On Tue, Jul 14, 2015 at 09:09:00PM +0200, Patrik Lundquist wrote: > On 14 July 2015 at 20:41, Hugo Mills <hugo@xxxxxxxxxxxxx> wrote: > > On Tue, Jul 14, 2015 at 01:57:07PM +0200, Patrik Lundquist wrote: > >> On 24 June 2015 at 12:46, Duncan <1i5t5.duncan@xxxxxxx> wrote: > >> > > >> > Regardless of whether 1 or huge -t means maximum defrag, however, the > >> > nominal data chunk size of 1 GiB means that 30 GiB file you mentioned > >> > should be considered ideally defragged at 31 extents. This is a > >> > departure from ext4, which AFAIK in theory has no extent upper limit, so > >> > should be able to do that 30 GiB file in a single extent. > >> > > >> > But btrfs or ext4, 31 extents ideal or a single extent ideal, 150 extents > >> > still indicates at least some remaining fragmentation. > >> > >> So I converted the VMware VMDK file to a VirtualBox VDI file: > >> > >> -rw------- 1 plu plu 28845539328 jul 13 13:36 Windows7-disk1.vmdk > >> -rw------- 1 plu plu 28993126400 jul 13 14:04 Windows7.vdi > >> > >> $ filefrag Windows7.vdi > >> Windows7.vdi: 15 extents found > >> > >> $ btrfs filesystem defragment -t 3g Windows7.vdi > >> $ filefrag Windows7.vdi > >> Windows7.vdi: 24 extents found > >> > >> How can it be less than 28 extents with a chunk size of 1 GiB? > > > > I _think_ the fragment size will be limited by the block group > > size. This is not the same as the chunk size for some RAID levels -- > > for example, RAID-0, a block group can be anything from 2 to n chunks > > (across the same number of devices), where each chunk is 1 GiB, so > > potentially you could have arbitrary-sized block groups. The same > > would apply to RAID-10, -5 and -6. > > > > (Note, I haven't verified this, but it makes sense based on what I > > know of the internal data structures). > > It's a raid1 filesystem, so the block group ought to be the same size > as the chunk, right? Yes. > A 2GiB block group would suffice to explain it though. Not with RAID-1 -- I'd expect the block group size to be 1 GiB. Hugo. -- Hugo Mills | There isn't a noun that can't be verbed. hugo@... carfax.org.uk | http://carfax.org.uk/ | PGP: E2AB1DE4 |
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