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Re: [linux-lvm] LVM snapshots and filesystem quiescence



On 2013-04-25 2:35 PM, Tom Coughlan <coughlan redhat com> wrote:
The LVM management tools will automatically suspend the file system
before taking the snapshot. Refer to the following, from the RHEL 6.4
Storage Administration Guide:

----------

To suspend or resume write activity to a file system, use xfs_freeze.
Suspending write activity allows hardware-based device snapshots to be
used to capture the file system in a consistent state.
Note
The xfs_freeze utility is provided by the xfsprogs package, which is
only available on x86_64.

Hmmm... thanks Tom, but googling seems to reveal that xfs_freeze has been replaced with fsfreeze (the code for which is based on xfs_freeze) on modern linux systems?

http://linux.die.net/man/8/fsfreeze

And the full answer to my question from the man page:

"fsfreeze is unncessary for device-mapper devices. The device-mapper (and LVM) automatically freezes filesystem on the device when a snapshot creation is requested. For more details see the dmsetup(8) man page."

and

"This option requests the specified a filesystem to be frozen from new modifications. When this is selected, all ongoing transactions in the filesystem are allowed to complete, new write system calls are halted, other calls which modify the filesystem are halted, and all dirty data, metadata, and log information are written to disk. Any process attempting to write to the frozen filesystem will block waiting for the filesystem to be unfrozen."

But thanks for the pointer... :)


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