Re: [Qemu-devel] [PATCHv2] block: add native support for NFS

[Paolo Bonzini]

Il 18/12/2013 18:21, Peter Lieven ha scritto:
> Not completely I think, but please correct me if I am wrong.
> 
> If cache=writeback is set we issue just a write. In libnfs or libiscsi case
> that guarantees that the request has been successfully executed
> on the target / server. This is enough to be consistent in case of
> migration because consecutive reads will be answered correctly.
> 
> But the target / server is still free to just keep this
> data in memory so we should only set this if we know that the target / server
> e.g. has a battery backup or we know that the filesystem uses e.g. barriers 
> and
> issues a flush at important points.

Yes.  However, the same holds for cache=none (both on
libnfs/libiscsi/NBD/others, where it is a nop, and on regular I/O, where
it uses O_DIRECT *but not O_SYNC or O_DSYNC*).

> If cache=none is set we issue a flush after every single write.

That's what you get with cache=writethrough or cache=directsync.  Here
the flush is added automatically by the block layer, so it works with
every driver.

Paolo