On 2016-07-07 20:23, Austin S. Hemmelgarn wrote:
[...]
> FWIW, I've pretty much always been of the opinion that the device discovery
> belongs in a mount helper. The auto-discovery from udev (and more
> importantly, how the kernel handles being told about a device) is much of the
> reason that it's so inherently dangerous to do block level copies. There's
> obviously no way that can be changed now without breaking something, but
> that's on the really short list of things that I personally feel are worth
> breaking to fix a particularly dangerous pitfall. The recent discovery that
> device ready state is write-once when set just reinforces this in my opinion.
>
> Here's how I would picture the ideal situation:
> * A device is processed by udev. It detects that it's part of a BTRFS array,
> updates blkid and whatever else in userspace with this info, and then stops
> without telling the kernel.
> * The kernel tracks devices until the filesystem they are part of is
> unmounted, or a mount of that FS fails.
> * When the user goes to mount the a BTRFS filesystem, they use a mount helper.
> 1. This helper queries udev/blkid/whatever to see which devices are part of
> an array.
> 2. Once the helper determines which devices are potentially in the
> requested FS, it checks the following things to ensure array integrity:
> - Does each device report the same number of component devices for the
> array?
> - Does the reported number match the number of devices found?
> - If a mount by UUID is requested, do all the labels match on each device?
> - If a mount by LABEL is requested, do all the UUID's match on each
> device?
> - If a mount by path is requested, do all the component devices reported
> by that device have matching LABEL _and_ UUID?
> - Is any of the devices found already in-use by another mount?
^^^^^^^^^^^^^^^^^ It is possible to mount two time the same device.
I add my favorite:
- is there a conflict of disk-uuid (i.e two different disk with the
same uuid) ?
Anyway the point 2 has to be in loop until timeout: i.e. if systemd ask to
mount a filesystem when the first device appear, wait for all devices appear.
> 4. If any of the above checks fails, and the user has not specified an
> option to request a mount anyway, report the error and exit with non-zero
> status _before_ even talking to the kernel.
> 5. If only the second check fails (the check verifying the number of
> devices found), and it fails because the number found is less than required
> for a non-degraded mount, ignore that check if and only if the user specified
> -o degraded.
> 6. If any of the other checks fail, ignore them if and only if the user
> asks to ignore that specific check.
> 7. Otherwise, notify the kernel about the devices and call mount(2).
> * The mount helper parses it's own set of special options similar to the
> bg/fg/retry options used by mount.nfs to allow for timeouts when mounting, as
> well as asynchronous mounts in the background.
> * btrfs device scan becomes a no-op
> * btrfs device ready uses the above logic minus step 7 to determine if a
> filesystem is probably ready.
>
> Such a situation would probably eliminate or at least reduce most of our
> current issues with device discovery, and provide much better error reporting
> and general flexibility.
>
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