On Sun, Nov 23, 2014 at 12:26:38AM +0100, Patrik Lundquist wrote: > On 22 November 2014 at 23:26, Marc MERLIN <m...@merlins.org> wrote: > > > > This one hurts my brain every time I think about it :) > > I'm new to Btrfs so I may very well be wrong, since I haven't really > read up on it. :-) > > > > So, the bigger the -dusage number, the more work btrfs has to do. > > Agreed. > > > > -dusage=0 does almost nothing > > -dusage=100 effectively rebalances everything > > And -dusage=0 effectively reclaims empty chunks, right? > > > > But saying saying "less than 95% full" for -dusage=95 would mean > > rebalancing everything that isn't almost full, > > But isn't that what rebalance does? Rewriting chunks <=95% full to > completely full chunks and effectively defragmenting chunks and most > likely reduce the number of chunks. > > A -dusage=0 rebalance reduced my number of chunks from 1173 to 998 and > dev_item.bytes_used went from 1593466421248 to 1491460947968. > > > > Now, just to be sure, if I'm getting this right, if your filesystem is > > 55% full, you could rebalance all blocks that have less than 55% space > > free, and use -dusage=55 > > I realize that I interpret the usage parameter as operating on blocks > (chunks? are they the same in this case?) that are <= 55% full while > you interpret it as <= 55% free. > > Which is correct?
Less than or equal to 55% full. 0 gives you less than or equal to 0% full -- i.e. the empty block groups. 100 gives you less than or equal to 100% full, i.e. all block groups. A chunk is the part of a block group that lives on one device, so in RAID-1, every block group is precisely two chunks; in RAID-0, every block group is 2 or more chunks, up to the number of devices in the FS. A chunk is usually 1 GiB in size for data and 250 MiB for metadata, but can be smaller under some circumstances. Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- And what rough beast, its hour come round at last / slouches --- towards Bethlehem, to be born?
signature.asc
Description: Digital signature
