Abysmal write performance on HW RAID5

Abysmal write performance on HW RAID5

[ChristopherD]

In the process of upgrading my RAID5 array, I've run into a brick wall (<
4MB/sec avg write perf!) that I could use some help figuring out.  I'll
start with the quick backstory and setup.

Common Setup:

Dell Dimension XPS T800, salvaged from Mom. (i440BX chipset, Pentium3 @
800MHZ)
768MB DDR SDRAM @ 100MHZ FSB  (3x256MB DIMM)
PCI vid card (ATI Rage 128)
PCI 10/100 NIC (3Com 905)
PCI RAID controller (LSI MegaRAID i4 - 4 channel PATA)
4 x 250GB (WD2500) UltraATA drives, each connected to separate channels on
the controller
Ubuntu Feisty Fawn

In the LSI BIOS config, I setup the full capacity of all four drives as a
single logical disk using RAID5 @ 64K strips size.  I installed the OS from
the CD, allowing it to create a 4GB swap partition (sda2) and use the rest
as a single ext3 partition (sda1) with roughly 700GB space.

This setup ran fine for months as my home fileserver.  Being new to RAID at
the time, I didn't know or think about tuning or benchmarking, etc, etc.  I
do know that I often moved ISO images to this machine from my gaming rig
using both SAMBA and FTP, with xfer limited by the 100MBit LAN (~11MB/sec).

About a month or so ago, I hit capacity on the partition.  I dumped some
movies off to a USB drive (500GB PATA) and started watching the drive aisle
at Fry's.  Last week, I saw what I'd been waiting for: Maxtor 500GB drives @
$99 each.  So, I bought three of them and started this adventure.


I'll skip the details on the pain in the butt of moving 700GB of data onto
various drives of various sizes...the end result was the following change to
my setup:

3 x Maxtor 500GB PATA drives (7200rpm, 16MB cache)
1 x IBM/Hitachi Deskstar 500GB PATA (7200rpm, 8MB cache)

Each drive still on a separate controller channel, this time configured into
two logical drives:
Logical Disk 1:  RAID0, 16GB, 64K stripe size (sda)
Logical Disk 2:  RAID5, 1.5TB, 128K stripe size (sdb)


I also took this opportunity to upgrade to the newest Ubuntu 7.10 (Gutsy),
and having done some reading, planned to make some tweaks to the partition
formats.  After fighting with the standard CD, which refused to install the
OS without also formatting the root partition (but not offering any control
of the formatting), i downloaded the "alternate CD" and used the textmode
installer.

I set up the partitions like this:
sda1: 14.5GB ext3, 256MB journal (mounted data_ordered), 4K block size,
stride=16, sparse superblocks, no resize_inode, 1GB reserved for root
sda2: 1.5GB linux swap
sdb1: 1.5TB ext2, largefile4 (4MB per inode), stride=32, sparse superblocks,
no resize_inode, 0 reserved for root

The format command was my first hint of a problem.  The block group creation
counter spun very rapidly up to 9800/11600 and then paused and I heard the
drives thrash.  The block groups completed at a slower pace, and then the
final creation process took several minutes.

But the real shocker was transferring my data onto this new partition.  FOUR
MEGABYTES PER SECOND?!?!

My initial plan was to plug a single old data drive into the motherboard's
ATA port, thinking the transfer speed within a single machine would be the
fastest possible mechanism.  Wrong.  I ended up mounting the drives using
USB enclosures to my laptop (RedHat EL 5.1) and sharing them via NFS.

So, deciding the partition was disposable (still unused), I fired up dd to
run some block device tests:
dd if=/dev/zero of=/dev/sdb bs=1M count=25

This ran silently and showed 108MB/sec??  OK, that beats 4...let's try
again!  Now I hear drive activity, and the result says 26MB/sec.  Running it
a third time immediately brought the rate down to 4MB/sec.  Apparently, the
first 64MB or so runs nice and fast (cache? the i4 only has 16MB onboard).

