Re: unbelievably bad performance: 2.6.27.37 and raid6

[Thomas Fjellstrom <tfjellstrom@shaw.ca>]

On Sat October 31 2009, Jon Nelson wrote:
> 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