On 2016-09-18 13:28, Chris Murphy wrote:
On Sun, Sep 18, 2016 at 2:34 AM, Anand Jain <anand.j...@oracle.com> wrote:


(updated the subject, was [1])

IMO the hot-spare feature makes most sense with the raid56,


  Why. ?

Raid56 is not scalable, has less redundancy in most all
configurations, rebuild impacts the entire array performance, and in
the case of raid6 two drives lost means incredibly slow rebuild. All
of that adds up to more disk for raid56 to be mitigated with a hot
spare being available for immediate rebuild.

Who currently would use hot spare right now? Problem 1 is Btrfs raid10
is not scalable like other raid10 implementations (mdadm, lvm,
hardware). Problem 2 is Btrfs the raid56 parity scrub bug; and
arguably also partial stripe writes not being CoW. I think hot spare
is pointless with those two problems still being true, and the way to
mitigate them right now is a clusterfs. Hot spare doesn't mitigate
these Btrfs weaknesses.



which is stuck where it is, so we need to get it working first.



  We need at least one RAID which does not have the availability
  issue. We could achieve that with raid1, there are patches
  which needs maintainer time.

I agree with the idea of degraded raid1 chunks. It's a nasty surprise
to realize this only once it's too late and there's data loss. That
there is a user space work around, maybe makes it less of a big deal?
But I don't think it's documented on gotchas page with the soft
conversion work around to do the rebuild properly: scrub/balance alone
is not correct.

I kinda think we need a list of priorities for multiple device stuff,
and honestly hot spare while important I think is bottom of the list.

1. multiple fs UUID dev UUID corruption problem (the cloned device problem)
2. degraded volumes new bg's are single profile (Anand's April patchset)
3. raid56 bad parity created during scrub when data strip is bad and gets fixed
4. better faulty device tolerance (no crashing)
5. raid10 scaling, needs a way for even number block devices of the
same size to get fixed mirroring so it can tolerate multiple drive
failures so long as a mirrored pair don't fail
6. raid56 partial stripe RMW need to be CoW, doesn't matter if it
slows things down, if you don't like it, use raid10
7. raid1 threaded/async reads (whatever the correct term is to read
from all raid1 drives rather than PID based)
8. better faulty device notifications
9. raid56 parity needs to be checksummed
10. hotspare
FWIW, I'd probably list the faulty device tolerance and notifications (in that order) immediately after the first two items, put the raid1 threaded reads at the end of the list (after hot-spares), and put the raid10 scaling after raid1 threading (anyone who's actually concerned with performance and has done their homework is more likely to be using BTRFS in raid1 mode on top of a pair of RAID0 arrays (most likely MD or LVM based) instead of BTRFS raid10 mode, not only because of the reliability factor, but also because it gets significantly better performance than BTRFS raid10 mode, and will continue to do so until we get proper load-balancing of reads on raid1 and raid10 profiles). I'd also add that we should be parallelizing reads of stripe components in raid0, raid10, raid5, and raid6 modes (ie, if we're using raid10 mode and need to read both halves of a stripe, both reads should get dispatched at the same time), but that would likely go in with the raid1 performance stuff.


2 and 3 might seem tied. Both can result in data loss, both have user
space work arounds (undocumented); but 2 has a greater chance of
happening than 3.
2 also impacts things other than raid5/6, which means (at least IMO) it should be higher priority.

4 is probably worse than 3, but 4 is much more nebulous and 3 produces
a big negative perception.

I'm sure someone could argue hotspare could get squeezed in between 4
and 5; but that's really my one bias in the list, I don't care about
hot spare. I think it's more scalable to take advantage of Btrfs
uniqueness to shrink the file system to drop the bad drive to regain
full redundancy, rather than do hot spares, this is faster, and
doesn't waste a drive that's not doing any work.
This isn't just you, I'm pretty much of the same opinion on this particular item.

I see shrink as more scalable with hard drives than hot spares,
especially in the case of data single profile with clusterfs's: drop
the bad device and its data, autodelete the lost files, rebuild
metadata to regain complete fs redundancy,  inform the cluster of
partial data loss - boom the array is completely fixed, let the
cluster figure out what to do next. Plus each brick isn't spinning an
unused hot spare. There is in effect a hot spare *somewhere* partially
used somewhere else in a cluster fs anyway. I see hot spare as an edge
case need, especially with hard drives. It's not a general purpose
need.
I agree on this too to a certain extent, except:
1. There aren't any clustered filesystems that have this functionality. As far as the big three, I have zero personal experience with Hadoop, it would be impractical to try with Ceph (they're moving to using just flat block devices instead of filesystem backed storage), and while it may be doable on Gluster, I'm not 100% certain (I don't have a very concrete understanding of how exactly GlusterFS works at a low level, which ideally needs to change considering that we're using it where I work and I'm using it for storing backups for my personal systems). 2. There is one very specific use case where a hot-spare has an advantage over shrinking the FS: automatic repair. If I have to set something up to automatically repair a storage array, I almost certainly want a hot-spare, not something that will reduce the size of the array, and I'd be willing to bet almost anyone else you ask will give the same opinion on that.

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