On Wed, Jul 16, 2014 at 12:50:30PM +0200, Sebastian Ochmann wrote: > 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. Oops, we've deeply made users confused in this old documents, will update it then. thanks, -liubo > > Best regards > Sebastian > > > >thanks, > >-liubo > > > >> > >>Kernel version 3.15.5, btrfs progs 3.14.2, Arch Linux. > >> > >>Best regards, > >>Sebastian > >>-- > >>To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > >>the body of a message to majordomo@xxxxxxxxxxxxxxx > >>More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
