linux-ext4 - Re: Extent Depth Histogram Fsck

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Message-ID: <20120626200026.GA28831@thunk.org>
Date:	Tue, 26 Jun 2012 16:00:26 -0400
From:	Ted Ts'o <tytso@....edu>
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 Tue, Jun 26, 2012 at 06:07:10PM +0000, Nelson, John R 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?

You can put 4 entries in the inode, and then there can be 340 entries
in each 4k extent tree block.  Each entry can either be a leaf entry
or an index entry (i.e., pointing to another extent tree block).

So it's pretty rare for there extent tree to have a depth greater than
1.  In order to actually test the code, what I do is use a 1k block
size (so you can only fit 84 entries in each ETB), and then use a
small file system, and then do something like this:

touch /tmp/test.img
mke2fs -t ext4 -O ^has_journal -F -b 1024 -N 131084 /tmp/test.img 150M
mount -o loop /tmp/test.img /mnt
cd /mnt
seq 1 131072 | xargs -n 1 fallocate -l 1024
seq 1 2 131072 | xargs rm
fallocate -l 64M test-file
cd ..
umount /mnt

i.e., create a maximally fragmented file system, and then create a
large file.  But this is very rare in real-life workloads.

      	     	      	      - Ted
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