Matthias Bodenbinder wrote on 2016/04/21 07:22 +0200:
Am 20.04.2016 um 09:25 schrieb Qu Wenruo:
Unfortunately, this is the designed behavior.
The fs is rw just because it doesn't hit any critical problem.
If you try to touch a file and then sync the fs, btrfs will become RO immediately.
....
Btrfs fails to read space cache, nor make a new dir.
The failure on cow_block in mkdir is ciritical, and btrfs become RO.
All expected behavior so far.
You may try use degraded mount option, but AFAIK it may not handle case like yours.
This really scares me. "Expected bevahour"?
So you are saying: If one of the drives in the raid1 is going dead without noticing btrfs, the redundancy is lost.
Lets say, the power unit of a disc is going dead. This disc will disappear from the raid1 pretty much as suddenly as in my test case here. No difference.
You are saying that in this case, btrfs should exactly behave like this? If that is the case I eventually need to rethink my interpretation of redundancy.
Matthias
The "expected behavior" just means the abort transaction behavior for
critical error is expected.
And you should know, btrfs is not doing full block level RAID1, it's
doing RAID at chunk level.
Which needs to consider more things than full block level RAID1, and
it's more flex than block level raid1.
(For example, you can use 3 devices with different sizes to do btrfs
RAID1 and get more available size than mdadm raid1)
You may think the behavior is totally insane for btrfs RAID1, but don't
forget, btrfs can have different metdata/data profile.
(And even more, there is already plan to support different profile for
different subvolumes)
In case your metadata is RAID1, your data can still be RAID0, and in
that case a missing devices can still cause huge problem.
There are already unmerged patches which will partly do the mdadm level
behavior, like automatically change to degraded mode without making the
fs RO.
The original patchset:
http://comments.gmane.org/gmane.comp.file-systems.btrfs/48335
Or the latest patchset inside Anand Jain's auto-replace patchset:
http://thread.gmane.org/gmane.comp.file-systems.btrfs/55446
Thanks,
Qu
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