Re: Plugin for corruption resistance?

[David Masover <ninja@slaphack.com>]

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First, let me second the original idea.  So long as the hash isn't
updated until that attribute is read, it should be fine.

Tom Vier wrote:
| On Fri, Feb 11, 2005 at 01:58:59PM -0500, Gregory Maxwell wrote:
|
|>1. Lots of applications today (such a P2P sharing systems) need the
|>hashes of files.. it's inefficient to keep recomputing them.  The file
|>system always knows when a file changes, so it can be setup to always
|>return the correct hash.
|
|
| That should be done in userland, imho. Especially since different apps use
| lots of different hashes.

For no particularly good reason, imho.  md4/md5 are reasonably fast,
sha1 is reasonably secure.  What else do you need?

| I was thinking about this kind of stuff (ECC plugin for r4) not too long
| ago. Hashing the whole file is too slow; if you update a single block, the
| whole file has to be read in to recalculate. Adding, say, one sector
of crc
| for each block would be a lot more feasible.

If it was actually to work like ECC.  But this doesn't sound like it
would be checked at every read, but rather when fsck is run, or when
some app needs it.  If it is supposed to be checked at every read, your
below suggestion is better.

| I think the best way to do this though, would be to write a virtual blk
| driver that works like loop back (ie, uses a backing file/dev), and
shortens
| the overall size by one sector * number of blocks. Actually, you could
| probably copy the raid5 md code and rewrite it to only use one device. I'd
| try that first.

Been done.  Think it's part of dev-mapper or evms or some disk magic
like that.  But it's also useful for individual files, for application
reasons.

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