosched/fifo_batch
16
/sys/block/sdc/queue/iosched/front_merges
1
/sys/block/sdc/queue/iosched/read_expire
500
/sys/block/sdc/queue/iosched/write_expire
5000
/sys/block/sdc/queue/iosched/writes_starved
2


sdd:

/sys/block/sdd/queue/hw_sector_size
512
/sys/block/sdd/queue/max_hw_sectors_kb
32767
/sys/block/sdd/queue/max_sectors_kb
512
/sys/block/sdd/queue/nomerges
0
/sys/block/sdd/queue/nr_requests
128
/sys/block/sdd/queue/read_ahead_kb
16384
/sys/block/sdd/queue/scheduler
noop anticipatory [deadline] cfq 


/sys/block/sdd/queue/iosched/fifo_batch
16
/sys/block/sdd/queue/iosched/front_merges
1
/sys/block/sdd/queue/iosched/read_expire
500
/sys/block/sdd/queue/iosched/write_expire
5000
/sys/block/sdd/queue/iosched/writes_starved
2


The deadline parameters are the default ones. Setting writes_starved much
higher I expected a change in the read/write ratio, but didn't see any change.



Ralf

Re: io-scheduler tuning for better read/write ratio

[Al Boldi]

Ralf Gross wrote:
> The main problem is with bacula. It reads/writes from/to two
> spoolfiles on the same device.
>
> I get the same behavior with 2 dd processes, one reading from disk, one
> writing to it.
>
> Here's the output from dstat (5 sec intervall).
>
> --dsk/md1--
> _read _writ
>   26M   95M
>   31M   96M
>   20M   85M
>   31M  108M
>   28M   89M
>   24M   95M
>   26M   79M
>   32M  115M
>   50M   74M
>  129M   15k
>  147M 1638B
>  147M    0
>  147M    0
>  113M    0
>
>
> At the end I stopped the dd process that is writing to the device, so you
> can see that the md device is capable of reading with >120 MB/s.
>
> I did this with these two commands.
>
> dd if=/dev/zero of=test bs=1MB
> dd if=/dev/md1 of=/dev/null bs=1M

Try changing /proc/sys/vm/dirty_ratio = 1


Thanks!

--
Al

Re: io-scheduler tuning for better read/write ratio

[Ralf Gross]

Al Boldi schrieb:
> Ralf Gross wrote:
> > The main problem is with bacula. It reads/writes from/to two
> > spoolfiles on the same device.
> >
> > I get the same behavior with 2 dd processes, one reading from disk, one
> > writing to it.
> >
> > Here's the output from dstat (5 sec intervall).
> >
> > --dsk/md1--
> > _read _writ
> >   26M   95M
> >   31M   96M
> >   20M   85M
> >   31M  108M
> >   28M   89M
> >   24M   95M
> >   26M   79M
> >   32M  115M
> >   50M   74M
> >  129M   15k
> >  147M 1638B
> >  147M    0
> >  147M    0
> >  113M    0
> >
> >
> > At the end I stopped the dd process that is writing to the device, so you
> > can see that the md device is capable of reading with >120 MB/s.
> >
> > I did this with these two commands.
> >
> > dd if=/dev/zero of=test bs=1MB
> > dd if=/dev/md1 of=/dev/null bs=1M
> 
> Try changing /proc/sys/vm/dirty_ratio = 1

$cat /proc/sys/vm/dirty_ratio
1


$dstat -D md1 -d 5
--dsk/md1--
_read _writ
  18M   18M
   0     0 
 820k  101M
  18M  113M
  26M   73M
  26M  110M
  32M  100M
  19M  111M
  13M  117M
  13M  142M
  32M   88M
  26M   99M
  38M   58M

No change. Even setting dirty_ratio to 100 didn't show any difference.


With the cfq scheduler and slice_idle = 24 (trial and error) I get better
results. Itried this before, but the overall throughput was a bit lower than
with deadline.

It seems that I can not tune deadline to get the samet behaviour.

