On 2017-05-02 15:50, Goffredo Baroncelli wrote:
On 2017-05-02 20:49, Adam Borowski wrote:
It could be some daemon that waits for btrfs to become complete. Do we
have something?
Such a daemon would also have to read the chunk tree.
I don't think that a daemon is necessary. As proof of concept, in the past I developed a mount helper [1] which handled the mount of a btrfs filesystem:
this handler first checks if the filesystem is a multivolume devices, if so it waits that all the devices are appeared. Finally mount the filesystem.
It's not so simple -- such a btrfs device would have THREE states:
1. not mountable yet (multi-device with not enough disks present)
2. mountable ro / rw-degraded
3. healthy
My mount.btrfs could be "programmed" to wait a timeout, then it mounts the filesystem as degraded if not all devices are present. This is a very simple strategy, but this could be expanded.
I am inclined to think that the current approach doesn't fit well the btrfs requirements. The roles and responsibilities are spread to too much layer (udev, systemd, mount)... I hoped that my helper could be adopted in order to concentrate all the responsibility to only one binary; this would reduce the interface number with the other subsystem (eg systemd, udev).
The primary problem is that systemd treats BTRFS like a block-layer
instead of a filesystem (so it assumes all devices need to be present),
and that it doesn't trust the kernel's mount function to work correctly.
As a result, it assumes that the mount operation will fail if it
doesn't see all the devices instead of just trying it like it should.
For example, it would be possible to implement a sane check that prevent to mount a btrfs filesystem if two devices exposes the same UUID...
BR
G.Baroncelli
[1] See "[RFC][PATCH v2] mount.btrfs helper" thread ( https://marc.info/?l=linux-btrfs&m=141736989508243&w=2 )
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