Re: ext4 vs btrfs performance on SSD array

On 09/05/2014 10:40 AM, Jeff Moyer wrote:

Christoph Hellwig [off-list ref] writes:

quoted

On Wed, Sep 03, 2014 at 10:01:58AM +1000, NeilBrown wrote:

quoted

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.

quoted

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/.

quoted

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?

The reason it's 256 right (or since forever, actually) is that this is
one single 4kb page. If you go higher, that would require a higher order
allocation. Not impossible, but it's definitely a potential issue. It's
a lot saner to string bios at that point, with separate 0 order allocs.

-- 
Jens Axboe