Re: ext4 vs btrfs performance on SSD array

[Jens Axboe <axboe@kernel.dk>]

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

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