I also ran iostat -dx in the background during a 26GB directory copy
operation, reporting on 60-sec intervals.  This is a typical output:

Device:    rrqm/s  wrqm/s    r/s    w/s    rMB/s  wMB/s  avgrq-sz  avgqu-sz 
await    svctm  %util
sda          0.00     0.18      0.00  0.48   0.00   0.00        11.03   
0.01         21.66    16.73   0.61
sdb          0.00     0.72      0.03  64.28  0.00   3.95       125.43  
137.57    2180.23  15.85   100.02


So, the RAID5 device has a huge queue of write requests with an average wait
time of more than 2 seconds @ 100% utilization?  Or is this a bug in iostat?

At this point, I'm all ears...I don't even know where to start.  Is ext2 not
a good format for volumes of this size?  Then how to explain the block
device xfer rate being so bad, too?  Is it that I have one drive in the
array that's a different brand?  Or that it has a different cache size?

Anyone have any ideas?
-- 
View this message in context: http://www.nabble.com/Abysmal-write-performance-on-HW-RAID5-tf4884768.html#a13980960
Sent from the linux-raid mailing list archive at Nabble.com.

Re: Abysmal write performance on HW RAID5

[Mikael Abrahamsson]

On Tue, 27 Nov 2007, ChristopherD wrote:

> At this point, I'm all ears...I don't even know where to start.  Is ext2 
> not a good format for volumes of this size?  Then how to explain the 
> block device xfer rate being so bad, too?  Is it that I have one drive 
> in the array that's a different brand?  Or that it has a different cache 
> size?

Well, I have seen 3ware hwraid volumes slow down to 10 megabyte/s when 
writing, so I guess you're seeing a similar problem. It's due to the way 
raid5 works (needing to read before writing parity) and small caches.

If you need to write quickly, you should look into using software raid 
instead, then the raid5 sw raid subsystem has access to the entire ram 
block cache and hopefully don't need to read as much from the drives.

I don't know why you're only getting 4 MB/s, if you would have said 10-15 
MB/s I wouldn't have been surprised at all, but 4 does seem to be on the 
low side. When I tried with 3ware, the type of filesystem chosen made 
very small difference.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se

Re: Abysmal write performance on HW RAID5

[Bill Davidsen]

ChristopherD wrote:
> In the process of upgrading my RAID5 array, I've run into a brick wall (<
> 4MB/sec avg write perf!) that I could use some help figuring out.  I'll
> start with the quick backstory and setup.
>
> Common Setup:
>
> Dell Dimension XPS T800, salvaged from Mom. (i440BX chipset, Pentium3 @
> 800MHZ)
> 768MB DDR SDRAM @ 100MHZ FSB  (3x256MB DIMM)
> PCI vid card (ATI Rage 128)
> PCI 10/100 NIC (3Com 905)
> PCI RAID controller (LSI MegaRAID i4 - 4 channel PATA)
> 4 x 250GB (WD2500) UltraATA drives, each connected to separate channels on
> the controller
> Ubuntu Feisty Fawn
>
> In the LSI BIOS config, I setup the full capacity of all four drives as a
> single logical disk using RAID5 @ 64K strips size.  I installed the OS from
> the CD, allowing it to create a 4GB swap partition (sda2) and use the rest
> as a single ext3 partition (sda1) with roughly 700GB space.
>
> This setup ran fine for months as my home fileserver.  Being new to RAID at
> the time, I didn't know or think about tuning or benchmarking, etc, etc.  I
> do know that I often moved ISO images to this machine from my gaming rig
> using both SAMBA and FTP, with xfer limited by the 100MBit LAN (~11MB/sec).
>
> About a month or so ago, I hit capacity on the partition.  I dumped some
> movies off to a USB drive (500GB PATA) and started watching the drive aisle
> at Fry's.  Last week, I saw what I'd been waiting for: Maxtor 500GB drives @
> $99 each.  So, I bought three of them and started this adventure.
>
>
> I'll skip the details on the pain in the butt of moving 700GB of data onto
> various drives of various sizes...the end result was the following change to
> my setup:
>
> 3 x Maxtor 500GB PATA drives (7200rpm, 16MB cache)
> 1 x IBM/Hitachi Deskstar 500GB PATA (7200rpm, 8MB cache)
>
> Each drive still on a separate controller channel, this time configured into
> two logical drives:
> Logical Disk 1:  RAID0, 16GB, 64K stripe size (sda)
> Logical Disk 2:  RAID5, 1.5TB, 128K stripe size (sdb)
>
>
> I also took this opportunity to upgrade to the newest Ubuntu 7.10 (Gutsy),
> and having done some reading, planned to make some tweaks to the partition
> formats.  After fighting with the standard CD, which refused to install the
> OS without also formatting the root partition (but not offering any control
> of the formatting), i downloaded the "alternate CD" and used the textmode
> installer.
>
> I set up the partitions like this:
> sda1: 14.5GB ext3, 256MB journal (mounted data_ordered), 4K block size,
> stride=16, sparse superblocks, no resize_inode, 1GB reserved for root
> sda2: 1.5GB linux swap
> sdb1: 1.5TB ext2, largefile4 (4MB per inode), stride=32, sparse superblocks,
> no resize_inode, 0 reserved for root
>
> The format command was my first hint of a problem.  The block group creation
> counter spun very rapidly up to 9800/11600 and then paused and I heard the
> drives thrash.  The block groups completed at a slower pace, and then the
> final creation process took several minutes.
>
> But the real shocker was transferring my data onto this new partition.  FOUR
> MEGABYTES PER SECOND?!?!
>
> My initial plan was to plug a single old data drive into the motherboard's
> ATA port, thinking the transfer speed within a single machine would be the
> fastest possible mechanism.  Wrong.  I ended up mounting the drives using
> USB enclosures to my laptop (RedHat EL 5.1) and sharing them via NFS.
>   

