On 2016-07-06 05:51, Andrei Borzenkov wrote:
On Tue, Jul 5, 2016 at 11:10 PM, Chris Murphy <lists@xxxxxxxxxxxxxxxxx> wrote:
I started a systemd-devel@ thread since that's where most udev stuff
gets talked about.
https://lists.freedesktop.org/archives/systemd-devel/2016-July/037031.html
Before discussing how to implement it in systemd, we need to decide
what to implement. I.e.
1) do you always want to mount filesystem in degraded mode if not
enough devices are present or only if explicit hint is given?
2) do you want to restrict degrade handling to root only or to other
filesystems as well? Note that there could be more early boot
filesystems that absolutely need same treatment (enters separate
/usr), and there are also normal filesystems that may need be mounted
even degraded.
3) can we query btrfs whether it is mountable in degraded mode?
according to documentation, "btrfs device ready" (which udev builtin
follows) checks "if it has ALL of it’s devices in cache for mounting".
This is required for proper systemd ordering of services.
To be entirely honest, if it were me, I'd want systemd to fsck off. If
the kernel mount(2) call succeeds, then the filesystem was ready enough
to mount, and if it doesn't, then it wasn't, end of story. The whole
concept of trying to track in userspace something the kernel itself
tracks and knows a whole lot more about is absolutely stupid. It makes
some sense when dealing with LVM or MD, because that is potentially a
security issue (someone could inject a bogus device node that you then
mount instead of your desired target), but it makes no sense here,
because there's no way to prevent the equivalent from happening in BTRFS.
As far as the udev rules, I'm pretty certain that _we_ ship those with
btrfs-progs, I have no idea why they're packaged with udev in CentOS (oh
wait, I bet they package every single possible udev rule in that package
just in case, don't they?).
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