Dne 28. 09. 23 v 14:23 Jean-Marc Saffroy napsal(a):
On Wed, Sep 27, 2023 at 5:41 PM Zdenek Kabelac <zdenek.kabelac@xxxxxxxxx> wrote:
What is the role of "dmsetup suspend"? I am having trouble finding
decent documentation about its purpose and how it's related to
snapshots. I did not need it in my experiments, so I am curious.
Suspend is freezing device's i/o queue (together with freezing FS layer - so
the snapshot should be easily mountable without requiring extensive fsck
operation as it would be missing some important metadata to be written on disk)
So the goal of a suspend is to take a 'good point in time' where the content
of snapshot is having all 'committed' transaction on disk in valid state.
Is this still required or useful with a journaling FS like ext4? It is
robust to pulling the plug at any time, so any point in time should be
good, no?
Wondering where do you came to the idea that journaling FS can rescue such
scenario flawlessly. Sure 'FS' should not completely broke itself if you
avoid this suspension & fsfreeze - but on the other hand the internal
inconsistency within a snapshot would require some repairing operation to
happen - and potential risk of valid data loss as even ext4 by default
journals only it's metadata, and 'data' are journaled only in 'data=journal'
mode - which is however used only by very small group of users who are willing
to give-up performance for this feature.
In all other cases - you want to get FS into frozen state before taking its
snapshot - so there is maximal consistency.
That said, I am curious about what can be achieved with dmsetup
commands. By any chance, do you have pointers to documentation besides
what's in the kernel (Documentation/admin-guide/device-mapper/)?
There are some DM talks available on the net describing some target logic in
greater details with some drawn boxes describing I/O flow - but other then
that I'm not sure what other kind of help would be needed here?
Regards
Zdenek
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