Re: unbelievably bad performance: 2.6.27.37 and raid6

[Andrew Dunn <andrew.g.dunn@gmail.com>]

Are we to expect some resolution in newer kernels?

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?

Thomas Fjellstrom wrote:
> 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.
>
>   

-- 
Andrew Dunn
http://agdunn.net