On 2015-08-15 02:30, Duncan wrote:
I've also found that BTRFS raid5/6 on top of MD RAID0 mitigates (to a certain extent that is) the performance penalty of doing raid5/6 if you aren't on ridiculously fast storage, probably not something that should be used in production yet, but it's how I've got the near-line backups setup on my home server system. It may also be worth pointing out that BTRFS raid6 lets you use 4 disks minimum, as opposed to most other raid6 implementations that (unnecessarily, as a 4 disk RAID6 is not a degenerate form) require 5.Austin S Hemmelgarn posted on Fri, 14 Aug 2015 15:58:30 -0400 as excerpted:FWIW, running BTRFS on top of MDRAID actually works very well, especially for BTRFS raid1 on top of MD-RAID0 (I get an almost 50% performance increase for this usage over BTRFS raid10, although most of this is probably due to how btrfs dispatches I/O's to disks in multi-disk stetups).Of course that's effectively a raid01, which is normally supposed to most often be a mistakenly reversed raid10 implementation, mistakenly, due to the IO cost of the rebuild should a device fail, since the whole raid0 of the one raid1 side would have to be rereplicated to the other, vs only having to rereplicate one device to the other locally, in a raid10 arrangement. However, in this case it's a very smart arrangement, actually, the only md-raid-under-btrfs-raid arrangement that makes real sense (well, other than raid00, raid0 at both levels, perhaps), in particular because the btrfs raid1 on top still gives you the full benefit of btrfs file integrity features as well as the usual raid1 redundancy, tho in this case it's only at the one raid0 against the other as the pair of btrfs raid1 copies. And the mdraid0 is much better optimized than btrfs raid0, so there's that bonus, while at the same time the btrfs raid1 redundancy nicely balances the usual "Russian Roulette" quality of raid0. Very nice configuration! =:^) Thanks for mentioning it, as I guess I was effectively ruling it out as an option before even really considering it due to the usual raid10's better than raid01 thing, and thus was entirely blind to the possibility. Which was bad, because as I alluded to, mdraid's lack of file integrity features and thus lack of any way to have btrfs scrub properly filter down to the mdraid level when there's mdraid level redundancy, kind of makes a mess of things, otherwise. But btrfs raid1 on mdraid0 effectively balances and eliminates the negatives at each level with the strengths of the other level, and is really a quite awesome solution, that until now I was entirely blinded to! =:^)
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