Hi all,
Am 10.05.2026 um 14:41 schrieb [email protected]:
On Sun, May 10, 2026 at 01:54:10PM +0200, Thomas Schmitt wrote:
Hi,
Jochen Spieker wrote:
I was always under the impression that an LVM snapshot is not better
than an unclean filesystem, like for example after a power loss. But
this 20 year old document proved me wrong:
https://tldp.org/HOWTO/LVM-HOWTO/snapshotintro.html
[email protected]:
Is there any reference for that? AFAIK LVM doesn't "know" about the
underlying file system (if there is any).
In a later mail:
This sounds very hypothetical: how should LVM that there is a file
system /at all/ on the logical volume being snapshotted? How that
this file system is mounted?
Obviously one can inquire the list of mounted filesystems and their
storage devices [...]
...then fsfreeze
Woah, Thomas: you definitely rock :)
Just what we all needed.
Do we? On one of my debian systems, man fsfreeze gets me
DESCRIPTION
fsfreeze suspends or resumes access to a filesystem.
fsfreeze halts any new access to the filesystem and creates a
stable image on disk. fsfreeze is intended to be used with
hardware RAID devices that support the creation of snapshots.
fsfreeze is unnecessary for device-mapper devices. The device-
mapper (and LVM) automatically freezes a filesystem on the device
when a snapshot creation is requested. For more details see the
dmsetup(8) man page.
Although LVM is rather limited in its support for snapshots, and XFS
being somewhat paranoid (which is good for file systems) make this
challenging in many cases.
Because, to get back to the original topic -- an LVM snapshot is *not*
an exact copy of the original volume, and of course much less so of the
partition, disk, or anything else. So a block level dump of an LVM
snapshot is not something you can just dump back as a disaster recovery
step.
And. of course, it doesn't let gaps between block storage consistency,
file system consistency, file system "cleanness", file contents
consistency and application data consistency vanish.
Cheers,
Arno
Thank you (and for all your other insightful
posts here!)
Cheers
--
Arno Lehmann
IT-Service Lehmann
Sandstr. 6, 49080 Osnabrück
