linux-ext4 - Re: Extent Depth Histogram Fsck

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Date:	Tue, 26 Jun 2012 14:07:51 -0600
From:	Andreas Dilger <aedilger@...il.com>
To:	"Nelson, John R" <John_Nelson@...dent.uml.edu>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Extent Depth Histogram Fsck

On 2012-06-26, at 12:07, "Nelson, John R" <John_Nelson@...dent.uml.edu> wrote:

> ok i see!
> So when there are like
> 3/3/4 that means double index blocks?? How many extents can a single extent index hold in a block?

The header takes 12 bytes, and each extent or index pointer takes 12 bytes, so for 4kB blocksize there can be (4096 / 12) - 1 = 340 extents per block.

The maximum extent size for 4kB blocks is 2^15*4kB = 128MB, so each index block can map up to ~42GB, so a two-level tree can map just over 14TB under ideal conditions. 

Cheers, Andreas

> ________________________________________
> From: Andreas Dilger [adilger@...ger.ca]
> Sent: Tuesday, June 26, 2012 11:40 AM
> To: Nelson, John R
> Cc: 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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