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

From: Mark Trumpold
Subject: [Qemu-discuss] 'qemu-nbd' explicite flush to disk
Date: Mon, 10 Sep 2012 18:35:40 -0700


I am experimenting with the '-cache=writeback' feature of Qemu version 1.2.0.  
I am using 'qcow2' loop images on top of a 'nilfs' filesystem (for snapshots).  
The default Qemu mode (with no '-cache' option) keeps data flushed to disk, but 
creates excessive 'nilfs' checkpoints - based on the Qemu synchronous writes.

The Qemu '-cache=writeback' alleviates the excessive checkpoint symptom with 
'nilfs', however I am having trouble explicitly flushing data to disk before 
taking a 'nilfs' snapshot.

'qemu-nbd -d /dev/nbdx' disconnects my Qemu image and things get flushed to 
disk, but in my case I don't want to disconnect before snapshotting.  I've 
tried the following in the 'ndb.c' which is similar to the 'disconnect' 
function (this doesn't work):

        int nbd_flush(int fd)
              ioctl(fd, NBD_CLEAR_QUE);
              ioctl(fd, NBD_CLEAR_SOCK);
              return 0;
         int nbd_disconnect(int fd)
               ioctl(fd, NBD_CLEAR_QUE);
               ioctl(fd, NBD_DISCONNECT);
               ioctl(fd, NBD_CLEAR_SOCK);
               return 0;

I've also been reading in your forum that the "bdrv_co_flush" might be an 
option, but I am not sure how to interface with it.

Any advice would be greatly appreciated.

Thank you,
Mark Trumpold

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