Hi Hugo,
I can reproduce this behavior, and the solution is quite simple: do a sync
after the removing the device.
Let me explain what (I think) happen.
The command "btrfs filesystem show" reads every block device and shows the
relevant information. This is performed without passing for the btrfs kernel
code.
Instead the command "btrfs device del..." performs the command doing an ioctl.
In this case the btrfs kernel code is involved.
I am saying that there is a delay between the time of "btrfs device del"
command execution and when the data are available with a reading of the
involved block device.
The same is true for every other commands: the data returned by the "btrfs
filesystem show command" are the ones evaluated on the basis of reading the
block devices. And these data may be not in sync with the "kernel data".
An umount and/or a sync may solve.
Your email suggest me to renew a request to Josef Bacik: he is working on an
interface in order to get the list of the device of a btrfs filesystem. I
think that this interface should report not only a list of devices, but also
other information like the ones reported by the btrfs filesystem show. The
Hugo email shows that we cannot really trust of the data returned by the
command "btrfs filesystem show" as implemented now.
Regards
G.Baroncelli
On Saturday, 09 October, 2010, Hugo Mills wrote:
> I've just encountered some odd behaviour with regard to removed
> devices. Brief summary:
>
> - It's hard (in some sense) to tell a btrfs filesystem that a device
> has been removed permanently, and seems to require an
> unmount/remount, or resize to do so.
>
> - Removed devices break btrfs dev scan
>
> Details follow:
>
> # mkfs.btrfs -d raid10 -m raid10 -L btest /dev/primary/btrtest{1,2,3,4,5}
> # mount /dev/primary/btrtest1 /mnt
> # sudo btrfs fi show btest
> failed to read /dev/sr0
> Label: 'btest' uuid: 908f87d7-23d8-453e-ab04-ae1426306e0f
> Total devices 5 FS bytes used 28.00KB
> devid 1 size 1.00GB used 276.00MB path /dev/dm-21
> devid 2 size 1.00GB used 136.00MB path /dev/dm-22
> devid 3 size 1.00GB used 136.00MB path /dev/dm-23
> devid 4 size 1.00GB used 264.00MB path /dev/dm-24
> devid 5 size 1.00GB used 264.00MB path /dev/dm-25
>
> Btrfs v0.19-35-g1b444cd
>
> All well and good so far.
>
> # btrfs dev del /dev/primary/btrtest4 /mnt
> # btrfs fi show btest
> failed to read /dev/sr0
> Label: 'btest' uuid: 908f87d7-23d8-453e-ab04-ae1426306e0f
> Total devices 5 FS bytes used 100.13MB
> devid 1 size 1.00GB used 190.38MB path /dev/dm-21
> devid 2 size 1.00GB used 170.38MB path /dev/dm-22
> devid 3 size 1.00GB used 170.38MB path /dev/dm-23
> devid 5 size 1.00GB used 170.38MB path /dev/dm-25
> *** Some devices missing
>
> Btrfs v0.19-35-g1b444cd
>
> Now, it's claiming that some devices are missing, but what if I
> wanted to make this a permanent change? Say, the additional device was
> one added temporarily to the array as part of a migration to new
> hardware?
>
> On IRC, it was suggested that a rescan would fix it:
>
> # btrfs dev scan
> Scanning for Btrfs filesystems
> failed to read /dev/sr0
> # btrfs fi show btest
> failed to read /dev/sr0
> Label: 'btest' uuid: 908f87d7-23d8-453e-ab04-ae1426306e0f
> Total devices 5 FS bytes used 100.13MB
> devid 1 size 1.00GB used 190.38MB path /dev/dm-21
> devid 2 size 1.00GB used 170.38MB path /dev/dm-22
> devid 3 size 1.00GB used 170.38MB path /dev/dm-23
> devid 5 size 1.00GB used 170.38MB path /dev/dm-25
> *** Some devices missing
>
> Btrfs v0.19-35-g1b444cd
>
> Nope. What about explicitly scanning the devices?
>
> # btrfs dev scan /dev/primary/btrtest*
> Scanning for Btrfs filesystems in '/dev/primary/btrtest1'
> Scanning for Btrfs filesystems in '/dev/primary/btrtest2'
> Scanning for Btrfs filesystems in '/dev/primary/btrtest3'
> Scanning for Btrfs filesystems in '/dev/primary/btrtest4'
> ERROR: unable to scan the device '/dev/primary/btrtest4'
>
> Note that it's stopped the scan immediately on encountering the
> removed device, so btrtest5 hasn't been picked up. Maybe it's
> something left in the device?
>
> # dd if=/dev/zero of=/dev/primary/btrtest4
> # btrfs dev scan /dev/primary/btrtest*
> Scanning for Btrfs filesystems in '/dev/primary/btrtest1'
> Scanning for Btrfs filesystems in '/dev/primary/btrtest2'
> Scanning for Btrfs filesystems in '/dev/primary/btrtest3'
> Scanning for Btrfs filesystems in '/dev/primary/btrtest4'
> ERROR: unable to scan the device '/dev/primary/btrtest4'
> # btrfs fi show btest
> failed to read /dev/sr0
> Label: 'btest' uuid: 908f87d7-23d8-453e-ab04-ae1426306e0f
> Total devices 5 FS bytes used 100.13MB
> devid 1 size 1.00GB used 190.38MB path /dev/dm-21
> devid 2 size 1.00GB used 170.38MB path /dev/dm-22
> devid 3 size 1.00GB used 170.38MB path /dev/dm-23
> devid 5 size 1.00GB used 170.38MB path /dev/dm-25
> *** Some devices missing
>
> Btrfs v0.19-35-g1b444cd
>
> Zeroing the device has no effect. However, unmounting it does work,
> partially:
>
> # umount /mnt
> # btrfs dev scan
> Scanning for Btrfs filesystems
> failed to read /dev/sr0
> # btrfs dev scan /dev/primary/btrtest*
> Scanning for Btrfs filesystems in '/dev/primary/btrtest1'
> Scanning for Btrfs filesystems in '/dev/primary/btrtest2'
> Scanning for Btrfs filesystems in '/dev/primary/btrtest3'
> Scanning for Btrfs filesystems in '/dev/primary/btrtest4'
> ERROR: unable to scan the device '/dev/primary/btrtest4'
> # btrfs fi show btest
> failed to read /dev/sr0
> Label: 'btest' uuid: 908f87d7-23d8-453e-ab04-ae1426306e0f
> Total devices 4 FS bytes used 100.13MB
> devid 1 size 1.00GB used 190.38MB path /dev/dm-21
> devid 2 size 1.00GB used 170.38MB path /dev/dm-22
> devid 3 size 1.00GB used 170.38MB path /dev/dm-23
> devid 5 size 1.00GB used 170.38MB path /dev/dm-25
>
> Btrfs v0.19-35-g1b444cd
>
> So, you need to unmount/remount the FS to make it believe that the
> device removal is permanent. (As an aside, I also found that resizing
> down by 1M then back to max has the same effect, if you don't want to
> unmount). However, the explicit scan of the block devices is still
> broken by the removed device, even after all data on it has been
> zeroed.
>
> Hugo.
>
> --
> === Hugo Mills: h...@... carfax.org.uk | darksatanic.net | lug.org.uk ===
> PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
> --- I hate housework. You make the beds, you wash the dishes, and ---
> six months later you have to start all over again.
>
--
gpg key@ keyserver.linux.it: Goffredo Baroncelli (ghigo) <kreij...@inwind.it>
Key fingerprint = 4769 7E51 5293 D36C 814E C054 BF04 F161 3DC5 0512
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