[uml-devel] Dynamic remount with variable COW stacking/merging needed, support for snapshot repilication

["Stephen D. Williams" <sdw@lig.net>]

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When a UML instance is running a always-on service, such as a web 
server, an administrator needs to be able to make live backups, or 
replications, of a running system.

This can be done using LVM snapshots, although that is not always 
appropriate for just this feature. A UML instance can be paused and then 
restarted, but this can cause severe delays while large partitions are 
replicated.

The COW ability is a great basis for an ideal solution, but I believe we 
need to identify and implement some additional features.

What I propose as a useful solution is:

A UML instance mounts filesystems directly or based on a COW image.
When an administrator invokes a console snapshot mode, the UML instance 
causes a quick freeze, new delta COWs to be created and stacked on 
existing mounts, then resumes.
When the administrator completes whatever snapshot backup is needed, 
they invoke a console unsnapshot command which pauses the instance, 
merges the delta COWs, remounts the original images with updates, and 
resumes.

This relies on COW stacking, which I saw was added in a patch last year 
and I assume is still present.
The downtime for the instance would be measured in seconds generally.

Can this be done now?  What needs to be added to support it?

sdw

-- 
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Stephen D. Williams 703-724-0118W 703-995-0407Fax 20147-4622 AIM: sdw



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