Re: Linux 2.6.29

Chris Mason
2009-04-03T11:36:29Z

On Thu, 2009-04-02 at 20:34 -0700, Linus Torvalds wrote:
>
> On Thu, 2 Apr 2009, Jeff Garzik wrote:
> >
> > I was really surprised the performance was so high at first, then fell off so
> > dramatically, on the SSD here.
>
> Well, one rather simple explanation is that if you hadn't been doing lots
> of writes, then the background garbage collection on the Intel SSD gets
> ahead of the game, and gives you lots of bursty nice write bandwidth due
> to having a nicely compacted and pre-erased blocks.
>
> Then, after lots of writing, all the pre-erased blocks are gone, and you
> are down to a steady state where it needs to GC and erase blocks to make
> room for new writes.
>
> So that part doesn't suprise me per se. The Intel SSD's definitely
> flucutate a bit timing-wise (but I love how they never degenerate to the
> "ooh, that _really_ sucks" case that the other SSD's and the rotational
> media I've seen does when you do random writes).
>

23MB/s seems a bit low though, I'd try with O_DIRECT. ext3 doesn't do
writepages, and the ssd may be very sensitive to smaller writes (what
brand?)

> The fact that it also happens for the regular disk does imply that it's
> not the _only_ thing going on, though.
>

Jeff if you blktrace it I can make up a seekwatcher graph. My bet is
that pdflush is stuck writing the indirect blocks, and doing a ton of
seeks.

You could change the overwrite program to also do sync_file_range on the
block device ;)

-chris