On 2017-01-30 23:58, Duncan wrote:
Oliver Freyermuth posted on Sat, 28 Jan 2017 17:46:24 +0100 as excerpted:
Just don't count on restore to save your *** and always treat what it
can often bring to current as a pleasant surprise, and having it fail
won't be a down side, while having it work, if it does, will always be
up side.
=:^)
I'll keep that in mind, and I think that in the future, before trying
any "btrfs check" (or even repair)
I will always try restore first if my backup was not fresh enough :-).
That's a wise idea, as long as you have the resources to actually be able
to write the files somewhere (as people running btrfs really should,
because it's /not/ fully stable yet).
One of the great things about restore is that all the writing it does is
to the destination filesystem -- it doesn't attempt to actually write or
repair anything on the filesystem it's trying to restore /from/, so it's
far lower risk than anything that /does/ actually attempt to write to or
repair the potentially damaged filesystem.
That makes it /extremely/ useful as a "first, to the extent possibke,
make sure the backups are safely freshened" tool. =:^)
It also has the interesting side effect that you can (mostly) safely run
restore against a mounted filesystem. I've never tried this myself, but
providing that restore doesn't check if the FS is mounted first (might
be something to add an option to disable if it does), the worst that
could happen is getting a corrupted file out or having restore crash on you.
Meanwhile, FWIW, restore can also be used as a sort of undelete tool.
Remember, btrfs is COW and writes any changes to a new location. The old
location tends to stick around, not any more referenced by anything
"live", but still there until some other change happens to overwrite it.
Note that this becomes harder the more active the FS is. This is the
case for most filesystems, but it's a much bigger factor for COW
filesystems, and even more so for BTRFS (because it will preferentially
pack data into existing chunks instead of allocating new ones).
Just like undelete on a more conventional filesystem, therefore, as long
as you notice the problem before the old location has been overwritten
again, it's often possible to recover it, altho the mechanisms involved
are rather different on btrfs. Basically, you use btrfs-find-root to get
a list of old roots, then point restore at them using the -t option.
There's a page on the wiki that goes into some detail in a more desperate
"restore anything" context, but here, once you found a root that looked
promising, you'd use restore's regex option to restore /just/ the file
you're interested in, as it existed at the time that root was written.
There's actually a btrfs-undelete script on github that turns the
otherwise multiple manual steps into a nice, smooth, undelete operation.
Or at least it's supposed to. I've never actually used it, tho I have
examined the script out of curiosity to see what it did and how, and it /
looks/ like it should work. I've kept that trick (and knowledge of where
to look for the script) filed away in the back of my head in case I need
it someday. =:^)
I've not used the script itself before, but I've used the method before
on a couple of occasions to pull out old versions of files that I should
have had under some kind of VCS but didn't, and the method does work
reliably as long as you do it soon.
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