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Added a comment: more details on coreutils cp treatment of reflink

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yarikoptic 2019-03-06 16:00:35 +00:00 • committed by admin
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[[!comment format=mdwn
username="yarikoptic"
avatar="http://cdn.libravatar.org/avatar/f11e9c84cb18d26a1748c33b48c924b4"
subject="more details on coreutils cp treatment of reflink"
date="2019-03-06T16:00:35Z"
content="""
> git-annex looks at the file's stat() and only if the device id is the same
<details>
<summary>They are indeed not the same across subvolumes of the same BTRFS file system</summary>
```
$> time cp --reflink=auto home/yoh/reprotraining.ova scrap/tmp
cp --reflink=auto home/yoh/reprotraining.ova scrap/tmp 0.00s user 0.00s system 92% cpu 0.004 total
$> stat home/yoh/reprotraining.ova scrap/tmp/reprotraining.ova
File: home/yoh/reprotraining.ova
Size: 5081213952 Blocks: 9924248 IO Block: 4096 regular file
Device: 2fh/47d Inode: 61771704 Links: 1
Access: (0600/-rw-------) Uid: (47521/ yoh) Gid: (47522/ yoh)
Access: 2018-06-14 19:23:25.000000000 -0400
Modify: 2018-06-11 15:35:57.000000000 -0400
Change: 2018-06-14 19:23:25.891351983 -0400
Birth: -
File: scrap/tmp/reprotraining.ova
Size: 5081213952 Blocks: 9924248 IO Block: 4096 regular file
Device: 30h/48d Inode: 190040764 Links: 1
Access: (0600/-rw-------) Uid: (47521/ yoh) Gid: (47522/ yoh)
Access: 2019-03-06 10:38:02.610657786 -0500
Modify: 2019-03-06 10:38:02.610657786 -0500
Change: 2019-03-06 10:38:02.610657786 -0500
Birth: -
```
</details>
`cp` seems to just to attempt a cheap clone
```
/* Perform the O(1) btrfs clone operation, if possible.
Upon success, return 0. Otherwise, return -1 and set errno. */
static inline int
clone_file (int dest_fd, int src_fd)
{
#ifdef FICLONE
return ioctl (dest_fd, FICLONE, src_fd);
#else
(void) dest_fd;
(void) src_fd;
errno = ENOTSUP;
return -1;
#endif
```
and if that one fails, assumes that full copy is required:
```c
/* --attributes-only overrides --reflink. */
if (data_copy_required && x->reflink_mode)
{
bool clone_ok = clone_file (dest_desc, source_desc) == 0;
if (clone_ok || x->reflink_mode == REFLINK_ALWAYS)
{
if (!clone_ok)
{
error (0, errno, _(\"failed to clone %s from %s\"),
quoteaf_n (0, dst_name), quoteaf_n (1, src_name));
return_val = false;
goto close_src_and_dst_desc;
}
data_copy_required = false;
}
}
```
BTW, why `rsync` instead of a regular `cp` for local filesystem if it is across the devices?
"""]]