raid1 vs raid5

raid1 vs raid5

[Psalle]

Hello all and excuse me if this is a silly question. I looked around in 
the wiki and list archives but couldn't find any in-depth discussion 
about this:

I just realized that, since raid1 in btrfs is special (meaning only two 
copies in different devices), the effect in terms of resilience achieved 
with raid1 and raid5 are the same: you can lose one drive and not lose data.

So!, presuming that raid5 were at the same level of maturity, what would 
be the pros/cons of each mode?

As a corollary, I guess that if raid1 is considered a good compromise, 
then functional equivalents to raid6 and beyond could simply be 
implemented as "storing n copies in different devices", dropping any 
complex parity computations and making this mode entirely generic. Since 
this seems pretty obvious, I'd welcome your insights on what are the 
things I'm missing, since it doesn't exist (and it isn't planned to be 
this way, AFAIK). I can foresee consistency difficulties, but that seems 
hardly insurmountable if its being done for raid1?

Thanks in advance,
Psalle.

Re: raid1 vs raid5

[Sean Greenslade]

On Tue, Jan 05, 2016 at 05:24:31PM +0100, Psalle wrote:
> Hello all and excuse me if this is a silly question. I looked around in the
> wiki and list archives but couldn't find any in-depth discussion about this:
> 
> I just realized that, since raid1 in btrfs is special (meaning only two
> copies in different devices), the effect in terms of resilience achieved
> with raid1 and raid5 are the same: you can lose one drive and not lose data.
> 
> So!, presuming that raid5 were at the same level of maturity, what would be
> the pros/cons of each mode?

This is true for "classic" RAID: assume you have 3x 1TB disks. RAID1
will give you 1.5TB, whereas RAID5 will give you 2TB.

RAID1 = 1/2 total disk space (assuming equally-sized disks)
RAID5 = (N-1)*single disk space (same assumption)

> As a corollary, I guess that if raid1 is considered a good compromise, then
> functional equivalents to raid6 and beyond could simply be implemented as
> "storing n copies in different devices", dropping any complex parity
> computations and making this mode entirely generic.

This is akin to what has been mentioned on the list earlier as "N-way
mirroring" and I agree that it will be very nice to have once
implemented. However it is not the same as RAID5/6 since the parity
schemes are used to get more usable storage than just simple mirroring
would allow for.

Thus, the main pro of RAID5/6 is more usable storage, and the main con
is more computational complexity (and thus more cpu requirements, slower
access time, more fragile error states, etc.)

> Since this seems pretty obvious, I'd welcome your insights on what are
> the things I'm missing, since it doesn't exist (and it isn't planned
> to be this way, AFAIK). I can foresee consistency difficulties, but
> that seems hardly insurmountable if its being done for raid1?

Fixing an inconsistency in RAID1 is much easier than RAID5/6. No math,
just checking csums. Fixing an inconsistency in RAID5/6 involves busting
out the parity math. This is why repairing RAID5/6 only became possible
in btrfs relatively recently. Generating the parity data was relatively
easy, but rebuilding missing data with it was a more difficult task.

--Sean

Re: raid1 vs raid5

[Psalle]

I had forgotten about this post and on rethinking about it I realized 
the kind of brain fart I was having. So thanks for the merciful silence ;-)

On 05/01/16 17:24, Psalle wrote:
> Hello all and excuse me if this is a silly question. I looked around 
> in the wiki and list archives but couldn't find any in-depth 
> discussion about this:
>
> I just realized that, since raid1 in btrfs is special (meaning only 
> two copies in different devices), the effect in terms of resilience 
> achieved with raid1 and raid5 are the same: you can lose one drive and 
> not lose data.
>
> So!, presuming that raid5 were at the same level of maturity, what 
> would be the pros/cons of each mode?
>
> As a corollary, I guess that if raid1 is considered a good compromise, 
> then functional equivalents to raid6 and beyond could simply be 
> implemented as "storing n copies in different devices", dropping any 
> complex parity computations and making this mode entirely generic. 
> Since this seems pretty obvious, I'd welcome your insights on what are 
> the things I'm missing, since it doesn't exist (and it isn't planned 
> to be this way, AFAIK). I can foresee consistency difficulties, but 
> that seems hardly insurmountable if its being done for raid1?
>
> Thanks in advance,
> Psalle.