Raid5 reads and cpu

Raid5 reads and cpu

[Rob Bray]

This might be a dumb question, but what causes md to use a large amount of
cpu resources when reading a large amount of data from a raid1 array?
Examples are on a 2.4GHz AMD64, 2GB, 2.6.15.1 (I realize there are md
enhancements to later versions; I had some other unrelated issues and
rolled back to one I've run on for several months).

A given 7-disk raid0 array can read 450MB/s (using cat > null) and use
virtually no CPU resources. (Although cat and kswapd use quite a bit [60%]
munching on the data)

A raid5 array on the same drive set pulls in at 250MB/s, but md uses
roughly 50% of the CPU (the other 50% is spent dealing with the data,
saturating the processor).

A consistency check on the raid5 array uses roughly 3% of the cpu. It is
otherwise ~97% idle.
md11 : active raid5 sdi2[5] sdh2[4] sdf2[3] sde2[2] sdd2[1] sdc2[6] sdb2[0]
      248974848 blocks level 5, 256k chunk, algorithm 2 [7/7] [UUUUUUU]
      [==============>......]  resync = 72.2% (29976960/41495808)
finish=3.7min speed=51460K/sec
(~350MB/s aggregate throughput, 50MB/s on each device)

Just a friendly question as to why CPU utilization is significantly
different between a check and a real-world read on raid5? I feel like if
there was vm overhead getting the data into userland, the slowdown would
be present in raid0 as well. I assume parity calculations aren't done on a
read of the array, which leaves me at my question.

Thanks,
Rob

Raid5 reads and cpu

[Rob Bray]

This might be a dumb question, but what causes md to use a large amount of
cpu resources when reading a large amount of data from a raid1 array?
Examples are on a 2.4GHz AMD64, 2GB, 2.6.15.1 (I realize there are md
enhancements to later versions; I had some other unrelated issues and
rolled back to one I've run on for several months).

A given 7-disk raid0 array can read 450MB/s (using cat > null) and use
virtually no CPU resources. (Although cat and kswapd use quite a bit [60%]
munching on the data)

A raid5 array on the same drive set pulls in at 250MB/s, but md uses
roughly 50% of the CPU (the other 50% is spent dealing with the data,
saturating the processor).

A consistency check on the raid5 array uses roughly 3% of the cpu. It is
otherwise ~97% idle.
md11 : active raid5 sdi2[5] sdh2[4] sdf2[3] sde2[2] sdd2[1] sdc2[6] sdb2[0]
      248974848 blocks level 5, 256k chunk, algorithm 2 [7/7] [UUUUUUU]
      [==============>......]  resync = 72.2% (29976960/41495808)
finish=3.7min speed=51460K/sec
(~350MB/s aggregate throughput, 50MB/s on each device)

Just a friendly question as to why CPU utilization is significantly
different between a check and a real-world read on raid5? I feel like if
there was vm overhead getting the data into userland, the slowdown would
be present in raid0 as well. I assume parity calculations aren't done on a
read of the array, which leaves me at my question.

Thanks,
Rob

Re: Raid5 reads and cpu

[Neil Brown]

On Monday August 28, raid@bymeinc.com wrote:
> This might be a dumb question, but what causes md to use a large amount of
> cpu resources when reading a large amount of data from a raid1 array?

I assume you meant raid5 there.

md/raid5 shouldn't use that much CPU when reading.
It does use more than raid0 as it reads data in the stripe-cache and
then copies the data from the stripe cache into the read-buffer.  But
I wouldn't expect that to come anywhere near 50%.

Are you really seeing 'raid5d' using 50% of CPU in 'top' or similar?

