On 09/05/2014 10:40 AM, Jeff Moyer wrote: > Christoph Hellwig <h...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
