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Re: [libvirt] [PATCH RFC] storage: perform btrfs clone if possible




> -----Original Message-----
> From: Eric Blake [mailto:eblake redhat com]
> Sent: Tuesday, November 25, 2014 11:59 PM
> To: Chen, Hanxiao/陈 晗霄; Martin Kletzander
> Cc: libvir-list redhat com
> Subject: Re: [libvirt] [PATCH RFC] storage: perform btrfs clone if possible
> 
> On 11/25/2014 03:19 AM, Chen, Hanxiao wrote:
> >> I think it makes sense to expose this functionality; although I suspect
> >> it is better if we do so by having the user pass an explicit new flag
> >> value to existing API instead of doing it automatically.
> >>
> > Thanks for your clarification.
> >
> > But we've already had nocow in virStorageSource and <nocow> tags.
> > So I think if we do not specify <nocow> tags in XML,
> > we should try it according to 'nocow' in codes.
> >
> > Or do we need a new flags such as --reflink
> > for tools like virt-clone?
> 
> It's a design trade-off - do we make the default operation efficient
> when possible (but with possible surprises due to overcommit of
> storage), or safe (but slow)?  Regardless of the default, is it possible
> to explicitly request the opposite action?  To me, it seems backwards
> compatible to make an operation faster by using underlying clone hooks
> as long as the end result is the same, and as long as a user NOT
> specifying a specific use of clone or avoidance of clone does not get an
> error message when clone is attempted but not supported.  So at this
> point, I could go either way for the default as long as I can still have
> full control over the behavior, and we don't break existing users.
> 

FYI, in my limited test,
the suggestion of this patch did not break existing users.
If not supported, it just print some debug logs
and try the original way then:

+    if (!vol->target.nocow) {
+        if (btrfs_clone_file(fd, inputfd) == -1) {
+            if (errno == ENOTSUP)
+                VIR_DEBUG("btrfs clone not supported, try another way.");
+        } else {
+            VIR_DEBUG("btrfs clone findished.");
+            goto cleanup;
+        }
+    }
+

If I didn't miss something, I think this kind of operation
is acceptable.

Thanks,
- Chen


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