On 2019-02-24 12:32, Nemo wrote:
Hi,
I had a RAID1 disk failure recently, and a limitation in number of SATA
connectors meant I could not do a live replace. I'm still in the
progress of resolving the issue, but posting some feedback here on the
issues I faced and what could have helped.
**What all I did**
- remove the dying disk and mount in degraded mode. This didn't work,
but _didn't provide me a failure reason_
If you don't see a message on the console, check dmesg.
- swap the dead disk post boot with the new one. Tried a btrfs device
add, but it failed as well because of too many failing reads from the
dead disk (which wasn't even attached).
You can take this approach, but you need to use the replace command not
the add command. You can't really add devices to a partial array, for
exactly the reason you saw it fail here. In the case of replacing a
device that is missing, you want to do the following (though it only
works if you have only one missing device):
btrfs replace start -r missing /path/to/new/dev /path/to/fs
- Started a btrfs device delete, which finally pointed me to the root
cause:
`ERROR: error removing device '/dev/sdb1': No space left on device`
In short:
1. My 2.7TBx3 setup was storing 2.9TB of data
2. Removing any one the disks was not possible without losing some data
However, getting to this error message took me hours of effort, despite
having read the btrfs wiki.
**Feedback**
1. The `btrfs device delete` command should have failed immediately
instead of making me wait for ages.
I won't pretend that your statement isn't correct, but I can at least
explain _why_ it didn't.
Put simply, the device delete command is a special case of a balance
operation, and balancing doesn't do any check prior to running to make
sure that the requested state is valid. Part of this is that it's
actually useful in some cases to just run the balance until it fails
(because getting part of the work done is usually better than none), and
part of it is because it is very much non-trivial in a number of the
cases that balance needs to handle how the disk will look after running
the balance if it's successful.
2. A delete command should stop writes to a disk, unless avoidable. This
would have left some data on the disk, and a subsequent delete could
have cleaned it up much faster. It failed in the worst manner possible,
because all deletion progress reversed once it failed: it balanced back
to complete usage, causing more strain on a dying disk.
It essentially operates currently in a manner similar to the LVM2 pvmove
command WRT writes, just not as smart. AFAIUI:
* Any writes that go to a chunk that is on the device to be removed
still go through.
* Any writes that go to a chunk that is not on the device to be removed
don't touch it.
* Any writes that go to the chunk currently being moved off of the
device being removed go to the new location for that chunk.
3. A degraded mount failure should provide some error message.
Again, if you see a mount failure, check dmesg. This is the case for
pretty much any filesystem (hell, mount actually tells you to do this in
a lot of cases).
*Status*: I cleared up some disk space, and ran another delete which is
still ongoing. I don't have physical access to the system for a few more
days, so adding the new disk will take me some time.
---
## Debug Details
```
uname -a
Linux tatooine.captnemo.in 4.20.10-arch1-1-ARCH #1 SMP PREEMPT Fri Feb
15 17:49:06 UTC 2019 x86_64 GNU/Linux
btrfs --version
btrfs-progs v4.20.1
```
A few more details are on a reddit post[0] I made to help debug the
issue and a gist[1] has disk usage and filesystem details.
[0]:
https://www.reddit.com/r/archlinux/comments/asrlam/btrfscleaner_at_100_cpu_usage_on_raid1_setup/egwc047/
[1]: https://paste.ubuntu.com/p/GDNKNBqqFy/
