Jack Stone wrote:
Chris Snook wrote:
The underlying internal implementation of something like this wouldn't
be all that hard on many filesystems, but it's the interface that's the
problem. The ':' character is a perfectly legal filename character, so
doing it that way would break things.
But to work without breaking userspace it would need to be a character
that would pass through any path checking routines, ie be a legal path
character.
I think NetApp more or less got the interface right by putting a
.snapshot directory in each directory, with time-versioned
subdirectories each containing snapshots of that directory's contents
at those points in time. It keeps the backups under the same
hierarchy as the original files, to avoid permissions headaches,
it's accessible over NFS without modifying the client at all,
and it's hidden just enough to make it hard for users to do something
stupid.
My personal implementation idea was to store lots of files for the form
file:revision_number (I'll keep using that until somebody sugests
something better) on the file system itself, with a hard link form the
latest version to file (this is probably not a major imporvement and
having the hard link coudl make it hard to implement deltas). This could
mean no changes to the file system itself (except maybe a flag to say
its versioned). The kernel would then do the translation to find the
correct file, and would only show the latest version to userapps not
requesting a specific version.
I pointed out NetApp's .snapshot directories because that's a method that uses
legal path character, but doesn't break anything. With this method, userspace
tools will have to be taught that : is suddenly a special character. Userspace
already knows that files beginning with . are special and treat them specially.
We don't need a new special character for every new feature. We've got one,
and it's flexible enough to do what you want, as proven by NetApp's extremely
successful implementation. Perhaps you want a slightly different interface from
what NetApp has implemented, but what you're suggesting will change the default
behavior of basic tools like tar and ls. This is not a good thing.
If you want to do something like this (and it's generally not a bad
idea), make sure you do it in a way that's not going to change the
behavior seen by existing applications, and that is accessible to
unmodified remote clients. Hidden .snapshot directories are one way, a
parallel /backup filesystem could be another, whatever. If you break
existing apps, I won't touch it with a ten foot pole.
The whole interface would be designed to give existing behavior as
default for two reasons: users are used to opening a file and getting
the latest version and not to break userspace. I personally wouldn't
touch this either if it broke userspace. The only userspace change would
be the addition of tools to manage the revisions etc. Userspace could
later upgrade to take advantage of the new functionality but I cannot
see the worth in breaking it.
But what you're talking about *will* break userspace. If I do an ls in a
directory, and get pages upon pages of versions of just one file, that's broken.
If I tar up a directory and get a tarball that's hundreds of times larger than
it should be, that's broken. If you want the files to be hidden from userspace
applications that don't know about your backup scheme, (and it sounds like you
do) then use the existing convention for hidden files, the prepended '.' This
is the universal sign for "don't mess with me unless you know what you're doing".
-- Chris
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