--dsk/md1--
_read _writ
  18M   18M
  25M   77M
  51M   65M
  51M   47M
  62M   45M
  53M   28M
  45M   43M
  46M   47M
  47M   42M
  51M   41M
  38M   51M
  51M   40M
  45M   40M
  58M   42M
  69M   41M
  72M   42M
 122M    0
 141M  340k
--dsk/md1--
_read _writ
 139M  562k
 136M    0
 141M   13k
  64M    0
1638B  104M
   0   110M
   0   122M
   0   104M
   0   108M

The last numbers are for reading/writing only.

Ralf

Re: io-scheduler tuning for better read/write ratio

[Al Boldi]

Ralf Gross wrote:
> Al Boldi schrieb:
> > Try changing /proc/sys/vm/dirty_ratio = 1
>
> $cat /proc/sys/vm/dirty_ratio
> 1
>
>
> $dstat -D md1 -d 5
> --dsk/md1--
> _read _writ
>   18M   18M
>    0     0
>  820k  101M
>   18M  113M
>   26M   73M
>   26M  110M
>   32M  100M
>   19M  111M
>   13M  117M
>   13M  142M
>   32M   88M
>   26M   99M
>   38M   58M
>
> No change. Even setting dirty_ratio to 100 didn't show any difference.

What's your readahead?  Do a blockdev --getra /dev/sdX and /dev/mdX.
Try increasing it, while keeping dirty_ratio low.


Thanks!

--
Al

Re: io-scheduler tuning for better read/write ratio

[Ralf Gross]

Al Boldi schrieb:
> Ralf Gross wrote:
> > The main problem is with bacula. It reads/writes from/to two
> > spoolfiles on the same device.
> >
> > I get the same behavior with 2 dd processes, one reading from disk, one
> > writing to it.
> >
> > Here's the output from dstat (5 sec intervall).
> >
> > --dsk/md1--
> > _read _writ
> >   26M   95M
> >   31M   96M
> >   20M   85M
> >   31M  108M
> >   28M   89M
> >   24M   95M
> >   26M   79M
> >   32M  115M
> >   50M   74M
> >  129M   15k
> >  147M 1638B
> >  147M    0
> >  147M    0
> >  113M    0
> >
> >
> > At the end I stopped the dd process that is writing to the device, so you
> > can see that the md device is capable of reading with >120 MB/s.
> >
> > I did this with these two commands.
> >
> > dd if=/dev/zero of=test bs=1MB
> > dd if=/dev/md1 of=/dev/null bs=1M
> 
> Try changing /proc/sys/vm/dirty_ratio = 1

$cat /proc/sys/vm/dirty_ratio
1


$dstat -D md1 -d 5
--dsk/md1--
_read _writ
  18M   18M
   0     0 
 820k  101M
  18M  113M
  26M   73M
  26M  110M
  32M  100M
  19M  111M
  13M  117M
  13M  142M
  32M   88M
  26M   99M
  38M   58M

No change. Even setting dirty_ratio to 100 didn't show any difference.


With the cfq scheduler and slice_idle = 24 (trial and error) I get better
results. Itried this before, but the overall throughput was a bit lower than
with deadline.

It seems that I can not tune deadline to get the samet behaviour.

--dsk/md1--
_read _writ
  18M   18M
  25M   77M
  51M   65M
  51M   47M
  62M   45M
  53M   28M
  45M   43M
  46M   47M
  47M   42M
  51M   41M
  38M   51M
  51M   40M
  45M   40M
  58M   42M
  69M   41M
  72M   42M
 122M    0
 141M  340k
--dsk/md1--
_read _writ
 139M  562k
 136M    0
 141M   13k
  64M    0
1638B  104M
   0   110M
   0   122M
   0   104M
   0   108M

The last numbers are for reading/writing only.

