Subject: Re: Which snapshot strategy to use? was: How to capture all file
To: NetBSD Kernel Technical Discussion <tech-kern@netbsd.org>
From: Greywolf <greywolf@starwolf.com>
List: tech-kern
Date: 10/23/2003 10:42:27
Limited viewpoint here:  Do we want to do this at the block layer, the
partition layer, the physical drive layer or the volume (virtual drive)
layer?

I was going to ask, "this is snapshotting, not journaling, right?", but
then it occurred to me that if we're going to snapshot the filesystem,
wouldn't a way be to freeze write access, queue up the writes in a
journal, dump off the snapshot to wherever, release the access, and
then keep going?  That's journaling, effectively, unless I'm sitting
in left field waiting for the White Sox to come up to bat again (which
is entirely possible).

...or do you just keep write logs (be they PBL or whatever) and use
those as the snapshots?

[It just seems to me that if you're snapshotting a filesystem, at one point,
you're going to have to have the original filesystem and go forward from
there using additions, or you'll have to have logs leading up to the
current filesystem and go backwards from there using deletions which have
been logged.]

(How wide of the mark am I, here?)

				--*greywolf;
--
Instead of asking why a piece of software is using "1970s technology",
start asking why software is ignoring 30 years of accumulated wisdom.