linux-ext4 - Re: Extent Depth Histogram Fsck

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Message-Id: <55654AE5-17FE-44EC-B7FE-A8D5FAAB3560@dilger.ca>
Date:	Tue, 26 Jun 2012 09:40:59 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	"Nelson, John R" <John_Nelson@...dent.uml.edu>
Cc:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Extent Depth Histogram Fsck

On 2012-06-26, at 8:34 AM, Nelson, John R wrote:
> What does the extent depth histogram mean? Is it a measure of something?
> 
> like mine is 
> 
> 
> Extent depth histogram: 36010/81

This means that of all the extent-mapped files in the filesystem,
36010 files have an extent tree of depth 0 (i.e. they fit inside the inode)
81 files have an extent tree of depth 1 (i.e. there is a single index block)

Typically, files larger than 4 * 128MB = 512MB need an index block, but if
the maximum-sized extents cannot be allocated then an index block will be needed for smaller files.  Only if you have very large files (> 40GB),
or a very fragmented free space would you need more than a single level
of index blocks.

Cheers, Andreas

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