Anand Jain wrote on 2016/03/17 09:58 +0800:
In particular, I don't understand a
behavior I'm seeing which somehow takes part of the advantage away from
the idea of having a RAID for me.
Yep. Very true. I am trying fix them.
Generally, a storage/volume-manager need to match the Enterprise
storage data center requirements, and I do understand the reliability,
availability, serviceability (RAS) gaps in the current btrfs volume
manager.
The main issue I have is as follows. I can mount a RAID1 with missing
devices using the "degraded" option. So far so good. I know I should add
another device at that point, but let's say I don't have a device ready
but need to keep my system running. The problem is that once I write
some data to the device and unmount it, I cannot mount it degraded+rw
again but only degraded+ro.
A known issue and a critical (in the enterprise storage standard) bug.
There is a patch set to fix.
'btrfs: Hot spare and Auto replace'
I hoped it got reviewed/merged sooner.
IHMO, splitting them into per-chunk-degraded-mount and
hot-spare-auto-replace part will make things much easier to get merged.
Thanks,
Qu
As I am getting couple of other private request for the hot spare
patch set, also of latest I see there are some critical changes in
the replace part of the code, looks like its time to revive them.
Thanks for asking. I shall find some time in the early next week
for this.
And at that point I can't make changes such
as adding new devices and rebalancing as far as I see, rendering the
degraded RAID useless for me for keeping the system running.
Example:
- Create RAID1 (data+metadata) with 2 devices
- mount the filesystem
- btrfs fi df /something:
Data, RAID1: total=112.00MiB, used=40.76MiB
System, RAID1: total=8.00MiB, used=16.00KiB
Metadata, RAID1: total=32.00MiB, used=768.00KiB
GlobalReserve, single: total=16.00MiB, used=0.00B
- unmount
- Destroy one of the devices (dd over it or whatever)
- mount with "-o degraded"
- Write some data to the device (not sure if this is strictly necessary).
- unmount the filesystem
- Try to mount with "-o degraded" again. It doesn't allow you to do so
(if it does, try to unmount and mount again, sometimes it needs two
tries). dmesg says "missing devices(1) exceeds the limit(0), writable
mount is not allowed"
- mounting with "-o degraded,ro" works.
- btrfs fi df [dev]:
Data, RAID1: total=112.00MiB, used=40.76MiB
System, RAID1: total=8.00MiB, used=16.00KiB
System, single: total=8.00MiB, used=0.00B
Metadata, RAID1: total=32.00MiB, used=768.00KiB
Metadata, single: total=28.00MiB, used=80.00KiB
GlobalReserve, single: total=16.00MiB, used=0.00B
Now there is some RAID1 and some single data. Which could make sense
since writing to the device in degraded mode could only be done on the
single device. Still, I cannot make changes to the filesystem now and
can "only" recover the data from it, but that's not really the idea of a
RAID1 in my opinion.
Any advice?
Fix is in the hot-spare patch set. And it should mount RW with that.
Any test results will help.
Thanks, Anand
Versions:
- Kernel 4.4.5-1-ARCH
- btrfs-progs 4.4.1
Best regards
Sebastian
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