Avi Kivity wrote:
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).
I think that the btrfs plan is still to push more complicated RAID
schemes off to MD (RAID6, etc) so this is an issue even with a JBOD. It
will be interesting to map out the possible ways to use built in
mirroring, etc vs the external RAID and actually measure the utilized
capacity and performance (online & during rebuilds).
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. :-)
Unless he can pawn it off on some other lucky developer :-)
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.
I think that this is somewhat correct, but most likely offset by the
performance levels of streaming IO vs IO with any seeks (at least for
full file systems). Certainly, the spare capacity model is increasingly
better when you have really light utilized file systems...
Don't think that I am arguing against the model, just saying that it is
not always as clear cut as you might think....
ric
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