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

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

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Yes, but that doesn't meet the key requirement I am proposing: that the 
downtime be limited to a few seconds.  The only potential significant 
downtime with what I am proposing is merging COWs if there was a lot of 
disk I/O during the 'snapshot mode'.  The existing ability is certainly 
useful, but not sufficient to get backups with the least impact to a 
running system.  Snapshot mode is suspend and flush/sync with the 
additional semantics of an automatic temporary push/pop of a COW layer 
on all filesystem images.  The remaining issue of merging COW's could be 
handled by RAID-recovery like gradual merging.

The problem is that if you have gigabytes of filesystem images, it takes 
time to copy them, even if using something like rsync to determine what 
has changed.  It also costs, unless there is a program that determines 
which blocks are actually utilized without reads, to scan zero holes in 
sparse files.

I should have also mentioned that it should be possible to get a 
reliable feed of what blocks, or ranges more likely have changed since a 
certain event.  This would allow very efficient, near-realtime 
replication.  At the very least this could be used in suspend or 
'snapshot mode' for efficiency, but with proper push of blocks, buffer 
visibility, or some kind of write-through notification, synchronization 
could be realtime.

sdw

Jeff Dike wrote:

>On Sat, Jan 17, 2004 at 04:13:39PM -0500, Stephen D. Williams wrote:
>  
>
>>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.
>>
>>Can this be done now?  What needs to be added to support it?
>>    
>>
>
>Have you seen the stop, sysrq s, cp, go trick described at 
>http://user-mode-linux.sourceforge.net/mconsole.html ?
>
>				Jeff
>  
>


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