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From Peter Schuller <peter.schul...@infidyne.com>
Subject Re: Backup/Restore: Coordinating Cassandra Nodetool Snapshots with Amazon EBS Snapshots?
Date Thu, 23 Jun 2011 15:54:36 GMT
> Actually, I'm afraid that's not true (unless I'm missing something). Even if
> you have only 1 drive, you still need to stop writes to the disk for the
> short time it takes the low level "drivers" to snapshot it (i.e., marking
> all blocks as clean so you can do CopyOnWrite later). I.e., you need to give
> a chance to LVM, or the EBS low level 'modules' in the hypervisor ( whatever
> you use underneath...), to have exclusive control of the drive for a moment.
> Now, that being said, some systems (like LVM)  will do a freeze themselves,
> so technically speaking you don't need to explicitly do a freeze
> yourself...but that's not to say that a freeze is not required for
> snapshotting.

This doesn't make sense unless you can provide some specific reason
why this would be required. If a file system is crash-consistent,
relying on write barriers to work, and given that the setup (kernel,
mounts opts, device driver etc) is such that write barriers are not
broken, it is directly implied that a consistent snapshot of the under
lying device is equivalent to a sudden halt (power off, sudden reboot,
etc).

If taking an atomic snapshot of the device on which a file system is
located on, assuming the file system is designed to be crash
consistent, it *has* to result in a consistent snapshot. Anything else
would directly violate the claim that the file system is crash
consistent, making the premise false.

-- 
/ Peter Schuller

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