[dm-devel] turning the root volume into a snapshot stalls (ioctl hangs)

[Hauke Laging]

Am Dienstag, 8. November 2011, 14:21:21 schrieb Mike Snitzer:
> So you're somehow activating the snapshot to be the root volume.  And
> then later trying to merge the tmpfs COW into the origin (obviously
> before you reboot otherwise you'd lose the tmpfs).  This isn't going to
> work with the current snapshot-merge code given the constraints that
> both the origin and snapshot must be closed before merge.

Yes but not even the snapshot part works (I copied the code in the first mail 
from the wrong part of the script; it should have been "snaphot" instead of 
"snapshot-merge".


> The lvm tools both document and guard against this, from lvconvert(8):

OK, I don't use lvconvert thus I had not read that.


>    Merging a snapshot into an origin that cannot be closed, for
>    example a root filesystem, is deferred until the next time the origin
>    volume is activated.

I am curious: Is there a technical explanation of this problem? I do not 
understand what "close" is supposed to mean in this context and how the root 
volume is different from the home volume (with users logged in).

Could this restriction be avoided by using another dm layer? I have an LV 
linux2/rootfs. If I create a new dm device rootfs and have it point to 
/dev/mapper/linux2-rootfs would the restriction apply to both then? It is not 
necessary in this case that the file system gets flushed (and the superblock 
cleaned).


Thanks for your help

Hauke
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