Re: unbelievably bad performance: 2.6.27.37 and raid6

On Sun November 1 2009, Andrew Dunn wrote:

Are we to expect some resolution in newer kernels?

I assume all of the new per-bdi-writeback work going on in .33+ will have a 
large impact. At least I'm hoping.

I am going to rebuild my array (backup data and re-create) to modify the
chunk size this week. I hope to get a much higher performance when
increasing from 64k chunk size to 1024k.

Is there a way to modify chunk size in place or does the array need to
be re-created?

This I'm not sure about. I'd like to be able to reshape to a new chunk size 
for testing.

Thomas Fjellstrom wrote:

quoted

On Sat October 31 2009, Jon Nelson wrote:

quoted

I have a 4 disk raid6. The disks are individually capable of (at
least) 75MB/s on average.
The raid6 looks like this:

md0 : active raid6 sda4[0] sdc4[5] sdd4[4] sdb4[6]
      613409536 blocks super 1.1 level 6, 64k chunk, algorithm 2 [4/4]
 [UUUU]

The raid serves basically as an lvm physical volume.

While rsyncing a file from an ext3 filesystem to a jfs filesystem, I
am observing speeds in the 10-15MB/s range.
That seems really really slow.

Using vmstat, I see similar numbers (I'm averaging a bit, I'll see
lows of 6MB/s and highs of 18-20MB/s, but these are infrequent.)
The system is, for the most part, otherwise unloaded.

I looked at stripe_cache_size and increased it to 384 - no difference.
blockdev --getra reports 256 for all involved raid components.
I'm using the deadline I/O scheduler.

Am I crazy?  Is 12.5MB/s (average) what I should expect, here?  What
might I look at here?

I can't say I see numbers that bad.. But I do get 1/3 or less of the
performance with .29, .30, .31, and .32 than I get with .26. I haven't
tried any other kernels as these are the only ones I've been able to
grab from apt ;)

I get something on the order of 100MB/s write and read with newer
kernels, with really bursty behaviour, and with .26, its not as fast as
it COULD be, but at least I get 200-300MB/s, which is reasonable.

Now if your two file systems are on the same LVM VG, that could have an
impact on performance.

-- 
Thomas Fjellstrom
tfjellstrom@shaw.ca