Ralf

Re: io-scheduler tuning for better read/write ratio

[Wu Fengguang]

On Tue, Jun 23, 2009 at 03:42:46AM +0800, Jeff Moyer wrote:
> Ralf Gross <rg@STZ-Softwaretechnik.com> writes:
> 
> > Jeff Moyer schrieb:
> >> Jeff Moyer <jmoyer@redhat.com> writes:
> >> 
> >> > Ralf Gross <rg@stz-softwaretechnik.com> writes:
> >> >
> >> >> Casey Dahlin schrieb:
> >> >>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
> >> >>> > David Newall schrieb:
> >> >>> >> Ralf Gross wrote:
> >> >>> >>> write throughput is much higher than the read throughput (40 MB/s
> >> >>> >>> read, 90 MB/s write).
> >> >>> > 
> >> >>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
> >> >>> > to the device at the same time.
> >> >>> > 
> >> >>> > Ralf
> >> >>> 
> >> >>> How specifically are you testing? It could depend a lot on the
> >> >>> particular access patterns you're using to test.
> >> >>
> >> >> I did the basic tests with tiobench. The real test is a test backup
> >> >> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
> >> >> The jobs partially write to the device in parallel. Depending which
> >> >> spool file reaches the 30 GB first, one starts reading from that file
> >> >> and writing to tape, while to other is still spooling.
> >> >
> >> > We are missing a lot of details, here.  I guess the first thing I'd try
> >> > would be bumping up the max_readahead_kb parameter, since I'm guessing
> >> > that your backup application isn't driving very deep queue depths.  If
> >> > that doesn't work, then please provide exact invocations of tiobench
> >> > that reprduce the problem or some blktrace output for your real test.
> >> 
> >> Any news, Ralf?
> >
> > sorry for the delay. atm there are large backups running and using the
> > raid device for spooling. So I can't do any tests.
> >
> > Re. read ahead: I tested different settings from 8Kb to 65Kb, this
> > didn't help.
> >
> > I'll do some more tests when the backups are done (3-4 more days).
> 
> The default is 128KB, I believe, so it's strange that you would test
> smaller values.  ;)  I would try something along the lines of 1 or 2 MB.
> 
> I'm CCing Fengguang in case he has any suggestions.

Jeff, thank you for the forwarding (and sorry for the long delay)!

The read:write (or rather sync:async) ratio control is an IO scheduler
feature. CFQ has parameters slice_sync and slice_async for that.
What's more, CFQ will let async IO wait if there are any in flight
sync IO. This is good, but not quite enough. Normally sync IOs come
one by one, with some small idle time window in between. If we only
start dispatching async IOs after the last sync IO has completed for
eg. 1ms, then we may stop the async background write IOs when there
are active sync foreground read IO stream.

This simple patch aims to address the writes-push-aside-reads problem.
Ralf, you can try applying this patch and run your workload with this
(huge) CFQ parameter:

        echo 1000 > /sys/block/sda/queue/iosched/slice_sync 

The patch is based on 2.6.30, but can be trivially backported if you
want to use some old kernel.

It may impact overall (sync+async) IO throughput when there are one or
more ongoing sync IO streams, so requires considerable benchmarks and
adjustments.

Thanks,
Fengguang
---

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index a55a9bd..14011b7 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1064,7 +1064,6 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
 	if (blk_queue_nonrot(cfqd->queue) && cfqd->hw_tag)
 		return;
 
-	WARN_ON(!RB_EMPTY_ROOT(&cfqq->sort_list));
 	WARN_ON(cfq_cfqq_slice_new(cfqq));
 
 	/*
@@ -2175,8 +2174,6 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
 	 * or if we want to idle in case it has no pending requests.
 	 */
 	if (cfqd->active_queue == cfqq) {
-		const bool cfqq_empty = RB_EMPTY_ROOT(&cfqq->sort_list);
-
 		if (cfq_cfqq_slice_new(cfqq)) {
 			cfq_set_prio_slice(cfqd, cfqq);
 			cfq_clear_cfqq_slice_new(cfqq);
@@ -2190,8 +2187,8 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
 		 */
 		if (cfq_slice_used(cfqq) || cfq_class_idle(cfqq))
 			cfq_slice_expired(cfqd, 1);
-		else if (cfqq_empty && !cfq_close_cooperator(cfqd, cfqq, 1) &&
-			 sync && !rq_noidle(rq))
+		else if (sync && !rq_noidle(rq) &&
+			 !cfq_close_cooperator(cfqd, cfqq, 1))
 			cfq_arm_slice_timer(cfqd);
 	}
 

Re: io-scheduler tuning for better read/write ratio

[Jens Axboe]

