On Thu, Jan 20, 2011 at 03:53:41PM +0100, Andreas Philipp wrote: > On 20.01.2011 14:39, Hugo Mills wrote: > > On Thu, Jan 20, 2011 at 02:07:23PM +0100, Andreas Philipp wrote: > >> Hi, > >> > >> Maybe it is a very stupid question but I want to ask it anyway. In > >> general, 'btrfs filesystem balance' takes very long to finish and > >> produces lots of IO. So what are the classical usage scenarios, when > >> it is (really) worth doing a balance? > > The primary use-cases for balancing are to even out the filesystem > > after adding, removing or changing the size of one of the underlying > > volumes. > Ok, so this is "a little bit" like for example resyncing a "classical" > raid after it was in degraded mode etc. Pretty much exactly that. > > It will also be of use when we finally get around to allowing you > > to change RAID settings on the whole volume, to implement the > > requested changes to the RAID level. > > > Definitely, a nice feature. > > I'm in the process of implementing balance filters, so that some > > other cases where balancing is useful (reclaiming unused block groups) > > can be run more efficiently by only balancing the bits that need > > doing. > I have seen your post on balance filters. So then it will be (much) > faster just because less is done? Yes, that's the idea. If you've lost and replaced a drive from a 2-drive RAID-1 array, there's not much that filters can do for you: all your data will have to be read and rebuilt. However, if you're changing just your metadata from DUP to RAID-1, say, or recovering from the loss of one drive in an 8-drive RAID-1 array, it should be an awful lot faster with filters. > When you have a version for trying it out and you need someone for > testing I will give it a try. Thanks. I've got quite a bit reworked now to support multiple filter types, but I need to do a full review of what I'm doing, and test it myself first. I probably won't have much time to work on it before Monday, now. 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 --- emacs: Eats Memory and Crashes. ---
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