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Re: [dm-devel] Re: possible regression by the barrier patch in 2.6.30-rc2



Hi Alasdair,

On 10/29/2009 12:21 AM +0900, Alasdair G Kergon wrote:
> Well let's go back to first principles.
> 
> There are two types of suspend.
> 
> (1) I don't care about the ordering of the I/O on the disk relative to
> the suspend.  This one is easy: it's --noflush --nolockfs.
> 
> (2) I do want some control over the state of the device at the point
> of the suspend.
> 
> Break this second case down.
> If I have a filesystem, I require it to be consistent, so I require lockfs.
> 
> If the device belongs to a userspace database, I require it to be consistent,
> so I must pause the database, at which point there is no further I/O being
> issued to the device, then I suspend (and everything prior to this must be
> flushed) and resume etc.  This can be either a "suspend with flush" OR the
> flush could have been issued prior to the suspend (and any decent database
> would have done that).
> 
> If I have any other application owning the device that cares about the
> state on the device at the point of the suspend it *must* stop issuing
> I/O while the suspend ioctl is run.  If any I/O continues to arrive
> during the suspend, then that tells us we are necessarily in case (1).
> 
> So the implementation of 'suspend with flush' is simply 'issue a flush
> using the standard mechanism for doing this and then issue a suspend'.
> The 'suspend' itself does not need any special handling for 'flush'.
> In practice, anything that requires a flush will issue it prior to
> calling the suspend ioctl and not send I/O concurrently with the
> suspend.  For backwards compatibility we could still support the
> 'suspend with flush' as I described - issue a flush internally before
> entering the code that performs the suspend.

OK, I basically agree with the idea (which issues a flush before
the guts of the suspend code).
But I think issuing a flush after lock_fs() may be better like below:

	dm_suspend() {
		...
		if (!noflush && do_lockfs)
			lock_fs();

		if (!noflush)
			blkdev_issue_flush();
		...
		set_bit(DMF_BLOCK_IO_FOR_SUSPEND);
		...
		dm_wait_for_completion();
		...
	}

By doing this, we can add another feature, which guarantees
the flushed I/Os are written to the medium, for "flush" suspend,
not only for the backward compatibility of "--nolockfs && --flush".
(Maybe the "lock_fs() and blkdev_issue_flush()" can be moved out
 from the suspend code as a pair.)


By the way, I also think we need only one flag for "suspend with flush"
from application viewpoints.
Probably, we can drop "lockfs" flag because its naming is internal 
and applications shouldn't know what "lockfs" is.

Thanks,
Kiyoshi Ueda


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