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SubjectRe: Linux 2.6.29


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).

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

> Unfortunately I cannot trash these blkdevs, so the raw blkdev numbers are not
> immediately measurable.

Hey, understood. I don't think raw block accesses are even all that
interesting. But you might try to write the file backwards, and see if you
see the same pattern.

Linus


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