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Re: [linux-lvm] F7 will not boot after running backup w/snapshot



> Once the device with the snapshot disappears, I'm not sure I would trust
> that snapshot even if the device comes back online.

The COW operation implies you can trust that snapshot if, and only if, nothing was written to the origin volume between when the snapshot disappeared and when it came back on line. Perhaps a "Has been written" flag could be maintained for the origin volume starting at reboot.
 
Larry Dickson
 
On 5/2/08, Gerry Reno <greno verizon net> wrote:
Marek Podmaka wrote:
Hi Gerry,

Thursday, May 1, 2008, 22:21:00, Gerry Reno wrote:

 
At least in the case where the snapshot is read-only (LVM1 default,
LVM2 by config?), if the snapshot is lost, invalid, not removed from
VG prior to reboot, when LVM comes back up it should see this and
immediately know that it can just vgreduce VG --removemissing for
the old snapshot. In the case of rw (no LVM1, LVM2 default), it
should be a user choice and LVM should prompt the user at boot as to
whether to remove this old snapshot so it can attempt to activate
the VG. Unless the user knows that there were non-backup related lvm
mods written during the snapshot (eg: pvmove) then the user will
just answer yes and the system should boot. This is how LVM should
operate in this scenario.
   

I don't think that automatically making changes to the VG is a good
idea. Imagine that you have a snapshot on disk mapped from SAN storage
array and that becomes temporary unavailable (SAN switch failure or
whatever). It would be a bad idea to remove this PV from VG just
because it is TEMPORARY unavailable.
This was part of my point about the ramdisk, in that other types of devices can also disappear not just ramdisk.
Once the device with the snapshot disappears, I'm not sure I would trust that snapshot even if the device comes back online.
The backup process would try to keep running and it would probably get all kinds of errors at this point so to me it
makes sense to consider that snapshot invalid from that standpoint of the backup. Now if it was a read-write snapshot
and you've used some lvm commands to pvmove something onto the snapshot then that is why I suggest that the user
be able to make decision at reboot about whether to remove the PV. If they answer 'no' to the removal then the boot will
stop and they will have to perform some recovery through the rescue mode to see if they can recover some data from
the device before allowing the PV to be removed from the VG so the system can boot.

And how do you distinguish
between ram disk and normal disk and if it is totally gone or it is
only temp issue? Also if real HDD fails, you don't want to vgreduce
it, you want to replace it, vgcfgrestore it and sync it (in case you
are using mirroring). Also imagine that you can have also real data on
the same PV, not only snapshot.
 
You don't vgreduce the entire hdd. Only for the PV that contained the snapshot.
If you used an entire hdd to be the snapshot then you can restore it. Recover data
in the case of read-write and then vgreduce it from the VG so system can boot.

What would help is the ability to activate the VG also with missing
PVs,
You cannot activate a VG with missing PV's because at some point then LVM would attempt to write
on one of the missing PV and then you get panic.


Regards,
Gerry


I'm not sure if that is possible in linux LVM.



 

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