[Gluster-devel] GlusterFS Spare Bricks?

[Gordan Bobic]

On 10/04/2012 09:39, 7220022 wrote:
> Are there plans to add provisioning of spare bricks in a replicated (or
> distributed-replicated) configuration? E.g., when a brick in a mirror
> set dies, the system rebuilds it automatically on a spare, similar to
> how it’d done by RAID controllers.
>
> Nor would it only improve the practical reliability, especially of large
> clusters, but it’d also make it possible to make better-performing
> clusters off less expensive components. For example, instead of having
> slow RAID5 bricks on expensive RAID controllers one uses cheap HBA-s and
> stripes a few disks per brick in RAID0 – that’s faster for writes than
> RAID 5/6 by an order of magnitude (and, by the way, should improve
> rebuild times in Gluster many are complaining about.).A failure of one
> such striped brick is not catastrophic in a mirrored Gluster – but it’s
> better to have spare bricks standing by strewn across cluster heads.
>
> A more advanced setup at a hardware level involves creating “hybrid
> disks” whereas HDD vdisks are cached by enterprise-class SSD-s.It works
> beautifully and makes HDD-s amazingly fast for random transactions.The
> technology’s become widely available for many $500 COTS
> controllers.However, it is not widely known that the results with HDD-s
> in RAID0 under SSD cache are 10 to 20 (!!) times better than with RAID 5
> or 6.

On reads the difference should be negligible unless the array is 
degraded. If it's not, your RAID controller is unfit for purpose.

Having said that, a lot of RAID controllers are pretty useless.

> There is no way to use RAID0 in commercial storage, the main reason
> being the absence of hot-spares.If on the other hand the spares are
> handled by Gluster in a form of (cached hardware-RAID0) pre-fabricated
> bricks both very good performance and reasonably sufficient redundancy
> should be easily achieved.

So why not use ZFS instead? The write performance is significantly 
better than traditional RAID equivalents and you get vastly more 
flexibility than with any hardware RAID solution. And it supports 
caching data onto SSDs.

Gordan