How to get the devid of a missing device

Hello,

I have a raid10 with one device missing. I would like to use btrfs replace to replace it. However, I am unsure on how to obtain the devid of the missing device. Having the filesystem mounted in degraded mode under mnt, btrfs fs show /mnt returns

sudo btrfs filesystem show /mnt
Label: 'dataPool' uuid: b5f082e2-2ce0-4f91-b54b-c2d26185a635

        Total devices 4 FS bytes used 665.88GiB
        devid    2 size 931.51GiB used 336.03GiB path /dev/sdb
        devid    4 size 931.51GiB used 336.03GiB path /dev/sdd
        devid    5 size 931.51GiB used 336.03GiB path /dev/sde
        *** Some devices missing

but does not mention the devid of the missing device. The device stat is

sudo btrfs device stats /mnt

[/dev/sdb].write_io_errs   0 
[/dev/sdb].read_io_errs    0 
[/dev/sdb].flush_io_errs   0 
[/dev/sdb].corruption_errs 0 
[/dev/sdb].generation_errs 0 
[(null)].write_io_errs   1448 
[(null)].read_io_errs    0 
[(null)].flush_io_errs   0 
[(null)].corruption_errs 0 
[(null)].generation_errs 0 
[/dev/sdd].write_io_errs   0 
[/dev/sdd].read_io_errs    0 
[/dev/sdd].flush_io_errs   0 
[/dev/sdd].corruption_errs 0 
[/dev/sdd].generation_errs 0 
[/dev/sde].write_io_errs   0 
[/dev/sde].read_io_errs    0 
[/dev/sde].flush_io_errs   0 

[/dev/sde].corruption_errs 0
[/dev/sde].generation_errs 0

Btw, the file system is too old for skinny metadata and extended inode refs. If I do a btrfs replace or a btrfs device add, must I myself ensure that the new features are not enables for the new device which is to be added?

Thank!
Wolfgang

Wolfgang Mader posted on Sun, 26 Apr 2015 20:39:34 +0200 as excerpted:

The devid is if the device is still active in the filesystem. If it's missing...

btrfs device delete missing

That, along with a bunch of other likely helpful information, is covered on the wiki:

https://btrfs.wiki.kernel.org

Specifically for that (reassemble from the wrap, too lazy to fiddle with it on my end):

https://btrfs.wiki.kernel.org/index.php/
Using_Btrfs_with_Multiple_Devices#Replacing_failed_devices

But since it was a four-device raid10 and four devices is the minimum for that, I don't believe it'll let you delete until you device add, first.

Of course that means probably some hours for the add, then some more hours for the delete missing, during which you obviously hope not to lose another device. Of course the sysadmin's backup rule of thumb applies -- if you don't have a backup, by definition, loss of the data isn't a big deal, or you'd have it backed up. (And the corollary, it's not a backup until it's tested, applies as well.) So you shouldn't have to worry about loss of a second device during that time, because it's either backed up or the data isn't worth the trouble to backup and thus loss of it with the loss of a second device isn't a big deal.

That page doesn't seem to cover direct replacement, probably because the replace command is new and it hasn't been updated. But AFAIK replace doesn't work with a missing device anyway; it's the fast way to replace a still listed device, so you don't have to add and then delete, but the device has to still be there in ordered to use that shortcut. (You could try using missing with the -r option tho, just in case it works now. The wiki/manpage is a bit vague on that point.)

I don't believe there's any way to set that manually even if you wanted to -- you don't use mkfs on it and the add/replace would overwrite existing if you did. The new device should just take on the attributes of the filesystem you're adding it to.

Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

Thanks for your answer. I know the stuff of that page, but since it is possible to use btrfs replace on a missing device I wanted to try that approach.

Well, of course, there is a backup on a different machine. Its in the same room, but who has the luxury for off-site backups in a home-use setting! :)

Good to know.

Thanks

At the user end there is no way. unless you want to use gdb and dump the fs_uuids and check.

Submitted these patches as of now to obtain from the logs. Will help in the situation when device is missing at the time of mount.

Btrfs: check error before reporting missing device and add uuid
Btrfs: log when missing device is created

For long term we have sysfs interface, patches are in the ML if you want to tests.

Good luck.

Anand

Great. Thank you for patching this in.

Best,
Ẃolfgang