On Fri, Jun 26 2009, Wu Fengguang wrote:
> On Tue, Jun 23, 2009 at 03:42:46AM +0800, Jeff Moyer wrote:
> > Ralf Gross <rg@STZ-Softwaretechnik.com> writes:
> > 
> > > Jeff Moyer schrieb:
> > >> Jeff Moyer <jmoyer@redhat.com> writes:
> > >> 
> > >> > Ralf Gross <rg@stz-softwaretechnik.com> writes:
> > >> >
> > >> >> Casey Dahlin schrieb:
> > >> >>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
> > >> >>> > David Newall schrieb:
> > >> >>> >> Ralf Gross wrote:
> > >> >>> >>> write throughput is much higher than the read throughput (40 MB/s
> > >> >>> >>> read, 90 MB/s write).
> > >> >>> > 
> > >> >>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
> > >> >>> > to the device at the same time.
> > >> >>> > 
> > >> >>> > Ralf
> > >> >>> 
> > >> >>> How specifically are you testing? It could depend a lot on the
> > >> >>> particular access patterns you're using to test.
> > >> >>
> > >> >> I did the basic tests with tiobench. The real test is a test backup
> > >> >> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
> > >> >> The jobs partially write to the device in parallel. Depending which
> > >> >> spool file reaches the 30 GB first, one starts reading from that file
> > >> >> and writing to tape, while to other is still spooling.
> > >> >
> > >> > We are missing a lot of details, here.  I guess the first thing I'd try
> > >> > would be bumping up the max_readahead_kb parameter, since I'm guessing
> > >> > that your backup application isn't driving very deep queue depths.  If
> > >> > that doesn't work, then please provide exact invocations of tiobench
> > >> > that reprduce the problem or some blktrace output for your real test.
> > >> 
> > >> Any news, Ralf?
> > >
> > > sorry for the delay. atm there are large backups running and using the
> > > raid device for spooling. So I can't do any tests.
> > >
> > > Re. read ahead: I tested different settings from 8Kb to 65Kb, this
> > > didn't help.
> > >
> > > I'll do some more tests when the backups are done (3-4 more days).
> > 
> > The default is 128KB, I believe, so it's strange that you would test
> > smaller values.  ;)  I would try something along the lines of 1 or 2 MB.
> > 
> > I'm CCing Fengguang in case he has any suggestions.
> 
> Jeff, thank you for the forwarding (and sorry for the long delay)!
> 
> The read:write (or rather sync:async) ratio control is an IO scheduler
> feature. CFQ has parameters slice_sync and slice_async for that.
> What's more, CFQ will let async IO wait if there are any in flight
> sync IO. This is good, but not quite enough. Normally sync IOs come
> one by one, with some small idle time window in between. If we only
> start dispatching async IOs after the last sync IO has completed for
> eg. 1ms, then we may stop the async background write IOs when there
> are active sync foreground read IO stream.
> 
> This simple patch aims to address the writes-push-aside-reads problem.
> Ralf, you can try applying this patch and run your workload with this
> (huge) CFQ parameter:
> 
>         echo 1000 > /sys/block/sda/queue/iosched/slice_sync 
> 
> The patch is based on 2.6.30, but can be trivially backported if you
> want to use some old kernel.
> 
> It may impact overall (sync+async) IO throughput when there are one or
> more ongoing sync IO streams, so requires considerable benchmarks and
> adjustments.
> 
> Thanks,
> Fengguang
> ---
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index a55a9bd..14011b7 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1064,7 +1064,6 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
>  	if (blk_queue_nonrot(cfqd->queue) && cfqd->hw_tag)
>  		return;
>  
> -	WARN_ON(!RB_EMPTY_ROOT(&cfqq->sort_list));
>  	WARN_ON(cfq_cfqq_slice_new(cfqq));
>  
>  	/*
> @@ -2175,8 +2174,6 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
>  	 * or if we want to idle in case it has no pending requests.
>  	 */
>  	if (cfqd->active_queue == cfqq) {
> -		const bool cfqq_empty = RB_EMPTY_ROOT(&cfqq->sort_list);
> -
>  		if (cfq_cfqq_slice_new(cfqq)) {
>  			cfq_set_prio_slice(cfqd, cfqq);
>  			cfq_clear_cfqq_slice_new(cfqq);
> @@ -2190,8 +2187,8 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
>  		 */
>  		if (cfq_slice_used(cfqq) || cfq_class_idle(cfqq))
>  			cfq_slice_expired(cfqd, 1);
> -		else if (cfqq_empty && !cfq_close_cooperator(cfqd, cfqq, 1) &&
> -			 sync && !rq_noidle(rq))
> +		else if (sync && !rq_noidle(rq) &&
> +			 !cfq_close_cooperator(cfqd, cfqq, 1))
>  			cfq_arm_slice_timer(cfqd);
>  	}

