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.
--
error compiling committee.c: too many arguments to function
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