[OpenAFS] Re: why afs backup is so poorly supported

Matt Benjamin matt@linuxbox.com
Tue, 10 Oct 2006 17:24:26 -0400

Cool as the transactional piece would be, iirc from our discussion in 
2004, putting a postgresql behind every fileserver sounds kind of 
heavyweight, doesn't it?

Plus, is there a difference between transactional metadata updates and 
transactional file data updates?


Marcus Watts wrote:
> Jeffrey Hutzelman <jhutz@cmu.edu> replied:
> ...
>> It's an interesting idea, though probably more suitable for discussion on 
>> openafs-devel than in this forum.  To handle StoreData, you'd need the 
>> ability to update only part of a blob.  Also, how efficiently are large 
>> blobs handled even by those databases that support them?
> That most likely depends on the database & API.  At a quick glance,
> the command line tool for postgresql only imports & exports to
> "a local file" (which has interesting similarities to the origins
> of AFS).  The libpq C interface to postgresql supports read, write,
> lseek from/to the server, much like regular file access.  The internals
> of postgresql implement blobs as chunks in a special table which can be
> randomly accessed.  Blobs can be up to 2G in size.  There are also
> "toasted" objects, which probably aren't as useful.  I'm afraid
> to ask if one can toast blobs.
> My recollection is that oracle has some sort of chunk-wise access
> to blobs.  I assume other db systems (db2, etc.) have similar
> functions, at least if they intend to implement the "l" in blob.
> One interesting limitation with blobs may have to do with rollback segment
> size.  I know that's an issue with oracle, not sure about postgresql.
> The basic problem is that if you update 2G of stuff, you might not be
> able to do it as one atomic transaction.  Probably you shouldn't want
> to in any case, but it might spoil chas's vision of atomic commits.
> 				-Marcus
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Matt Benjamin

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