I'm not sure you were wrong about internal being faster, but you clearly 
have tuning issues. The two obvious things which should be done are to 
(a) use blockdev to set the read ahead for the source drives to 
something large based on your memory size, 16384 is probably a 
reasonable starting value. Then set the stripe_cache_size in /sys, files 
like
  /sys/block/md1/md/stripe_cache_size
should get a fairly large value, see man pages and discussion for ideas 
on "fairly large" or start with 8192 just to see if it make a visible 
improvement. Finally, there are tunables in /proc/sys/vm which can help, 
but other things can be tried first.
> So, deciding the partition was disposable (still unused), I fired up dd to
> run some block device tests:
> dd if=/dev/zero of=/dev/sdb bs=1M count=25
>
> This ran silently and showed 108MB/sec??  OK, that beats 4...let's try
> again!  Now I hear drive activity, and the result says 26MB/sec.  Running it
> a third time immediately brought the rate down to 4MB/sec.  Apparently, the
> first 64MB or so runs nice and fast (cache? the i4 only has 16MB onboard).
>
> I also ran iostat -dx in the background during a 26GB directory copy
> operation, reporting on 60-sec intervals.  This is a typical output:
>
> Device:    rrqm/s  wrqm/s    r/s    w/s    rMB/s  wMB/s  avgrq-sz  avgqu-sz 
> await    svctm  %util
> sda          0.00     0.18      0.00  0.48   0.00   0.00        11.03   
> 0.01         21.66    16.73   0.61
> sdb          0.00     0.72      0.03  64.28  0.00   3.95       125.43  
> 137.57    2180.23  15.85   100.02
>   

This would have been nicer unwrapped, but shows the problem. Make 
changes and rerun?
>
> So, the RAID5 device has a huge queue of write requests with an average wait
> time of more than 2 seconds @ 100% utilization?  Or is this a bug in iostat?
>
> At this point, I'm all ears...I don't even know where to start.  Is ext2 not
> a good format for volumes of this size?  Then how to explain the block
> device xfer rate being so bad, too?  Is it that I have one drive in the
> array that's a different brand?  Or that it has a different cache size?
>
> Anyone have any ideas?
>   
You will get more and maybe better, but this is a start just to see if 
the problem responds to obvious changes.

