linux-kernel - Re: I/O and pdflush

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Message-ID: <20090829101247.GA20786@localhost>
Date:	Sat, 29 Aug 2009 18:12:47 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Fernando Silveira <fsilveira@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: I/O and pdflush

On Sat, Aug 29, 2009 at 05:48:40AM +0800, Fernando Silveira wrote:
> On Sun, Jul 12, 2009 at 05:04, Wu Fengguang<fengguang.wu@...el.com> wrote:
> > On Sat, Jul 11, 2009 at 02:27:25PM -0300, Fernando Silveira wrote:
> >> I'm having a hard time with an application that writes sequentially
> >> 250GB of non-stop data directly to a solid state disk (OCZ SSD CORE
> >> v2) device and I hope you can help me. The command "dd if=/dev/zero
> >> of=/dev/sdc bs=4M" reproduces the same symptoms I'm having and writes
> >> exactly as that application does.
> >
> > What's your kernel version?  Can the following patch help?
> 
> Sorry for the delay, but I could not test it before.
> 
> Unfortunately it did not help or change any symptoms at all. The
> kernel version I tested was a non-patched 2.6.30.1 version.
> 
> Do you have any other hint?

Sure. Attached is a writeback debug patch. It will generate lots of
kernel messages. You can just stop your klogd, start your workload
and monitor the SSD writeback throughput via tools like iostat/dstat.
When it goes into the suboptimal 25MB/s state for several seconds, run
        dmesg > dmesg-writeback
and send me the log.

And it is advised to turn on the kconfig option CONFIG_PRINTK_TIME=y.

Thanks,
Fengguang

View attachment "writeback-debug.patch" of type "text/x-diff" (2686 bytes)