What's the purpose of this patch? If you have requests pending you don't
want to arm the idle timer and wait, you want to dispatch those.

-- 
Jens Axboe

Re: io-scheduler tuning for better read/write ratio

[Wu Fengguang]

On Fri, Jun 26, 2009 at 06:44:06PM +0800, Jens Axboe wrote:
> On Fri, Jun 26 2009, Wu Fengguang wrote:
> > On Tue, Jun 23, 2009 at 03:42:46AM +0800, Jeff Moyer wrote:
> > > Ralf Gross <rg@STZ-Softwaretechnik.com> writes:
> > > 
> > > > Jeff Moyer schrieb:
> > > >> Jeff Moyer <jmoyer@redhat.com> writes:
> > > >> 
> > > >> > Ralf Gross <rg@stz-softwaretechnik.com> writes:
> > > >> >
> > > >> >> Casey Dahlin schrieb:
> > > >> >>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
> > > >> >>> > David Newall schrieb:
> > > >> >>> >> Ralf Gross wrote:
> > > >> >>> >>> write throughput is much higher than the read throughput (40 MB/s
> > > >> >>> >>> read, 90 MB/s write).
> > > >> >>> > 
> > > >> >>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
> > > >> >>> > to the device at the same time.
> > > >> >>> > 
> > > >> >>> > Ralf
> > > >> >>> 
> > > >> >>> How specifically are you testing? It could depend a lot on the
> > > >> >>> particular access patterns you're using to test.
> > > >> >>
> > > >> >> I did the basic tests with tiobench. The real test is a test backup
> > > >> >> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
> > > >> >> The jobs partially write to the device in parallel. Depending which
> > > >> >> spool file reaches the 30 GB first, one starts reading from that file
> > > >> >> and writing to tape, while to other is still spooling.
> > > >> >
> > > >> > We are missing a lot of details, here.  I guess the first thing I'd try
> > > >> > would be bumping up the max_readahead_kb parameter, since I'm guessing
> > > >> > that your backup application isn't driving very deep queue depths.  If
> > > >> > that doesn't work, then please provide exact invocations of tiobench
> > > >> > that reprduce the problem or some blktrace output for your real test.
> > > >> 
> > > >> Any news, Ralf?
> > > >
> > > > sorry for the delay. atm there are large backups running and using the
> > > > raid device for spooling. So I can't do any tests.
> > > >
> > > > Re. read ahead: I tested different settings from 8Kb to 65Kb, this
> > > > didn't help.
> > > >
> > > > I'll do some more tests when the backups are done (3-4 more days).
> > > 
> > > The default is 128KB, I believe, so it's strange that you would test
> > > smaller values.  ;)  I would try something along the lines of 1 or 2 MB.
> > > 
> > > I'm CCing Fengguang in case he has any suggestions.
> > 
> > Jeff, thank you for the forwarding (and sorry for the long delay)!
> > 
> > The read:write (or rather sync:async) ratio control is an IO scheduler
> > feature. CFQ has parameters slice_sync and slice_async for that.
> > What's more, CFQ will let async IO wait if there are any in flight
> > sync IO. This is good, but not quite enough. Normally sync IOs come
> > one by one, with some small idle time window in between. If we only
> > start dispatching async IOs after the last sync IO has completed for
> > eg. 1ms, then we may stop the async background write IOs when there
> > are active sync foreground read IO stream.
> > 
> > This simple patch aims to address the writes-push-aside-reads problem.
> > Ralf, you can try applying this patch and run your workload with this
> > (huge) CFQ parameter:
> > 
> >         echo 1000 > /sys/block/sda/queue/iosched/slice_sync 
> > 
> > The patch is based on 2.6.30, but can be trivially backported if you
> > want to use some old kernel.
> > 
> > It may impact overall (sync+async) IO throughput when there are one or
> > more ongoing sync IO streams, so requires considerable benchmarks and
> > adjustments.
> > 
> > Thanks,
> > Fengguang
> > ---
> > 
> > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> > index a55a9bd..14011b7 100644
> > --- a/block/cfq-iosched.c
> > +++ b/block/cfq-iosched.c
> > @@ -1064,7 +1064,6 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
> >  	if (blk_queue_nonrot(cfqd->queue) && cfqd->hw_tag)
> >  		return;
> >  
> > -	WARN_ON(!RB_EMPTY_ROOT(&cfqq->sort_list));
> >  	WARN_ON(cfq_cfqq_slice_new(cfqq));
> >  
> >  	/*
> > @@ -2175,8 +2174,6 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
> >  	 * or if we want to idle in case it has no pending requests.
> >  	 */
> >  	if (cfqd->active_queue == cfqq) {
> > -		const bool cfqq_empty = RB_EMPTY_ROOT(&cfqq->sort_list);
> > -
> >  		if (cfq_cfqq_slice_new(cfqq)) {
> >  			cfq_set_prio_slice(cfqd, cfqq);
> >  			cfq_clear_cfqq_slice_new(cfqq);
> > @@ -2190,8 +2187,8 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
> >  		 */
> >  		if (cfq_slice_used(cfqq) || cfq_class_idle(cfqq))
> >  			cfq_slice_expired(cfqd, 1);
> > -		else if (cfqq_empty && !cfq_close_cooperator(cfqd, cfqq, 1) &&
> > -			 sync && !rq_noidle(rq))
> > +		else if (sync && !rq_noidle(rq) &&
> > +			 !cfq_close_cooperator(cfqd, cfqq, 1))
> >  			cfq_arm_slice_timer(cfqd);
> >  	}
> 
> What's the purpose of this patch? If you have requests pending you don't
> want to arm the idle timer and wait, you want to dispatch those.

