Re: [Qemu-devel] PATCH: Support for multi-file raw images

[Ryan Lortie <desrt@desrt.ca>]

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On Fri, 2006-12-05 at 09:35 -0500, Troy Benjegerdes wrote:
> Have you tried making a read-only 'base' image and using qcow images
> instead? I'm not convinced that splitting things up is going to help a
> lot. You might end up writing 1 512 byte block each to 500 files.. in
> the qcow image case, that is writing 256K, and with 10mb files, that's
> 5GB.

The problem with this approach is that I get an ever-growing changes
file which must eventually be merged (and then I have to copy the 20GB
again).

> at the very least, the console should print an error. If you can keep
> all the files open, deleting the file won't be a problem.

I agree -- console error should definitely occur.

Opening all the files at the start is something to consider.  It does
fly in the face of two things, though:

1 -- Limiting the number of fds.  Obviously opening all file parts at
the very start will use up a whack of fds right away.

2 -- Not modifying files that haven't been modified.  If you open a file
read-write then its modification time is automatically updated and rsync
will want to checksum/copy it again (it quickly checks local files based
on modtime and size).  If you open all files read-only then the file may
not be there when you go to reopen it read-write.  This is why the
current implementation is as lazy as possible in terms of opening files.

Cheers

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