On 10/13/2017 01:27 PM, Duncan wrote:
Misono, Tomohiro posted on Wed, 11 Oct 2017 11:18:50 +0900 as excerpted:
Add 'btrfs remove missing-all' to remove all the missing devices at once
for improving usability.
Example:
sudo mkfs.btrfs -f -d raid1 /dev/sdb1 /dev/sdb2 /dev/sdb3 /dev/sdb4
sudo wipefs -a /dev/sdb1 /dev/sdb3
sudo mount -o degraded /dev/sdb2 /mnt <--
I agree with Duncan here. This step itself will fail even with RO
option. Do you have any patch that is not in the ML which will
make this step a success in the first place ?
Thanks, Anand
sudo btrfs filesystem show /mnt
sudo btrfs device remove missing-all /mnt
sudo btrfs filesystem show /mnt
There's a reason remove missing-all hasn't yet been implemented.
Note that the above would be very unlikely to work once a filesystem has
been used in any significant way, because raid1 and raid10 are explicitly
chunk pairs, *NOT* duplicated N times across N devices. So with two
devices missing, chances are that both copies of some chunks will be
missing as well, so the filesystem would no longer be mountable degraded-
writable, only degraded-readonly, in which case device remove won't work
at all because the filesystem is readonly.
In fact, until the recent per-chunk check patches went in, it was
impossible to mount-writable a raid1 missing two devices at all, because
the safeguards simply assumed some chunks would be entirely missing.
The only case in which more than a single device missing is likely to be
mountable degraded-writable (so device remove will work at all) is raid6,
tho with recent patches there's narrow cases in which it /might/ be
doable with raid1 as well.
Now you may still wish to implement remove missing-all for raid6 mode and
for the unusual corner-case raid1/raid10 in which it might work, but the
documentation should be pretty clear that save for raid6 it can't be
expected to work in most cases.
Given that, I think remove missing-all hasn't been implemented as it
simply hasn't been considered to be worth the bother for the narrow use-
cases in which it will actually work.
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