linux-kernel - Re: Linux and SSD and how to decrease io delay : question, please

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Date:	Thu, 28 Feb 2008 15:30:03 -0500
From:	Bill Davidsen <davidsen@....com>
To:	Mariella Petrini <mariellapetrini@...oo.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Linux and SSD and how to decrease io delay : question, please

Mariella Petrini wrote:
> Hi All,
> 
> I have been running Linux kernel 2.6.23 with Debian on
> 
> a 2 CPUs Dual-Core AMD Opteron(tm) Processor 2218.
> I have been using an SSD drive (MTRON) connected to a
> raid controller (LSI      MegaRAID 8704ELP) to contain
> data stored on several databases.
> Each database contains 20,000 files and there are 50
> databases total (1,000,000 files distributed across 50
> directories on the filesystem on the ssd).
> 
> The database server uses the databases very "write"
> intensively.
> An approximate percentage could be 20% reads versus
> 80% random writes.
> 
> No matter which filesystem type is used (I have tried
> ext3, xfs, jfs, reiserfs) I see every few seconds
> snapshots (the could last in the worst case up to 200
> seconds, depending on the filesystem type) 
> where the iowait on the ssd device goes approximately
> over 50% (up to 100%) and 
> the database server threads go to wait (get
> suspended).
> 
> Is there any Linux kernel parameter or any Linux
> parameter that could be varied that could help to
> decrease the time spent to sync all the data to the
> SSD device ?
> 
I may misunderstand the problem, but have you tried tuning the 
parameters /proc/sys/vm/dirty_* to help this? It sounds as if you may be 
filling write cache and then flushing. Of course you didn't measure the 
disk i/o rates along with this, so it's hard to see what's happening.

-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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