[CentOS] Who does snapshots other than Network Appliance?

John R Pierce pierce at hogranch.com
Wed Aug 22 18:47:07 UTC 2007


Jiann-Ming Su wrote:
> On 8/22/07, Scott Ehrlich <scott at mit.edu> wrote:
>   
>> Having snapshot technology is great.   Who else supports it?
>>
>>     
>
> Roll your own:
>
> http://www.mikerubel.org/computers/rsync_snapshots/
> http://www.dirvish.org/
>   

those aren't even close to the same thing, more like traditional 
backups, rsync or dirvish make copies of the original data.


'snapshotting' in this context refers to 'copy on write' technology.  
taking a snapshot is a very quick process, usually only a couple 
seconds, and no data actually gets copied.    when the file system is 
updated while a snapshot is active, THEN the block(s) behind the update 
are copied to new space, so that there are now two versions of that 
block, original (snapshot) and new (current).    If the snapshot is 
deleted, these copied blocks are released back into the free pool.  if 
the system is rolled back to the snapshot, the 'new' blocks are released.


A classic use of a snapshot is to make a  point-in-time backup of a live 
system to tape or an external near line disk storage system...  you 
snapshot the file system, let the applications keep running  on the live 
system, and backup the snapshot to your archival storage, then drop the 
snapshot.    Another use is as a checkpoint before doing something 
risky, like a system update.    snapshot the system, install the updates 
and test them.  if there's a problem, rollback to the snapshot.



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