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

[Chen, Hanxiao]

> -----Original Message-----
> From: Martin Kletzander [mailto:mkletzan at redhat.com]
> Sent: Monday, November 24, 2014 3:09 PM
> To: Chen, Hanxiao/陈 晗霄
> Cc: libvir-list at redhat.com
> Subject: Re: [libvirt] [PATCH RFC] storage: perform btrfs clone if possible
> 
> On Mon, Nov 24, 2014 at 02:11:47PM +0800, Chen Hanxiao wrote:
> >We already had nocow flags in virStorageSource.
> >But when creating RAW file, we don't take advantage
> >of clone of btrfs.
> >This file introduce btrfs_clone_file function,
> >and try to use it when !nocow.
> >
> 
> I'm not sure we want to do this, but I have nothing against that
> either.  So I'll just review the code without any other comments.
> 
> >Signed-off-by: Chen Hanxiao <chenhanxiao at cn.fujitsu.com>
> >---
> > src/storage/storage_backend.c | 31 +++++++++++++++++++++++++++++++
> > 1 file changed, 31 insertions(+)
> >
> >diff --git a/src/storage/storage_backend.c b/src/storage/storage_backend.c
> >index 98720f6..f5ea34c 100644
> >--- a/src/storage/storage_backend.c
> >+++ b/src/storage/storage_backend.c
> >@@ -156,6 +156,27 @@ enum {
> > #define READ_BLOCK_SIZE_DEFAULT  (1024 * 1024)
> > #define WRITE_BLOCK_SIZE_DEFAULT (4 * 1024)
> >
> >+/*
> >+ * Perform the O(1) btrfs clone operation, if possible.
> >+ * Upon success, return 0.  Otherwise, return -1 and set errno.
> >+ */
> >+static inline int
> >+btrfs_clone_file(int dest_fd, int src_fd)
> 
> All the functions in this file use camelCase, this one might too.
> 
> >+{
> >+#ifdef __linux__
> >+# undef BTRFS_IOCTL_MAGICi
> 
> s/i$// ?
> 
> >+# define BTRFS_IOCTL_MAGIC 0x94
> >+# undef BTRFS_IOC_CLONE
> >+# define BTRFS_IOC_CLONE _IOW (BTRFS_IOCTL_MAGIC, 9, int)
> 
> Are you redefining those just in case they are not defined, but the
> support exists?  I'm always afraid of creating incompatibilities and
> would prefer #if for that and if anything is not defined, just don't
> use it.
> 
> >+    return ioctl(dest_fd, BTRFS_IOC_CLONE, src_fd);
> >+#else
> >+    (void) dest_fd;
> >+    (void) src_fd;
> 
> we use ignore_value(), but you don't need that if you do what's
> preferred ...
> 
> >+    errno = ENOTSUP;
> >+    return -1;
> >+#endif
> >+}
> >+
> 
> ... we prefer to split the whole definitions for functions to a
> working variant and a stub, in that case you can mark unused
> parameters in the stub function.  From a subjective point of view,
> it's more readable, also (you see right before the definition what you
> need for the function to work):
> 
> #if defined(__linux__) && defined(BTRFS_IOC_CLONE)
> 
> static inline int
> btrfs_clone_file(int dest_fd, int src_fd)
> {
>     return ioctl(dest_fd, BTRFS_IOC_CLONE, src_fd);
> }
> 
> #else
> 
> static inline int
> btrfs_clone_file(int dest_fd, int src_fd)
> {
>     errno = ENOTSUP;
>     return -1;
> }
> 
> #endif

Thanks for your explanation in such a detail way.
That's very helpful. :) 

> 
> >@@ -200,6 +221,16 @@ virStorageBackendCopyToFD(virStorageVolDefPtr vol,
> >         goto cleanup;
> >     }
> >
> >+    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.");
> 
> s/findished/finished/
> 
> As I said, I'm not commenting on whether we want this in or not, so
> for that you should wait for someone's response.  I bet there's a
> (good) reason behind libvirt not using some lvm/zfs/btrfs features,
> but I am too lazy to search for it since it'd be inaccurate anyway.
> 

Let's wait for other's comments on whether we need this.
But features like btrfs clone really cool
if we could get full use of it.
It's slow to copy huge VM images by tools such as virt-clone.

Thanks,
- Chen