You are right, please ignore this mindless hacking patch.

Ralf, you can do the read/write ratio in the CFQ scheduler by tuning
the slice_sync/slice_async parameters.

For example,

        echo 10 > /sys//block/sda/queue/iosched/slice_async
        echo 100 > /sys//block/sda/queue/iosched/slice_sync 

gives

-dsk/total-
 read  writ
  66M   25M
  65M   20M
  49M   32M
  84M   19M
  46M   28M
  61M   23M
  55M   25M
  67M   23M
  76M   18M
  46M   31M
  56M   29M
  54M   23M
  76M   20M

while 

        echo 10 > /sys//block/sda/queue/iosched/slice_async
        echo 300 > /sys//block/sda/queue/iosched/slice_sync 

gives

-dsk/total-
 read  writ
 102M   11M
  82M   10M
 100M   12M
  86M   10M
  95M   11M
 102M 3168k
  96M   11M
  88M   10M
  96M   12M

However too large slice_sync may not be desirable.

Thanks,
Fengguang

Re: io-scheduler tuning for better read/write ratio

[Ralf Gross]

Wu Fengguang schrieb:
> On Fri, Jun 26, 2009 at 06:44:06PM +0800, Jens Axboe wrote:
> > On Fri, Jun 26 2009, Wu Fengguang wrote:
> > > On Tue, Jun 23, 2009 at 03:42:46AM +0800, Jeff Moyer wrote:
> > > > Ralf Gross <rg@STZ-Softwaretechnik.com> writes:
> > > > 
> > > > > Jeff Moyer schrieb:
> > > > >> Jeff Moyer <jmoyer@redhat.com> writes:
> > > > >> 
> > > > >> > Ralf Gross <rg@stz-softwaretechnik.com> writes:
> > > > >> >
> > > > >> >> Casey Dahlin schrieb:
> > > > >> >>> On 06/16/2009 02:40 PM, Ralf Gross wrote:
> > > > >> >>> > David Newall schrieb:
> > > > >> >>> >> Ralf Gross wrote:
> > > > >> >>> >>> write throughput is much higher than the read throughput (40 MB/s
> > > > >> >>> >>> read, 90 MB/s write).
> > > > >> >>> > 
> > > > >> >>> > Hm, but I get higher read throughput (160-200 MB/s) if I don't write
> > > > >> >>> > to the device at the same time.
> > > > >> >>> > 
> > > > >> >>> > Ralf
> > > > >> >>> 
> > > > >> >>> How specifically are you testing? It could depend a lot on the
> > > > >> >>> particular access patterns you're using to test.
> > > > >> >>
> > > > >> >> I did the basic tests with tiobench. The real test is a test backup
> > > > >> >> (bacula) with 2 jobs that create 2 30 GB spool files on that device.
> > > > >> >> The jobs partially write to the device in parallel. Depending which
> > > > >> >> spool file reaches the 30 GB first, one starts reading from that file
> > > > >> >> and writing to tape, while to other is still spooling.
> > > > >> >
> > > > >> > We are missing a lot of details, here.  I guess the first thing I'd try
> > > > >> > would be bumping up the max_readahead_kb parameter, since I'm guessing
> > > > >> > that your backup application isn't driving very deep queue depths.  If
> > > > >> > that doesn't work, then please provide exact invocations of tiobench
> > > > >> > that reprduce the problem or some blktrace output for your real test.
> > > > >> 
> > > > >> Any news, Ralf?
> > > > >
> > > > > sorry for the delay. atm there are large backups running and using the
> > > > > raid device for spooling. So I can't do any tests.
> > > > >
> > > > > Re. read ahead: I tested different settings from 8Kb to 65Kb, this
> > > > > didn't help.
> > > > >
> > > > > I'll do some more tests when the backups are done (3-4 more days).
> > > > 
> > > > The default is 128KB, I believe, so it's strange that you would test
> > > > smaller values.  ;)  I would try something along the lines of 1 or 2 MB.
