On Tue, Feb 21, 2012 at 11:45:51AM +1100, Wes wrote: > I've noticed similar behavior when even RAID0'ing an odd number of > devices which should be even more trivial in practice. > You would expect something like: > sda A1 B1 > sdb A2 B2 > sdc A3 B3 This is what it should do -- it'll use as many disks as it can find to put stripes across at the time the allocator is asked to make another block group. > or at least, if BTRFS can only handle block pairs, > > sda A1 B2 > sdb A2 C1 > sdc B1 C2 > > But the end result was that disk usage and reporting went all out of > whack, allocation reporting got confused and started returning > impossible values, and very shortly after the entire FS was corrupted. > Rebalancing messed everything up royally and in the end I concluded > to simply not use an odd number of drives with BTRFS. I can't see why that should have happened. What kernel were you doing this with? > I also tried RAID1 with an odd number of drives, expecting to have 2 > redundant mirrors. This isn't a valid expectation. Or rather, you can expect it, but it's not what btrfs is designed to deliver. Btrfs's RAID-1 implementation is *precisely two* copies. Hence it isn't really much like RAID-1, as you've found out. > Instead the end result was that the blocks were > still only allocated in pairs, and since they were allocated > round-robbin on the drives I completely lost the ability to remove any > single drive from the array without data loss. > > ie: > Instead of: > sda A1 B1 > sdb A1 B1 > sdc A1 B1 > > it ended up doing: > > sda A1 B1 > sdb A1 C1 > sdc B1 C1 > > meaning removing any 1 drive would result in lost data. (Any 2 drives, as you corrected in your subsequent email) However, you can remove any one drive, and your data is fine, which is what btrfs's RAID-1 guarantee is. I understand that there will be additional features coming along Real Soon Now (possibly at the same time that RAID-5 and -6 are integrated) which will allow the selection of larger numbers of copies. 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 --- People are too unreliable to be replaced by machines. ---
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