Re: [GIT PULL] Ext3 latency fixes

Linus Torvalds
2009-04-04T17:46:56Z

On Sat, 4 Apr 2009, Jens Axboe wrote:
> >
> > I'm sorry, but that fsync thing _is_ a real-world case, and it's the one
> > that a hell of a lot more people care about than some idiotic sqlite
> > throughput issue.
>
> sqlite is just one case, I'm sure there are others. My point is that we
> should make sure that we don't regress on the throughput side. It's a
> trade off, we don't want throughput to fall through the floor either.

Jens, we _have_ regressed on the latency side. Everybody agrees.

Also, I may be odd, but I really do think latency is more important than
throughput. When my disk has latencies in the sub-milliseconds, I simply
do not think it is _acceptable_ to have hickups that affect my workload in
human-visible terms.

You say sqlite might regress by 4-5x. But Ted's numbers improve latencies
by mor than that. I haven't re-created them yet myself (still reading
email), but the point is, 4-5x may sound bad to you, but turn it around:
the current latency situation is _really_ bad. If we can fix it, we
definitely should.

> > Quite frankly, the fact that I can see _seconds_ of latencies with a
> > really good SSD is not acceptable. The fact that it is by design is even
> > less so.
>
> Agree, multi-second latencies is not acceptable.

I can literally send you strace output from my MUA, where it pauses for
ten seconds after it has written about 5kB (that's _kilobytes_) of data
and does a 'fsync'.

That's the load that Ted worked on and has a solution for.

Linus