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Re: [libvirt] [PATCH 7/8] save: add virDirectFd wrapper type



On Thu, Jul 14, 2011 at 08:34:46AM -0600, Eric Blake wrote:
> On 07/14/2011 08:24 AM, Eric Blake wrote:
> > O_DIRECT has stringent requirements - I/O must occur with buffers
> > that have both alignment and size as multiples of the file system
> > block size (used to be 512 bytes, but these days, 4k is safer, and
> > 64k allows for better throughput).  Rather than make lots of changes
> > at each site that wants to use O_DIRECT, it is easier to offload
> > the work through a helper process that mirrors the I/O between a
> > pipe and the actual direct fd, so that the other end of the pipe
> > no longer has to worry about constraints.
> > 
> 
> > +virDirectFdPtr
> > +virDirectFdNew(int *fd, const char *name)
> > +{
> > +    virDirectFdPtr ret = NULL;
> > +    bool output = false;
> > +    int pipefd[2] = { -1, -1 };
> > +    int mode = -1;
> > +
> > +    if (VIR_ALLOC(ret) < 0) {
> > +        virReportOOMError();
> > +        goto error;
> > +    }
> > +    if (!O_DIRECT)
> > +        return ret;
> 
> Question - should an attempt to use 'virsh save --direct' on a system
> that lacks O_DIRECT (think mingw) be rejected, rather than silently
> ignored?  My argument is that --direct is merely an optimization hint -
> it tries to reduce filesystem cache pollution (possibly at the expense
> of slower operation), but other than the cache effects, the end result
> is the same as if O_DIRECT is unavailable.  Hence, my decision of
> silently ignoring it rather than erroring out.

The motivation for using O_DIRECT is that allowing pollution of the
host cache causes stability problems for the host as a whole. As
such IMHO, apps would likely want an error back if O_DIRECT cannot
be supported,

NB, even some Linux filesystems can't do O_DIRECT, so this isn't an
obscure mingw32 issue.


Daniel
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