Re: ext3cow FS?

Scott R. Ehrlich wrote:
> I just found an article comparing Ubuntu Linux (7.04) to Vista...

Link?


> In that section they mention ext3cow...

Link?
Oh, here it is: http://www.ext3cow.com/  (Thanks Bill)
Also : http://en.wikipedia.org/wiki/Ext3cow


> ...which was the first time I'd heard of it.

Me too. Thanks for mentioning it.


> I haven't had to recompile a kernel in ages - it seems the default
> ones, for the most part, for Ubuntu and CentOS, anyway, are fully
> robust to handle whatever I throw at them.

And your point is? Are you implying use of ext3cow requires compiling a
custom kernel? The blog posting Bill linked to:

http://cmynhier.blogspot.com/2007/05/ext3cow-snapshots-for-ext3.html

says:

   One of the design features of ext3cow is that changes to support
   snapshotting were localized to the ext3 code itself... This certainly
   makes life easy, as it can be used with otherwise stock kernels.

It doesn't say explicitly, but I assume the author is implying that it
can be built as a module.


> What are people's insights of ext3cow?

Per this from the blog posting:

   Multiple versions of a file can share data blocks, as long as those
   data blocks haven't changed.

I'd say it fairs better than most other snapshotting technologies
available on Linux. The other options generally operate at the file
level, rather than at the block level. This sounds comparable to what is
done in NetApp's WAFL[1] or Sun's ZFS[2], both file systems designed
with snapshotting in mind. You can do snapshots with LVM, but I'm pretty
sure it isn't as efficient. To quote Wikipedia, "Block-level
snapshotting is almost always less space-efficient than direct file
system support for snapshots."[3]

1. http://en.wikipedia.org/wiki/Write_Anywhere_File_Layout
2. http://en.wikipedia.org/wiki/Zfs
3. http://en.wikipedia.org/wiki/Snapshot_%28computer_storage%29


What I don't see in either the ext3cow site or the blog posting is an
indication of the maturity or stability of the software, aside from the
0.2 version number. I'd have to assume it is still in an experimental state.


> So how does something like ext3cow for some kind of backup/restore fair?

I'd assume it supplements a backup strategy just as any other
snapshotting[3] technology does. It permits you to more quickly access
past versions of a file, and to efficiently store more versions. Of
course that's contingent upon the file system being accessible, and thus
you still need to pair it with traditional backup techniques.


> What other similar tools do people use?

rsnapshot[4] and other similar tools built around rsync have
traditionally been the approach to attaining snapshot functionality on
Linux. I see mention of LVM snapshots some too.

4. http://www.rsnapshot.org/

  -Tom

--
Tom Metro
Venture Logic, Newton, MA, USA
"Enterprise solutions through open source."
Professional Profile: http://tmetro.venturelogic.com/

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