linux-ext4 - Re: Large buffer cache in EXT4

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Date:	Sun, 17 Feb 2013 11:19:11 +0100
From:	Martin Steigerwald <Martin@...htvoll.de>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	Subranshu Patel <spatel.ml@...il.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Large buffer cache in EXT4

Am Sonntag, 17. Februar 2013 schrieb Andreas Dilger:
> On 2013-02-16, at 21:04, Subranshu Patel <spatel.ml@...il.com> wrote:
> > I created 2 filesystem on my system (RHEL 6.3, kernel version 2.6.32)
> > - XFS and EXT4 and mounted them.
> > 
> > On both the filesystem I executed the mdtest tool(opensource tool) for
> > 64 concurrent process. Each process performed the following:
> > - Create large number of directories
> > - Remove all the directories
> > 
> > During this time I monitored the memory usage of the system using sar
> > command. I checked the 3 components - kbmemused, kbbuffers and
> > kbcached
> > 
> > kbmemused - Amount of used memory in kilobytes. This does not take
> > into account memory used by the kernel itself.
> > kbbuffers - buffer cache
> > kbcached - page cache
> > 
> > While the kbmemused and kbcached component was almost similar in EXT4
> > and XFS (XFS being a little higher), the kbbuffer showed a totally
> > different trend.
> > 
> > For EXT4, kbbuffers was:
> > 390999KB for dir creation
> > 364803KB for dir removal
> > For XFS, kbbuffers was:
> > 
> > 1701KB for dir creation
> > 2738KB for dir removal
> > 
> > In kernel 2.6, both buffer cache and page cache are merged. The page
> > cache caches pages of files. The buffer cache caches disk blocks which
> > consists of mainly metadata (not file data).
> > 
> > Why is the buffer cache large in case of EXT4 and what is stored in
> > the buffer cache?
> 
> XFS does not use buffer cache, while ext[234] does use buffer cache.
> 
> This is just a different code design. Ext4 uses the buffer cache to track
> metadata for journaling.

Doesn´t XFS use its own mechanism with xfsbufd kernel thread?

Thanks,
-- 
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA  B82F 991B EAAC A599 84C7
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