Re: silent semantic changes with reiser4

David Masover
2004-08-26T18:54:28Z

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Christophe Saout wrote:

| Yes, the main compound stream unaware applications would see would just
| be a writable view of its contents. Probably not trivial to implement
| but doable.

Not the main stream, I think, but we do need a way to make "eight-stream
octopus" files portable. But there needs to be a copy(2) syscall, and
apps need to start supporting it. After all, copy-on-write was planned
for Reiser4, now 4.1*. And don't anyone dare suggest "userland library".

| It should be a simple format. Something simliar to tar. The worst thing

Simple, yes. Similar to tar, no.

How about a serializer plugin? foo.mp3/serialize (say) should return
some single stream which contains all of foo.mp3. It could work for
directories as well, recursively, at least on the same filesystem --
you'd back up by doing

cp /serialize /backup/20040826

This has two potential problems. One is security -- I want to be able
to serialize foo.mp3 without being root, but root doesn't want me to be
able to serialize /etc and thus get access to shadow.

The second problem is that in order to actually be used for backup, we
need snapshots, which are not done yet*. Currently, it'd have to lock
the file/directory (recursively), which you don't want to do to entire
live filesystems... So, no massive backups of live systems yet, but
definitely useful for sending around mp3s and such.

| that can happen is people start writing plugins for every existing
| compound format out there. It should be the other way around, agree on a
| simple compound format and encourage applications to use this one if the
| want to use this advantage.

For the format, I vote for whatever format we're using at the storage
layer. It'd be a lot faster that way, both in execution time (I think)
and in development time (I'm sure). Plus, imagine the restore:

cat /backup/20040826 > /dev/hda5
resizefs_reiser4 /dev/hda5

* Disclaimer -- all this is "I think, last I checked".

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