On Tue, Jun 12, 2012 at 06:53:00AM +0000, Santosh Hosamani wrote: > > Hi btrfs folks, > I am working on btrfs filesystem on how it manages the free > space. And found out btrfs maintain a ctree which manages the physical > location of the chunks and stripes of the filesystem. > Btrfs-debug-tree also gives the information on the chunk tree > > I created btrfs on single device and two device.I have attached the output of > both on running btrfs-debug-tree. > For single device sum of all the length in the chunks will add upto the total > used bytes which is expected behavior. > > But for two devices sum of all lengths in the chunks does not add to the > total bytes .Am I missing something .
Without actually seeing the details of your technique and expectations, I shall make a guess that you're not accounting for the double-counting of RAID-1 metadata. In other words, you will find that all of the metadata device extents (or chunks) will appear twice -- once on each device. Actually, this isn't quite right either -- what you really need to do is look at the RAID-1, RAID-10 and DUP bits in the chunk flags, add up all of those chunks, divide by two, and then add in the remaining (RAID-0 and single) chunks. That total should then add up to the total value of allocated space that you get from the output of "btrfs fi df". > Also I notice that for the second device the superblock location 0x10000 is > not considered as used . > > I would be really grateful if you folks can answer my query. > > I hav run these tests on SLES11-sp2-x86 > Kernel 3.0.13.0.27-default This is pretty old, but shouldn't affect the results. It will cause reliability problems if you try running it seriously. Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- "There's a Martian war machine outside -- they want to talk --- to you about a cure for the common cold."
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