Re: degraded permanent mount option

[Tomasz Pala <gotar@polanet.pl>]

On Sat, Jan 27, 2018 at 14:12:01 -0700, Chris Murphy wrote:

> doesn't count devices itself. The Btrfs systemd udev rule defers to
> Btrfs kernel code by using BTRFS_IOC_DEVICES_READY. And it's totally
> binary. Either they are all ready, in which case it exits 0, and if
> they aren't all ready it exits 1.
> 
> But yes, mounting whether degraded or not is sufficiently complicated
> that you just have to try it. I don't get the point of wanting to know
> whether it's possible without trying. Why would this information be

If you want to blind-try it, just tell the btrfs.ko to flip the IOCTL bit.

No shortcuts please, do it legit, where it belongs.

>> Ie, the thing systemd can safely do, is to stop trying to rule everything,
>> and refrain from telling the user whether he can mount something or not.
> 
> Right. Open question is whether the timer and timeout can be
> implemented in the systemd world and I don't see why not, I certainly

It can. The reasons why it's not already there follow:

1. noone created udev rules and systemd units for btrfs-progs yet (that
   is trivial),
2. btrfs is not degraded-safe yet (the rules would have to check if the
   filesystem won't stuck in read-only mode for example, this is NOT
   trivial),
3. there is not way to tell the kernel that we want degraded (probably
   some new IOCTL) - this is the path that timer would use to trigger udev
   event releasing systemd mount.

Let me repeat this, so this would be clear: this is NOT going to work
as some systemd-shortcut being "mount -o degraded", this must go through
the kernel IOCTL -> udev -> systemd path, i.e.:

timer expires -> executes IOCTL with "OK, give me degraded /dev/blah" ->
BTRFS_IOC_DEVICES_READY returns "READY" (or new value "DEGRADED") -> udev
catches event and changes SYSTEMD_READY -> systemd mounts the volume.


This is really simple. All you need to do is to pass "degraded" to the
btrfs.ko, so the BTRFS_IOC_DEVICES_READY would return "go ahead".

-- 
Tomasz Pala <gotar@pld-linux.org>