> 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