> > > > 
> > > > I'm CCing Fengguang in case he has any suggestions.
> > > 
> > > Jeff, thank you for the forwarding (and sorry for the long delay)!
> > > 
> > > The read:write (or rather sync:async) ratio control is an IO scheduler
> > > feature. CFQ has parameters slice_sync and slice_async for that.
> > > What's more, CFQ will let async IO wait if there are any in flight
> > > sync IO. This is good, but not quite enough. Normally sync IOs come
> > > one by one, with some small idle time window in between. If we only
> > > start dispatching async IOs after the last sync IO has completed for
> > > eg. 1ms, then we may stop the async background write IOs when there
> > > are active sync foreground read IO stream.
> > > 
> > > This simple patch aims to address the writes-push-aside-reads problem.
> > > Ralf, you can try applying this patch and run your workload with this
> > > (huge) CFQ parameter:
> > > 
> > >         echo 1000 > /sys/block/sda/queue/iosched/slice_sync 
> > > 
> > > The patch is based on 2.6.30, but can be trivially backported if you
> > > want to use some old kernel.
> > > 
> > > It may impact overall (sync+async) IO throughput when there are one or
> > > more ongoing sync IO streams, so requires considerable benchmarks and
> > > adjustments.
> > > 
> > > Thanks,
> > > Fengguang
> > > ---
> > > 
> > > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> > > index a55a9bd..14011b7 100644
> > > --- a/block/cfq-iosched.c
> > > +++ b/block/cfq-iosched.c
> > > @@ -1064,7 +1064,6 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
> > >  	if (blk_queue_nonrot(cfqd->queue) && cfqd->hw_tag)
> > >  		return;
> > >  
> > > -	WARN_ON(!RB_EMPTY_ROOT(&cfqq->sort_list));
> > >  	WARN_ON(cfq_cfqq_slice_new(cfqq));
> > >  
> > >  	/*
> > > @@ -2175,8 +2174,6 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
> > >  	 * or if we want to idle in case it has no pending requests.
> > >  	 */
> > >  	if (cfqd->active_queue == cfqq) {
> > > -		const bool cfqq_empty = RB_EMPTY_ROOT(&cfqq->sort_list);
> > > -
> > >  		if (cfq_cfqq_slice_new(cfqq)) {
> > >  			cfq_set_prio_slice(cfqd, cfqq);
> > >  			cfq_clear_cfqq_slice_new(cfqq);
> > > @@ -2190,8 +2187,8 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
> > >  		 */
> > >  		if (cfq_slice_used(cfqq) || cfq_class_idle(cfqq))
> > >  			cfq_slice_expired(cfqd, 1);
> > > -		else if (cfqq_empty && !cfq_close_cooperator(cfqd, cfqq, 1) &&
> > > -			 sync && !rq_noidle(rq))
> > > +		else if (sync && !rq_noidle(rq) &&
> > > +			 !cfq_close_cooperator(cfqd, cfqq, 1))
> > >  			cfq_arm_slice_timer(cfqd);
> > >  	}
> > 
> > What's the purpose of this patch? If you have requests pending you don't
> > want to arm the idle timer and wait, you want to dispatch those.
> 
> You are right, please ignore this mindless hacking patch.
> 
> Ralf, you can do the read/write ratio in the CFQ scheduler by tuning
> the slice_sync/slice_async parameters.
> 
> For example,
> 
>         echo 10 > /sys//block/sda/queue/iosched/slice_async
>         echo 100 > /sys//block/sda/queue/iosched/slice_sync 
> 
> gives
> 
> -dsk/total-
>  read  writ
>   66M   25M
>   65M   20M
>   49M   32M
>   84M   19M
>   46M   28M
>   61M   23M
>   55M   25M
>   67M   23M
>   76M   18M
>   46M   31M
>   56M   29M
>   54M   23M
>   76M   20M


