On 2015-08-15 02:30, Duncan wrote:
> 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! =:^)
>
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.