On 16.07.2014 09:53, Liu Bo wrote:
On Tue, Jul 15, 2014 at 11:17:26PM +0200, Sebastian Ochmann wrote:
Hello,
I have a VirtualBox hard drive image which is quite fragmented even
after very light use; it is 1.6 GB in size and has around 5000
fragments (I'm using "filefrag" to determine the number of
fragments). Doing a "btrfs fi defrag -f image.vdi" reduced the
number of fragments to 3749. Even doing a "btrfs fi defrag -f -t 1
image.vdi" which should make sure every extent is rewritten
(according to the btrfs-progs 3.14.2 manpage) does not yield any
better result and seems to return immediately. Copying the file,
however, yields a copy which has only 5 fragments (simply doing a cp
image.vdi image2.vdi; sync; filefrag image2.vdi).
What do I have to do to defrag the file to the minimal number of
fragments possible? Am I missing something?
So usually btrfs thinks of an extent whose size is bigger than 256K as a big
enough extent.
Another possible reason is that there is something wrong with btrfs_fiemap which
gives filefrag' a wrong output.
Would you please show us the 'filefrag -v' output?
Sure, I have pasted the output of "filefrag -v" here:
http://pastebin.com/kcZhVhkc
However, I think the problem is merely in the documentation (manpage of
btrfs-filesystem). The description of the "-t" option is different in
two locations and doesn't make sense in general, I think. It is first
described as follows:
"Any extent bigger than threshold given by -t option, will be considered
already defragged. Use 0 to take the kernel default, and use 1 to say
every single extent must be rewritten."
So I used "-t 1" because I thought it will defrag as much as possible.
However when thinking about it, any extent at least 1 byte (or 2 bytes?)
in size will be ignored this way, am I correct?
Further below, the "-t" option is described as follows:
"-t <size> defragment only files at least <size> bytes big"
Here, the option suddenly refers to the file size. In any case, doing a
"btrfs fi defrag -f -t 10G image.vdi" defragged my file to the 5 extents
I also get by simply copying the file. I think the documentation should
be updated to reflect what the -t option actually does.
Best regards
Sebastian
thanks,
-liubo
Kernel version 3.15.5, btrfs progs 3.14.2, Arch Linux.
Best regards,
Sebastian
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