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark 

RE: Abysmal write performance on HW RAID5

[Daniel Korstad]

> -----Original Message-----
> From: ChristopherD [mailto:christopherthe1@yahoo.com]
> Sent: Sunday, December 02, 2007 4:03 AM
> To: linux-raid@vger.kernel.org
> Subject: Abysmal write performance on HW RAID5
> 
> 
> In the process of upgrading my RAID5 array, I've run into a brick wall 
(<
> 4MB/sec avg write perf!) that I could use some help figuring out.  
I'll
> start with the quick backstory and setup.
> 
> Common Setup:
> 
> Dell Dimension XPS T800, salvaged from Mom. (i440BX chipset, Pentium3 
@
> 800MHZ)
> 768MB DDR SDRAM @ 100MHZ FSB  (3x256MB DIMM)
> PCI vid card (ATI Rage 128)
> PCI 10/100 NIC (3Com 905)
> PCI RAID controller (LSI MegaRAID i4 - 4 channel PATA)
> 4 x 250GB (WD2500) UltraATA drives, each connected to separate 
channels on
> the controller
> Ubuntu Feisty Fawn
> 
> In the LSI BIOS config, I setup the full capacity of all four drives 
as a
> single logical disk using RAID5 @ 64K strips size.  I installed the OS
> from
> the CD, allowing it to create a 4GB swap partition (sda2) and use the 
rest
> as a single ext3 partition (sda1) with roughly 700GB space.
> 
> This setup ran fine for months as my home fileserver.  Being new to 
RAID
> at
> the time, I didn't know or think about tuning or benchmarking, etc, 
etc.
> I
> do know that I often moved ISO images to this machine from my gaming 
rig
> using both SAMBA and FTP, with xfer limited by the 100MBit LAN
> (~11MB/sec).

That sounds about right; 11MB * 8 (bit/Byte) = 88Mbit on your 100M LAN.

> 
> About a month or so ago, I hit capacity on the partition.  I dumped 
some
> movies off to a USB drive (500GB PATA) and started watching the drive
> aisle
> at Fry's.  Last week, I saw what I'd been waiting for: Maxtor 500GB 
drives
> @
> $99 each.  So, I bought three of them and started this adventure.
> 
> 
> I'll skip the details on the pain in the butt of moving 700GB of data 
onto
> various drives of various sizes...the end result was the following 
change
> to
> my setup:
> 
> 3 x Maxtor 500GB PATA drives (7200rpm, 16MB cache)
> 1 x IBM/Hitachi Deskstar 500GB PATA (7200rpm, 8MB cache)
> 
> Each drive still on a separate controller channel, this time 
configured
> into
> two logical drives:
> Logical Disk 1:  RAID0, 16GB, 64K stripe size (sda)
> Logical Disk 2:  RAID5, 1.5TB, 128K stripe size (sdb)
> 
> 
> I also took this opportunity to upgrade to the newest Ubuntu 7.10 
(Gutsy),
> and having done some reading, planned to make some tweaks to the 
partition
> formats.  After fighting with the standard CD, which refused to 
install
> the
> OS without also formatting the root partition (but not offering any
> control
> of the formatting), i downloaded the "alternate CD" and used the 
textmode
> installer.
> 
> I set up the partitions like this:
> sda1: 14.5GB ext3, 256MB journal (mounted data_ordered), 4K block 
size,
> stride=16, sparse superblocks, no resize_inode, 1GB reserved for root
> sda2: 1.5GB linux swap
> sdb1: 1.5TB ext2, largefile4 (4MB per inode), stride=32, sparse
> superblocks,
> no resize_inode, 0 reserved for root
> 
> The format command was my first hint of a problem.  The block group
> creation
> counter spun very rapidly up to 9800/11600 and then paused and I heard 
the
> drives thrash.  The block groups completed at a slower pace, and then 
the
> final creation process took several minutes.
> 
> But the real shocker was transferring my data onto this new partition.
> FOUR
> MEGABYTES PER SECOND?!?!
> 
> My initial plan was to plug a single old data drive into the 
motherboard's
> ATA port, thinking the transfer speed within a single machine would be 
the
> fastest possible mechanism.  Wrong.  I ended up mounting the drives 
using
> USB enclosures to my laptop (RedHat EL 5.1) and sharing them via NFS.
> 
> So, deciding the partition was disposable (still unused), I fired up 
dd to
> run some block device tests:
> dd if=/dev/zero of=/dev/sdb bs=1M count=25
> 
> This ran silently and showed 108MB/sec??  OK, that beats 4...let's try
> again!  Now I hear drive activity, and the result says 26MB/sec.  
Running
> it
> a third time immediately brought the rate down to 4MB/sec.  
Apparently,
> the
> first 64MB or so runs nice and fast (cache? the i4 only has 16MB 
onboard).
> 
> I also ran iostat -dx in the background during a 26GB directory copy
> operation, reporting on 60-sec intervals.  This is a typical output:
> 
> Device:    rrqm/s  wrqm/s    r/s    w/s    rMB/s  wMB/s  avgrq-sz  
avgqu-
> sz
> await    svctm  %util
> sda          0.00     0.18      0.00  0.48   0.00   0.00        11.03
> 0.01         21.66    16.73   0.61
> sdb          0.00     0.72      0.03  64.28  0.00   3.95       125.43
> 137.57    2180.23  15.85   100.02
> 
> 
> So, the RAID5 device has a huge queue of write requests with an 
average
> wait
> time of more than 2 seconds @ 100% utilization?  Or is this a bug in
> iostat?
> 
> At this point, I'm all ears...I don't even know where to start.  Is 
ext2
> not
> a good format for volumes of this size?  Then how to explain the block
> device xfer rate being so bad, too?  Is it that I have one drive in 
the
> array that's a different brand?  Or that it has a different cache 
size?
> 
> Anyone have any ideas?
> 
> 
> UPDATE:
> I attached another drive to the motherboard's IDE port and installed
> Windows
> 2003 Server.  I used the swap partition on the RAID0 volume and shrunk 
the
> ext2 filesystem to create some room on the RAID5 volume...these areas
> served
> as testbeds for the Windows write performance.  I used a 750MB ISO 
file as
> my test object, transferring it from another machine on my LAN via FTP 
as
> well as from the lone IDE drive on the same machine.  The lone drive 
FTP'd
> the file @ 11.5MB/sec, so that was my baseline.  The RAID0 volume 
matched
> this (no surprise), but the RAID5 volume was about 4.5MB/sec.  Same 
for
> internal transfers.  So the problem is not with the Linux 
driver...it's
> something in the hardware.
> 
> Right now, I've replaced the one "odd" Deskstar drive with another 
Maxtor
> 500GB/16MB cache drive that matches the other 3 drives in the array 
and
> letting the controller rebuild it.  I'll run more performance tests 
when
> it's done, but it's going to take quite a while.  In the meantime, I'd
> still
> appreciate hearing from the folks here.
> --
> View this message in context: http://www.nabble.com/Abysmal-write-
> performance-on-HW-RAID5-tf4884768.html#a13980960
> Sent from the linux-raid mailing list archive at Nabble.com.
> 
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> To unsubscribe from this list: send the line "unsubscribe linux-raid" 
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Your LAN transfer sounds about right;
11MB * 8 (bits per Byte) = 88Mbit on your 100M LAN.

