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    SubjectRe: ext4 features (salvage)
    On Tue, 2006-07-04 at 13:35 +0200, Peter Zijlstra wrote:
    > On Tue, 2006-07-04 at 11:22 +0200, Petr Tesarik wrote:
    > > Yes and no. A simple mv is better done in userspace, but what I'd
    > > _really_ appreciate would be a true kernel salvage (similar to the way
    > > NetWare does things). That means marking the file as deleted in the
    > > directory, marking its blocks as deleted but avoiding the use of those
    > > blocks. The kernel would then prefer allocating new blocks from
    > > elsewhere but once the filesystem runs out of space, it would start
    > > allocating from the deleted files area and marking the blocks as well as
    > > the corresponding files purged.
    > >
    > > Salvaging files would be done with a separate tool. Of course, if you
    > > delete more files with the same name in the same directory, you'd need
    > > to tell that tool which one of them you want to salvage. Yes, I really
    > > mean you'd have more than one deleted file with the same name in the
    > > directory.
    > Wouldn't such a scheme interfere with the block allocator algorithms,
    > and hence increase the risk of fragmentation? Schemes like this realy
    > put my hairs on end,

    Yes, they would interfere. That's why I'm not proposing to add them to
    ext4 in the first place.

    > 1) if you don't want to lose your data, make backups;

    Generally, I agree.

    > 2) if I mean to delete a file, I want it gone proper. Silently keeping
    > it about is not unix like;

    Yes, this is a problem. Although you would of course have a tool for
    purging the files unconditionally, some programs may need the assumption
    that an unlinked file is gone forever.

    Regarding the second clause, well, Linux is not Unix-like in many
    respects and we want it like that. That's a weak argument.

    > 3) don't aid third parties in recovering your removed data. If I want
    > them to have it I'll give it to them.

    See 2. Explicit purging is of course possible. (Novell Netware also had
    a "purge" command.)

    Anyway, it seems that there is some functionality which many users want
    but which can't be provided in user space:

    - if files are moved to the recycle-bin-or-whatever-you-call-it, their
    size is added to disk free space and
    - automatically purging least recently deleted files.

    Petr Tesarik
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