RE: [Xen-users] Can't use DVD Writer in DomU

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> On Thursday 14 February 2008 08:41:05 pm James Harper wrote:
> > The block interface provided by blkback and blkfront is pretty
basic,
> > supporting only read, write, and write+barrier. There is no
provision
> > for the sort of commands required for doing writing.
> 
> What is 'write+barrier'? Thanx.

The barrier is a signal to pass on to the disk that all writes sent to
the disk before the barrier should be completed before any writes sent
after the barrier. The disk (or disk array etc) may re-order writes for
efficiency reasons, especially if write caching is happening. This can
be disastrous if the operating system has made an assumption about the
atomicity of its write operation.

James


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