Re: [Qemu-devel] [PATCH] Align file accesses with cache=off (O_DIRECT)

[Jamie Lokier]

Kevin Wolf wrote:
> I think we agree that it's mostly item 3 why one would use O_DIRECT with 
> qemu. In terms of reliability, it is important that the data really is 
> written to the disk when the guest OS thinks so. But when for example 
> qemu crashes, I don't think it's too important if 40% or 50% of a 
> snapshot have already been written - it's unusable anyway. A sync 
> afterwards could be enough there.

A call to fsync() after writing provides the same guarantee as
O_DIRECT, or stronger if fsync() does a hard disk cache flush, which
O_DIRECT does not.

-- Jamie