Re: Some very basic questions

[Avi Kivity <avi@redhat.com>]

Ric Wheeler wrote:
>>
>> Well, btrfs is not about duplicating how most storage works today.  
>> Spare capacity has significant advantages over spare disks, such as 
>> being able to mix disk sizes, RAID levels, and better performance.
>
> Sure, there are advantages that go in favour of one or the other 
> approaches. But btrfs is also about being able to use common hardware 
> configurations without having to reinvent where we can avoid it (if we 
> have a working RAID or enough drives to do RAID5 with spares or RAID6, 
> we want to be able to delegate that off to something else if we can).

Well, if you have an existing RAID (or have lots of $$$ to buy a new 
one), you needn't tell Btrfs about it.  Just be sure not to enable Btrfs 
data redundancy, or you'll have redundant redundancy, which is expensive.

What Btrfs enables with its multiple device capabilities is to assemble 
a JBOD into a filesystem-level data redundancy system, which is cheaper, 
more flexible (per-file data redundancy levels), and faster (no need for 
RMW, since you're always COWing).

> The major difficulty with the spare capacity model is that your 
> recovery is not as simple and well understood as RAID rebuilds. 

That's Chris's problem. :-)

> If you assume that whole drives fail under btrfs mirroring, you are 
> not really doing anything more than simple RAID, or do I misunderstand 
> your suggestion?

I do assume that whole drives fail, but RAIDing and rebuilding is file 
level.  So one extent on a failed disk might be part of a mirrored file, 
while another extent can be part of a 14-member RAID6 extent.

A rebuild would iterate over all disk extents (making use of the backref 
tree), determine which file contains that extent, and rebuild that 
extent using spare storage on other disks.

> I don't see the point about head seeking. In RAID, you also have the 
> same layout so you minimize head movement (just move more heads per IO 
> in parallel).

Suppose you have 5 disks with 1 spare.  Suppose you are reading from a 
full fs.  On a disk-level RAID, all disks are full.  So you have 5 
spindles seeking over 100% of the disk surface.  With spare capacity, 
you have 6 disks which are 5/6 full (retaining the same utilization as 
old-school RAID).  So you have 6 spindles, each with a seek range that 
is 5/6 of a whole disk, so more seek heads _and_ faster individual seeks.

-- 
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