[Qemu-devel] 'qemu-nbd' explicite flush to disk

[Mark Trumpold]

Hello,

I posted a similar message to 'qemu-discuss', and was thinking my question may 
require a developer level of expertise.  I will try to be as brief as possible.

My context is using 'qemu-nbd' and 'qemu-img' to manage 'qcow2' loop filesystem 
images being hosted on a 'nilfs' filesystem.  This way I can 'nilfs snapshot' 
the images.

The problem is 'nilfs' creates a 'checkpoint' for every synchronous write, and 
'qemu-nbd's default cache mode is 'writethrough'.   Disk space is consumed 
quickly with the 'checkpoint' overhead in this case.

So, I've been experimenting with 'qemu-nbd --cache=writeback ..'
This nicely eliminates the  'checkpoint' issue; however, I have as yet been 
unable to explicitely flush things to disk -- which I would like to do just 
before a 'nilfs' snapshot.

I've tried the 'bdrv_drain_all() / bdrv_flush_all()'.   I instrumented and 
found the 'bdrv_co_flush(bs)' doesn't get called as expected in the loop.  I 
had one image connected as follows for this test:
    qemu-nbd -v --cache=writeback -c /dev/nbd5 /images/root/snap.img

Subsequently I've been trying to call 'bdrv_co_flush(bs)' directly, but I can't 
figure out how to dereference 'bs' for the call.

I'm probably out in the weeds on this one.
Any guidance would be greatly appreciated.

I am running:
    qemu-1.2.0
    linux kernel 3.3.1

Thank you,
Mark Trumpold


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