linux-kernel - Re: Linux 2.6.29

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Date:	Fri, 03 Apr 2009 07:32:50 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jeff Garzik <jeff@...zik.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rees <drees76@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

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


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