Fwd: Re: Reiser FS performance thingy.

[Rogier Wolff <R.E.Wolff@BitWizard.nl>]

> OK Rogier, please search through the archive, and post the URL of your
> previous report.  That will be most helpful.

I searched my own mail archive, remembering that I got an Email
from someone named Hans. Do you know him? :-)

		Roger. 

----- Forwarded message from Hans Reiser <reiser@namesys.com> -----

From: Hans Reiser <reiser@namesys.com>
To: Rogier Wolff <R.E.Wolff@BitWizard.nl>
Date: Wed, 07 Aug 2002 00:03:00 +0400
Subject: Re: Reiser FS performance thingy.
X-Spam-Status: No, hits=-100.0 required=5.0 tests=A_FROM_IN_AUTO_WLIST version=2.01

Rogier Wolff wrote:

>Hi Hans,
>
>We've got a big filesystem on some pretty fast disks running reiserfs:
>
>Filesystem            Size  Used Avail Use% Mounted on
>/dev/md0              603G  466G  137G  78% /recover5
>
>/dev/md0 on /recover5 type reiserfs (rw)
>
>Just a couple of minutes ago, we happened to have only 60G left on
>that filesystem, and suddenly performance went down the drain. Like
>write 500k per second, while normally we're able to read (and write)
>around 100Mb per second off the disks.
>
>We know that you're not supposed to use the last 10% of a disk because
>of performance reasons, but this seems like an enormous penalty.
>
>My impression is that the "search for a free block" routine in
>reiserfs somehow uses the wrong starting point. It is customary to
>start searching where you found a free block last time and not say "at
>the ideal point if you consider the directory entry". Then you might
>end up searching 500G worth of block free flags for a spot for EVERY
>block allocated....
>
>The processes involved in writing to that partition were using LOTS of
>CPU time in system mode. Often top would tell me that over 180% of one
>CPU was spent in system mode.
>
>As you can see, we've solved the problem by doing a little cleaning,
>but you may want to look into this for the future.... 
>
>
>Thanks for the fast fsck. :-)
>
>
>		Roger. 
>
> 
>
Proper solutions from us to your problem include enhanced bitmap 
scanning code (see 2.4.20 when it comes out), and repacking (see 
Reiser4.1 when it comes out).

-- 
Hans




----- End forwarded message -----

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