linux-kernel - Re: Linux 2.6.29

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Message-ID: <9b1675090904021938q6600bb94m2f9fa1b23316162a@mail.gmail.com>
Date:	Thu, 2 Apr 2009 20:38:59 -0600
From:	"Trenton D. Adams" <trenton.d.adams@...il.com>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.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, Apr 2, 2009 at 8:01 PM, Jeff Garzik <jeff@...zik.org> wrote:
> Linus Torvalds wrote:
> The most interesting thing I found:  the SSD does 80 MB/s for the first ~1
> GB or so, then slows down dramatically.  After ~2GB, it is down to 32 MB/s.
>  After ~4GB, it reaches a steady speed around 23 MB/s.

Isn't that the kernel IO queue, and the dd averaging of transfer
speed?  For example, once you hit the dirty ratio limit, that is when
it starts writing to disk.  So, the first bit you'll see really fast
speeds, as it goes to memory, but it averages out over time to a
slower speed.  As an example...

tdamac ~ # dd if=/dev/zero of=/tmp/bigfile bs=1M count=1
1+0 records in
1+0 records out
1048576 bytes (1.0 MB) copied, 0.00489853 s, 214 MB/s

tdamac ~ # dd if=/dev/zero of=/tmp/bigfile bs=1M count=10
10+0 records in
10+0 records out
10485760 bytes (10 MB) copied, 0.242217 s, 43.3 MB/s

Those are with /proc/sys/vm/dirty_bytes set to 1M...
echo $((1024*1024*1)) > /proc/sys/vm/dirty_bytes

It's probably better to set it much higher though.
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