Re: strange filefrag output on btrfs

[Andreas Philipp <philipp.andreas@gmail.com>]

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On 14.06.2011 04:54, Li Zefan wrote:
> Andreas Philipp wrote:
>>
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>>
>> On 13.06.2011 13:50, David Sterba wrote:
>>> On Sat, Jun 11, 2011 at 05:39:15PM +0200, Andreas Philipp
>>> wrote:
>>>> On one of my btrfs volumes I see a strange output from
>>>> filefrag when run against a particular large (~8GB) file.
>>>> filefrag and filefrag -v give me a different number of
>>>> extents, see below.
>>>>
>>>> aph@thor /mnt/nutshell $ sudo filefrag -v funtoo.img | grep
>>>> extents funtoo.img: 2624 extents found aph@thor /mnt/nutshell
>>>> $ sudo filefrag funtoo.img | grep extents funtoo.img: 2653
>>>> extents found
>>>
>>> is the file open and being written to? did you run sync before
>>> the first command?
>> The file is not open. Yes, I have run sync before the first
>> command. Now, I tested again with a copy of file but the results
>> is more or less the same.
>>
>> aph@thor /mnt/nutshell $ cp funtoo.img funtoo.1.img aph@thor
>> /mnt/nutshell $ sync aph@thor /mnt/nutshell $ sudo filefrag -v
>> funtoo.img funtoo.1.img | grep extents funtoo.img: 2624 extents
>> found funtoo.1.img: 57 extents found aph@thor /mnt/nutshell $
>> sudo filefrag funtoo.img funtoo.1.img | grep extents funtoo.img:
>> 2653 extents found funtoo.1.img: 311 extents found
>>
>
> If you look into the source code of filefrag, you'll know why.
>
> There are two ways to calc the extent number, depending on whether
> verbose option is turned on or not.
>
> In the verbose mode, it will check if the next extent is adjacent
> to the prev extent in the physical position, and in this case they
> are considered to be one extent.
>
> That's why the number returned in verbose mode is smaller.
Thank you for this explanation.

Andreas
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