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Date: Thu, 23 Jun 2011 17:54:36 +0200
Message-ID: <BANLkTinkQkTsQonvgVYUrvUcV-ReP+xZ=Q@mail.gmail.com>
Subject: Re: Backup/Restore: Coordinating Cassandra Nodetool Snapshots with
 Amazon EBS Snapshots?
From: Peter Schuller <peter.schuller@infidyne.com>
To: user@cassandra.apache.org
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> 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 g=
ive
> a chance to LVM, or the EBS low level 'modules' in the hypervisor ( whate=
ver
> you use underneath...), to have exclusive control of the drive for a mome=
nt.
> Now, that being said, some systems (like LVM)=C2=A0 will do a freeze them=
selves,
> 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.

--=20
/ Peter Schuller