Re: unbelievably bad performance: 2.6.27.37 and raid6

[Michael Evans <mjevans1983@gmail.com>]

>
> A reshape is a fundamentally slow operation.  Each block needs to
> be read and then written somewhere else so there is little opportunity
> for streaming.
> An in-place reshape (i.e the array doesn't get bigger or smaller) is
> even slower as we have to take a backup copy of each range of blocks
> before writing them back out.  This limits streaming even more.
>
> It is possible to get it fast than it is by increasing the
> array's stripe_cache_size and also increasing the 'backup' size
> that mdadm uses.  mdadm-3.1.1 will try to do better in this respect.
> However it will still be significantly slower than e.g. a resync.
>
> So reshape will always be slow.  It is a completely different issue
> to filesystem activity on a RAID array being slow.  Recent reports of
> slowness are, I think, not directly related to md/raid.  It is either
> the filesystem or the VM or a combination of the two that causes
> these slowdowns.
>
>
> NeilBrown
>

stripe_cache_active = default 0?
stripe_cache_size = default 256? (1mb per disk)
cat /sys/block/md52/md/stripe_cache_*
0
8192

I bumped it up, but I haven't seen a real increase in the speed yet.
Do I need to mdadm -S /dev/md52 and then re-assemble it to have this
take effect, or is it the fact that the stripe cache active seems to
be 'deactivated'?  Can I safely set this to 1 mid-reshape?
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