Re: raid6 + caviar black + mpt2sas horrific performance

[Michael Tokarev <mjt@tls.msk.ru>]

30.03.2011 19:20, Louis-David Mitterrand wrote:
[]

> /dev/mapper/cmd1	/		xfs	defaults,inode64,delaylog,logbsize=262144	0	0
> /dev/mapper/cmd1	/		xfs	defaults,inode64,delaylog,logbsize=262144	0	0
> 
>> [root@vault t]# dd if=/dev/zero of=/backup/t/big.file bs=32k count=32000
> 
> 1048576000 octets (1,0 GB) copiés, 0,870001 s, 1,2 GB/s
> 1048576000 bytes (1.0 GB) copied, 9.11934 s, 115 MB/s

Are you sure your LVM volumes are aligned to raid stripe size correctly?
Proper alignment for raid levels with parity is _critical_, and with
8-drive raid6 you'll have 6 data disks in eash stripe, but since LVM
can only align volumes to a power of two, you'll have 2 unaligned
volumes after each aligned...  Verify that the volume starts at the
raid stripe boundary, maybe create new volume and recheck -- there
should be quite dramatic speed difference like the above when you
change from aligned to misaligned.

/mjt
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