Re: XATTRs in NFS?

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On Fri, 2013-10-25 at 10:08 -0400, J. Bruce Fields wrote:
> Can you give any more details about your use case?  (E.g. is there an
> article describing this system somewhere?)
Okay let me try to explain the motivation more elaborately.

The general idea is getting data integrity, i.e. being able to see
whether some files are in a valid state.

This is similar to what e.g. btrfs/zfs provide at (IIRC block/extent
level) with checksuming.
But a) btrfs is not yet fully production ready (IMHO) and zfs in Linux
has it's (license) issues and more important b) the checksuming there
happens always and automatically as soon as some valid filesystem
operations occur on the files.

So what you basically notice are errors on the physical medium (or at
least on block layers below the filesystem).


You won't notice many other cases of file "corruptions":

- when you have programs which do subtly manipulate the files and you
don't notice,...e.g. some image organisation programs store their
meta-data crap in the Exif/XMP fields, even when you don't actively tell
them to do so (and sometimes even when the files are read-only)

- when there are e.g. kernel bugs (in some other places of the kernel)
and you copy the files around. Some years ago I found a bug (not the
solution) where single bit errors happened more or less randomly every
40-100 GB of writing data. In the end it was found to be a IOMMU related
bug and a single one liner of flushing some buffers cleared the problem.
Since such errors would happen already at earlier stages, when writing
such files btrfs/zfs would simply write the checksums of the corrupted
data.

So the idea of my integrity data is, that I really manually say "now the
data is in the state where I consider it to be consistent and I want to
have checksums stored and attached to the files, for exactly that
state", e.g. after I have read out some images from the SD card (perhaps
even twice with the cache cleared and the results diffed) and placed in
my archive.
Afterwards I can regularly verify the whole archive and if at some stage
corruptions as the above would have happened, I can simply take the
respective files from backups.


Obviously that cannot be stored *in* the files,... and a database
solution fails since, it wouldn't track the changes when I move files
around within the archive.


So IMHO the best solution were XATTRs, which do work fine for that
purpose... just the problem that I can't have it via NFS ;)


Cheers,
Chris.

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