writing:

--dsk/md1--
_read _writ
   0   150M
   0   142M
   0   143M
   0   112M
   0   141M
   0   152M
   0   132M
   0   123M
   0   149M


reading:

--dsk/md1--
_read _writ
 143M    0 
 145M    0 
 160M    0 
 128M    0 
 148M    0 
 140M    0 
 158M    0 
 130M    0 
 122M    0 

reading + writing:

--dsk/md1--
_read _writ
  55M   76M
  41M   83M
  64M   81M
  64M   83M
  63M   68M
  56M  117M
  41M   61M
  64M   87M
  64M   69M
  61M   87M
  67M   81M
  64M   33M
  63M   68M
  56M   76M


 
> while 
> 
>         echo 10 > /sys//block/sda/queue/iosched/slice_async
>         echo 300 > /sys//block/sda/queue/iosched/slice_sync 
> 
> gives
> 
> -dsk/total-
>  read  writ
>  102M   11M
>   82M   10M
>  100M   12M
>   86M   10M
>   95M   11M
>  102M 3168k
>   96M   11M
>   88M   10M
>   96M   12M
> 
> However too large slice_sync may not be desirable.

writing:

--dsk/md1--
_read _writ
   0   131M
   0   136M
   0   145M
   0   136M
   0   128M
   0   150M
   0   127M
   0   149M
   0   127M
   0   156M
   0   125M
   0   142M

reading:

--dsk/md1--
_read _writ
 128M    0
 160M    0
 128M    0
 128M    0
 160M    0
 128M    0
 109M    0
 128M    0
 128M    0
 160M    0
 128M    0


writing:

--dsk/md1--
_read _writ
   0   183M
   0   142M
   0   137M
   0   147M
   0   135M
   0   147M
   0   117M
   0   135M
   0   156M
   0   120M
   0   147M
   0   135M

reading + writing:

--dsk/md1--
_read _writ
  96M   40M
  64M   38M
  96M   29M
  96M   24M
  96M   31M
  95M   35M
  97M   26M
  96M   23M
  96M   33M
  95M   73M
  91M   25M


Thanks, this seem to be what I was looking for. I'll change the scheduler
parameter for all spool devices and will run a backup with two concurrent
backups. This will show me if bacula behaves the same as the simple dd test
does.


Ralf