What does the extent depth histogram mean? Is it a measure of something?
like mine is
Extent depth histogram: 36010/81
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
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?
________________________________________
From: Andreas Dilger [[email protected]]
Sent: Tuesday, June 26, 2012 11:40 AM
To: Nelson, John R
Cc: [email protected]
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
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
On 2012-06-26, at 12:07, "Nelson, John R" <[email protected]> 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 [[email protected]]
> Sent: Tuesday, June 26, 2012 11:40 AM
> To: Nelson, John R
> Cc: [email protected]
> 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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>
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