Re: ext4 vs btrfs performance on SSD array

[Jeff Moyer <jmoyer@redhat.com>]

Christoph Hellwig <hch@infradead.org> writes:

> On Wed, Sep 03, 2014 at 10:01:58AM +1000, NeilBrown wrote:
>> Do we still need maximums at all?
>
> I don't think we do.  At least on any system I work with I have to
> increase them to get good performance without any adverse effect on
> throttling.
>
>> So can we just remove the limit on max_sectors and the RAID5 stripe cache
>> size?  I'm certainly keen to remove the later and just use a mempool if the
>> limit isn't needed.
>> I have seen reports that a very large raid5 stripe cache size can cause
>> a reduction in performance.  I don't know why but I suspect it is a bug that
>> should be found and fixed.
>> 
>> Do we need max_sectors ??

I'm assuming we're talking about max_sectors_kb in
/sys/block/sdX/queue/.

> I'll send a patch to remove it and watch for the fireworks..

:) I've seen SSDs that actually degrade in performance if I/O sizes
exceed their internal page size (using artificial benchmarks; I never
confirmed that with actual workloads).  Bumping the default might not be
bad, but getting rid of the tunable would be a step backwards, in my
opinion.

Are you going to bump up BIO_MAX_PAGES while you're at it?

Cheers,
Jeff

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