[Libguestfs] list-filesystems and btrfs snapshots

[Maxim Perevedentsev]

Richard and Cedric, thank you for your replies.


On 07/06/2016 09:11 PM, Richard W.M. Jones wrote:
> On Wed, Jul 06, 2016 at 05:33:01PM +0300, Maxim Perevedentsev wrote:
>> Hello everyone!
>>
>> I have an issue with OpenSUSE-42.1 (I guess this is the only distro
>> with btrfs snapshots by default).
>> As we remember, list_filesystems returns btrfs snapshots along with devices.
>>
>>> <fs> list-filesystems
>> /dev/sda1: swap
>> btrfsvol:/dev/sda2/@: btrfs
>> btrfsvol:/dev/sda2/@/.snapshots: btrfs
>> btrfsvol:/dev/sda2/@/.snapshots/1/snapshot: btrfs
>> btrfsvol:/dev/sda2/@/.snapshots/2/snapshot: btrfs
>> ...
>> btrfsvol:/dev/sda2/@/.snapshots/16/snapshot: btrfs
>> /dev/sda2: btrfs
>>
>>
>> The btrfs snapshots are not block devices, so an error is thrown by
>> RESOLVE_DEVICE macro in stubs.c. That means we will get an error
>> trying to call blockdev - oriented functions at these devices.
>>
>> =========================
>>
>> After grepping "list_filesystems" issues were found in:
>> - virt-sparsify (in-place and copying): is_lv
I'll send a patch soon.
>> - virt-filesystems: blockdev_getsize64
>> - virt-cat (inspect_os) treats this as multiboot system and refuses
>> to proceed
>> - virt-rescue --suggest suggests all snapshots (maybe this is correct)
>> - virt-sysprep --operation fs-uuids goes into infinite loop
>>
>> [ 144.2] Performing "fs-uuids" ...
>> virt-sysprep: warning: cannot set random UUID on filesystem
>> btrfsvol:/dev/sda2/@/.snapshots/16/snapshot type btrfs: set_uuid:
>> set_uuid_stub: btrfsvol:/dev/sda2/@/.snapshots/16/snapshot: expecting a
>> device name
> The specific problem is that set_uuid is declared to take a Device as
> the first parameter [see generator/action.ml], whereas I believe it
> should take a Mountable instead.
>
> Compare it to how vfs_uuid is declared in the same file.
>
> This is simply a bug in generator/action.ml, and I believe a simple
> one line patch should fix it.
I tried that, but this does not work.
btrfstune takes an unmounted filesystem as an argument.
It is impossible to change uuid for a subvolume. Passing a Mountable
results in setting different uuids to /dev/sda2  16 times.

I think it is better to skip subvolumes on this operation.

>
>> virt-sysprep: warning: cannot set random UUID on filesystem
>> btrfsvol:/dev/sda2/@/.snapshots/1/snapshot type btrfs: set_uuid:
>> set_uuid_stub: btrfsvol:/dev/sda2/@/.snapshots/1/snapshot: expecting a
>> device name
>> virt-sysprep: warning: cannot set random UUID on filesystem
>> btrfsvol:/dev/sda2/@/.snapshots type btrfs: set_uuid: set_uuid_stub:
>> btrfsvol:/dev/sda2/@/.snapshots: expecting a device name
>> virt-sysprep: warning: cannot set random UUID on filesystem
>> btrfsvol:/dev/sda2/@ type btrfs: set_uuid: set_uuid_stub:
>> btrfsvol:/dev/sda2/@: expecting a device name
>> [ 154.4] Performing "fs-uuids" ...
>>
>> =========================
>>
>> I'd like to ask for your comments on fixing this.
>> Should we
>> 1. add a new command (guestfs_list_filesystems_devices) to return
>> only device-like filesystems?
>> 2. add something like guestfs_is_device which handles subvolumes on
>> host side?
>> NB: guestfs_is_blockdev also fails on stub
>> 3. allow guestfs_is_(blockdev|chardev|...) to check subvolumes on
>> host side? (maybe together with 2?)
>> 4. anything else?
> Number 4.
>
> Rich.
>
Do I understand right that the right way of skipping btrfs subvolumes is
guestfs_mountable_device(fs) != NULL && guestfs_mountable_subvolume(fs) 
!= NULL.

And we should add this code to every place where we need Device?


-- 
Your sincerely,
Maxim Perevedentsev