I have twelve drives in my system.  Two are in a RAID 1 for the OS with 
ext2 and ten are in a RAID 6 for my data in xfs.  When I notice a 
significant drop in performance on one my raided md# drive, it is 
usually a drive failing, somewhere... (If you have smart running it will 
yell at you at this point too)

I run the following to get a measurement of each drive that is in a raid 
set.  Usually if I am have problems there will be one in the bunch that 
has very low Timing buffered disk reads.  They are usually around 
50MB/sec for me;

hdparm -tT /dev/sd*  <-- If you have IDE drives use; /dev/hd*

/dev/sda:
 Timing cached reads:   2208 MB in  2.00 seconds = 1102.53 MB/sec
 Timing buffered disk reads:  172 MB in  3.01 seconds =  57.10 MB/sec

/dev/sda1:
 Timing cached reads:   2220 MB in  2.00 seconds = 1110.51 MB/sec
 Timing buffered disk reads:  172 MB in  3.01 seconds =  57.17 MB/sec

/dev/sdb:
 Timing cached reads:   2108 MB in  2.00 seconds = 1052.77 MB/sec
 Timing buffered disk reads:  164 MB in  3.03 seconds =  54.12 MB/sec

/dev/sdb1:
 Timing cached reads:   2256 MB in  2.00 seconds = 1126.57 MB/sec
 Timing buffered disk reads:  164 MB in  3.03 seconds =  54.20 MB/sec
.
.
.
. 

If you are having problems with sata controller chipset/drives on your 
Motherboard, that is a different issue...