NeilBrown


> Examples are on a 2.4GHz AMD64, 2GB, 2.6.15.1 (I realize there are md
> enhancements to later versions; I had some other unrelated issues and
> rolled back to one I've run on for several months).
> 
> A given 7-disk raid0 array can read 450MB/s (using cat > null) and use
> virtually no CPU resources. (Although cat and kswapd use quite a bit [60%]
> munching on the data)
> 
> A raid5 array on the same drive set pulls in at 250MB/s, but md uses
> roughly 50% of the CPU (the other 50% is spent dealing with the data,
> saturating the processor).
> 
> A consistency check on the raid5 array uses roughly 3% of the cpu. It is
> otherwise ~97% idle.
> md11 : active raid5 sdi2[5] sdh2[4] sdf2[3] sde2[2] sdd2[1] sdc2[6] sdb2[0]
>       248974848 blocks level 5, 256k chunk, algorithm 2 [7/7] [UUUUUUU]
>       [==============>......]  resync = 72.2% (29976960/41495808)
> finish=3.7min speed=51460K/sec
> (~350MB/s aggregate throughput, 50MB/s on each device)
> 
> Just a friendly question as to why CPU utilization is significantly
> different between a check and a real-world read on raid5? I feel like if
> there was vm overhead getting the data into userland, the slowdown would
> be present in raid0 as well. I assume parity calculations aren't done on a
> read of the array, which leaves me at my question.
> 
> Thanks,
> Rob
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

-- 
VGER BF report: U 0.5

Re: Raid5 reads and cpu

[Bill Davidsen]

Rob Bray wrote:

>This might be a dumb question, but what causes md to use a large amount of
>cpu resources when reading a large amount of data from a raid1 array?
>Examples are on a 2.4GHz AMD64, 2GB, 2.6.15.1 (I realize there are md
>enhancements to later versions; I had some other unrelated issues and
>rolled back to one I've run on for several months).
>
>A given 7-disk raid0 array can read 450MB/s (using cat > null) and use
>virtually no CPU resources. (Although cat and kswapd use quite a bit [60%]
>munching on the data)
>
>A raid5 array on the same drive set pulls in at 250MB/s, but md uses
>roughly 50% of the CPU (the other 50% is spent dealing with the data,
>saturating the processor).
>
>A consistency check on the raid5 array uses roughly 3% of the cpu. It is
>otherwise ~97% idle.
>md11 : active raid5 sdi2[5] sdh2[4] sdf2[3] sde2[2] sdd2[1] sdc2[6] sdb2[0]
>      248974848 blocks level 5, 256k chunk, algorithm 2 [7/7] [UUUUUUU]
>      [==============>......]  resync = 72.2% (29976960/41495808)
>finish=3.7min speed=51460K/sec
>(~350MB/s aggregate throughput, 50MB/s on each device)
>
>Just a friendly question as to why CPU utilization is significantly
>different between a check and a real-world read on raid5? I feel like if
>there was vm overhead getting the data into userland, the slowdown would
>be present in raid0 as well. I assume parity calculations aren't done on a
>read of the array, which leaves me at my question.
>  
>
What are you stripe and cache sizes?

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979

Re: Raid5 reads and cpu

[Rob Bray]

> On Monday August 28, raid@bymeinc.com wrote:
>> This might be a dumb question, but what causes md to use a large amount
>> of
>> cpu resources when reading a large amount of data from a raid1 array?
>
> I assume you meant raid5 there.
>
> md/raid5 shouldn't use that much CPU when reading.
> It does use more than raid0 as it reads data in the stripe-cache and
> then copies the data from the stripe cache into the read-buffer.  But
> I wouldn't expect that to come anywhere near 50%.
>
> Are you really seeing 'raid5d' using 50% of CPU in 'top' or similar?
>
> NeilBrown

Sorry for the long response time -- email got lost.

top - 16:45:21 up 10 days, 17:41,  2 users,  load average: 0.58, 0.17, 0.05
Tasks: 113 total,   2 running, 111 sleeping,   0 stopped,   0 zombie
Cpu(s):  1.7% us, 87.7% sy,  6.3% ni,  0.0% id,  0.0% wa,  0.0% hi,  4.3% si
Mem:   2061564k total,  2044784k used,    16780k free,  1193384k buffers
Swap:  4257016k total,      552k used,  4256464k free,    24348k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
  945 root      10  -5     0    0    0 S 44.2  0.0   7:27.73 md11_raid5


