linux-ext4 - Re: Large buffer cache in EXT4

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Message-Id: <6F61E227-A6F7-4AD6-9B9C-39E9E9EAABFA@dilger.ca>
Date:	Sat, 16 Feb 2013 23:28:53 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Subranshu Patel <spatel.ml@...il.com>
Cc:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Large buffer cache in EXT4

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. 

Cheers, Andreas--
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