Re: abysmal performance

[Chris Mason <chris.mason@oracle.com>]

Excerpts from John Wyzer's message of 2011-04-29 10:46:08 -0400:
> Currently on 
> commit 7cf96da3ec7ca225acf4f284b0e904a1f5f98821
> Author: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
> Date:   Mon Apr 25 19:43:53 2011 -0400
>     Btrfs: cleanup error handling in inode.c
> 
> merged into 2.6.38.4
> 
> I'm on a btrfs filesystem that has been used for some time. Let's say nine 
> months. Very recently I noticed performance getting worse and worse.
> Most of the time it feels as if the system is just busy with iowait.
> Write and read performance during random access is mostly around 2MB/s,
> sometimes 1MB/s or slower. It's better for big files which can be read with about
> 6-9MB/s. The disk is a reasonably recent SATA disk (WDC_WD3200BEVT) so 30MB/s
> or 40MB/s linear reading should not be a problem.
> 
> rootfs                291G  242G   35G  88% /
> 
> I tried  btrfs filesystem defragment -v / but did not notice any improvement 
> after that.
> 
> Is this a known phenomenon? :-)
> 

Sounds like you're hitting fragmentation, which we can confirm with
latencytop.  Please run latencytop while you're seeing poor performance
and take a look at where you're spending most of your time.

-chris