Re: unbelievably bad performance: 2.6.27.37 and raid6

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]

Why would you use a 4 disk raid6? If 50% of raw capacity is enough
just go with raid10.
The default 64KiB chunk size has always been very slow for me, try
something larger, perhaps even 1MiB.

While rsyncing a file from an ext3 filesystem to a jfs filesystem,

rsync isn't really a good diagnostic, how's performance with something
simple, dd, bonnie++, whatever?

I looked at stripe_cache_size and increased it to 384

Much too low, I usually set it to 8192. Take care you don't run out of
RAM, though, it's in pages/disk (16KiB in your case).

blockdev --getra reports 256 for all involved raid components.

In my experience only the top component of a layered block device
matters at all, that would be the LV. 256 sectors / 128 KiB seems
awfully low, try something higher, 2MiB at least.

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

No, it's probably just that the default tunining options are not very good.

Cheers,

C.
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