About Filesystem freeze/thaw in freebsd

[Matthew Seaman]

On 02/16/15 09:07, zx zx wrote:
> Hi,               I am experimenting to do a live backup of FreeBSD
> VM. Question is do we have freeze/thaw interfaces in FreeBSD? I
> searched a lot in web and freebsd source code, just could not find
> the right interface.               As I know that in linux:VxFS
> provides ioctl interfaces to application programs to freeze and thaw
> VxFS file systems. The interfaces are VX_FREEZE, VX_FREEZE_ALL, and
> VX_THAW.About Freeze and thaw Freezing a file system temporarily
> blocks all I/O operations to a file system and then performs a sync
> on the file system. Current operations are completed and the file
> system is synchronized to disk. Freezing a file system is a necessary
> step for obtaining a stable and consistent image of the file system
> at the volume level. Consistent volume-level file system images can
> be obtained and used with a file system snapshot tool. The freeze
> operation flushes all buffers and pages in the file system cache that
> contain dirty metadata and user data. The operation then suspends any
> new activity on the file system until the file system is thawed.
> Any help would be appreciated, thanks a lot!  Andy Zhang

What you want is snapshotting.  You can create a snapshot of UFS or ZFS
filesystems, mount the snapshot and then back it up without needing to
worry about the filesystem changing while you're trying to back it up.

See mksnap_ffs(8) and the 'snapshot' entry in zfs(8)

The snapshot is mounted separately from the actual filesystem which can
carry on with normal activities in the mean time.

Snapshotting functionality is built into dump(8) for UFS filesystems
(See the -L flag in that man page) or you can use zfs send / recv to
dump filesystems to tape, which implies use of snapshots.

	Cheers,

	Matthew


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