Re: Raid1 with 3 drives

[Ravi Pinjala <ravi@p-static.net>]

On 03/05/10 15:49, Bart Noordervliet wrote:
> On Fri, Mar 5, 2010 at 21:31, Josef Bacik<josef@redhat.com>  wrote:
>>> Since I have three devices in a RAID1 pool, can it survive 2 drive failures?
>>
>> Yes, tho you won't be able to remove more than 1 at a time (since it wants you
>> to keep at least two disks around).  Thanks,
>>
>> Josef
>
> Hmm, I would expect the raid1 data mode to keep 2 copies of each file
> and thus yield 50% effective storage capacity, even with 3 disks. I
> see no real reason to stick with the full-disk mirroring mentality of
> previous raid systems since raid implemented in a filesystem works
> differently. Or would it be difficult to implement btrfs raid1 like
> this?
>
> Maybe it's worth to consider leaving the burdened raid* terminology
> behind and name the btrfs redundancy modes more clearly by what they
> do. For instance "-d double|triple" or "-d 2n|3n". And for raid5/6 "-d
> single-parity|double-parity" or "-d n+1|n+2".
>
> Regards,
>
> Bart
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This would be pretty excellent - there's a real need for a storage 
system where you can just give it a bunch of disks and a policy, and let 
the system worry about the details. Current RAID implementations are 
pretty inflexible, for example when dealing with disks of varying size.

--Ravi