Raz Ben-Jehuda(caro) wrote:

On 12/12/06, Bill Davidsen [off-list ref] wrote:

quoted

Neil Brown wrote:

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On Friday December 8, neilb@suse.de wrote:

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I have measured very slow write throughput for raid5 as well, though
2.6.18 does seem to have the same problem.  I'll double check and 

do a

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git bisect and see what I can come up with.

Correction... it isn't 2.6.18 that fixes the problem.  It is compiling
without LOCKDEP or PROVE_LOCKING.  I remove those and suddenly a
3 drive raid5 is faster than a single drive rather than much slower.

Bill:  Do you have LOCKDEP or PROVE_LOCKING enabled in your .config ??

YES and NO respectively. I did try increasing the stripe_cache_size and
got better but not anywhere near max performance, perhaps the
PROVE_LOCKING is still at fault, although performance of RAID-0 is as
expected, so I'm dubious. In any case, by pushing the size from 256 to
1024, 4096, and finally 10240 I was able to raise the speed to 82MB/s,
which is right at the edge of what I need. I want to read the doc on
stripe_cache_size before going huge, if that's K 10MB is a LOT of cache
when 256 works perfectly in RAID-0.

I noted that the performance really was bad using 2k write, before
increasing the stripe_cache, I will repeat that after doing some other
"real work" things.

Any additional input appreciated, I would expect the speed to be (Ndisk
- 1)*SingleDiskSpeed without a huge buffer, so the fact that it isn't
makes me suspect there's unintended serialization or buffering, even
when not need (and NOT wanted).

Thanks for the feedback, I'm updating the files as I type.
http://www.tmr.com/~davidsen/RAID_speed
http://www.tmr.com/~davidsen/FC6-config

-- 
bill davidsen [off-list ref]
 CTO TMR Associates, Inc
 Doing interesting things with small computers since 1979

Bill helllo
I have been working on raid5 performance write throughout.
The whole idea is the access pattern.
One should  buffers with respect to the size of stripe.
this way you will be able to eiliminate the undesired reads.
By accessing it correctly I have managed reach a write
throughout with respect to the number of disks in the raid.

I'm doing the tests writing 2GB of data to the raw array, in 1MB writes. 
The array is RAID-5 with 256 chunk size. I wouldn't really expect any 
reads, unless I totally misunderstand how all those numbers work 
together. I was really trying to avoid any issues there.However, the 
only other size I have tried was 2K blocks, so I can try other sizes. I 
have a hard time picturing why smaller sizes would be better, but that's 
what testing is for.

-- 
bill davidsen [off-list ref]
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979