On Fri, May 02, 2014 at 01:21:50PM -0600, Chris Murphy wrote: > > On May 2, 2014, at 2:23 AM, Duncan <1i5t5.dun...@cox.net> wrote: > > > > Something tells me btrfs replace (not device replace, simply replace) > > should be moved to btrfs device replace… > > The syntax for "btrfs device" is different though; replace is like balance: > btrfs balance start and btrfs replace start. And you can also get a status on > it. We don't (yet) have options to stop, start, resume, which could maybe > come in handy for long rebuilds and a reboot is required (?) although maybe > that just gets handled automatically: set it to pause, then unmount, then > reboot, then mount and resume. > > > Well, I'd say two copies if it's only two devices in the raid1... would > > be true raid1. But if it's say four devices in the raid1, as is > > certainly possible with btrfs raid1, that if it's not mirrored 4-way > > across all devices, it's not true raid1, but rather some sort of hybrid > > raid, raid10 (or raid01) if the devices are so arranged, raid1+linear if > > arranged that way, or some form that doesn't nicely fall into a well > > defined raid level categorization. > > Well, md raid1 is always n-way. So if you use -n 3 and specify three devices, > you'll get 3-way mirroring (3 mirrors). But I don't know any hardware raid > that works this way. They all seem to be raid 1 is strictly two devices. At 4 > devices it's raid10, and only in pairs. > > Btrfs raid1 with 3+ devices is unique as far as I can tell. It is something > like raid1 (2 copies) + linear/concat. But that allocation is round robin. I > don't read code but based on how a 3 disk raid1 volume grows VDI files as > it's filled it looks like 1GB chunks are copied like this > > Disk1 Disk2 Disk3 > 134 124 235 > 679 578 689 > > So 1 through 9 each represent a 1GB chunk. Disk 1 and 2 each have a chunk 1; > disk 2 and 3 each have a chunk 2, and so on. Total of 9GB of data taking up > 18GB of space, 6GB on each drive. You can't do this with any other raid1 as > far as I know. You do definitely run out of space on one disk first though > because of uneven metadata to data chunk allocation.
The algorithm is that when the chunk allocator is asked for a block group (in pairs of chunks for RAID-1), it picks the number of chunks it needs, from different devices, in order of the device with the most free space. So, with disks of size 8, 4, 4, you get: Disk 1: 12345678 Disk 2: 1357 Disk 3: 2468 and with 8, 8, 4, you get: Disk 1: 1234568A Disk 2: 1234579A Disk 3: 6789 Hugo. > Anyway I think we're off the rails with raid1 nomenclature as soon as we have > 3 devices. It's probably better to call it replication, with an assumed > default of 2 replicates unless otherwise specified. > > There's definitely a benefit to a 3 device volume with 2 replicates, > efficiency wise. As soon as we go to four disks 2 replicates it makes more > sense to do raid10, although I haven't tested odd device raid10 setups so I'm > not sure what happens. > > > Chris Murphy > -- === 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 --- Prisoner unknown: Return to Zenda. ---
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