>
>
>> Examples are on a 2.4GHz AMD64, 2GB, 2.6.15.1 (I realize there are md
>> enhancements to later versions; I had some other unrelated issues and
>> rolled back to one I've run on for several months).
>>
>> A given 7-disk raid0 array can read 450MB/s (using cat > null) and use
>> virtually no CPU resources. (Although cat and kswapd use quite a bit
>> [60%]
>> munching on the data)
>>
>> A raid5 array on the same drive set pulls in at 250MB/s, but md uses
>> roughly 50% of the CPU (the other 50% is spent dealing with the data,
>> saturating the processor).
>>
>> A consistency check on the raid5 array uses roughly 3% of the cpu. It is
>> otherwise ~97% idle.
>> md11 : active raid5 sdi2[5] sdh2[4] sdf2[3] sde2[2] sdd2[1] sdc2[6]
>> sdb2[0]
>>       248974848 blocks level 5, 256k chunk, algorithm 2 [7/7] [UUUUUUU]
>>       [==============>......]  resync = 72.2% (29976960/41495808)
>> finish=3.7min speed=51460K/sec
>> (~350MB/s aggregate throughput, 50MB/s on each device)
>>
>> Just a friendly question as to why CPU utilization is significantly
>> different between a check and a real-world read on raid5? I feel like if
>> there was vm overhead getting the data into userland, the slowdown would
>> be present in raid0 as well. I assume parity calculations aren't done on
>> a
>> read of the array, which leaves me at my question.
>>
>> Thanks,
>> Rob
>>
>> -
>> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>

Re: Raid5 reads and cpu

[Rob Bray]

> Rob Bray wrote:
>
>>This might be a dumb question, but what causes md to use a large amount
>> of
>>cpu resources when reading a large amount of data from a raid1 array?
>>Examples are on a 2.4GHz AMD64, 2GB, 2.6.15.1 (I realize there are md
>>enhancements to later versions; I had some other unrelated issues and
>>rolled back to one I've run on for several months).
>>
>>A given 7-disk raid0 array can read 450MB/s (using cat > null) and use
>>virtually no CPU resources. (Although cat and kswapd use quite a bit
>> [60%]
>>munching on the data)
>>
>>A raid5 array on the same drive set pulls in at 250MB/s, but md uses
>>roughly 50% of the CPU (the other 50% is spent dealing with the data,
>>saturating the processor).
>>
>>A consistency check on the raid5 array uses roughly 3% of the cpu. It is
>>otherwise ~97% idle.
>>md11 : active raid5 sdi2[5] sdh2[4] sdf2[3] sde2[2] sdd2[1] sdc2[6]
>> sdb2[0]
>>      248974848 blocks level 5, 256k chunk, algorithm 2 [7/7] [UUUUUUU]
>>      [==============>......]  resync = 72.2% (29976960/41495808)
>>finish=3.7min speed=51460K/sec
>>(~350MB/s aggregate throughput, 50MB/s on each device)
>>
>>Just a friendly question as to why CPU utilization is significantly
>>different between a check and a real-world read on raid5? I feel like if
>>there was vm overhead getting the data into userland, the slowdown would
>>be present in raid0 as well. I assume parity calculations aren't done on
>> a
>>read of the array, which leaves me at my question.
>>
>>
> What are you stripe and cache sizes?
>
> --
> bill davidsen <davidsen@tmr.com>
>   CTO TMR Associates, Inc
>   Doing interesting things with small computers since 1979
>
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>

md11 : active raid5 sdi2[5] sdh2[4] sdf2[3] sde2[2] sdd2[1] sdc2[6] sdb2[0]
      248974848 blocks level 5, 256k chunk, algorithm 2 [7/7] [UUUUUUU]

stripe_cache_size = 256
I've tried increasing it with the same result