Re: [PATCH] copy: adjust fiemap handling of sparse files

[Pádraig Brady]

On 09/02/11 23:35, Pádraig Brady wrote:
>>> Subject: [PATCH] copy: adjust fiemap handling of sparse files
>>>
>>> Don't depend on heuristics to detect sparse files
>>> if fiemap is available.  Also don't introduce new
>>> holes unless --sparse=always has been specified.

>>> * src/copy.c (extent_copy): Pass the user specified
>>> sparse mode, and handle as described above.
>>> Also a redundant lseek has been suppressed when
>>> there is no hole between two extents.
>>
>> Could that be done in two separate patches?

Here is the first patch split out to suppress redundant lseek()s.
It's expanded now to suppress lseek in the source as well as dest.

$ fallocate -l $((1<<29)) /tmp/t.f
$ strace -e _llseek cp-before /tmp/t.f /dev/null 2>&1 | wc -l
180
$ strace -e _llseek cp /tmp/t.f /dev/null 2>&1 | wc -l
0

Hmm, the above suggests to me that we could use the
FIEMAP_EXTENT_UNWRITTEN flag, so as to skip the read()
for these allocated but unwritten (zero) extents.
We could treat such extents like holes, except we
would only generate holes in the dest with SPARSE_ALWAYS.
I'll do patch for that this evening.

Anyway the following small reorg will also simplify
possible future changes to introduce fallocate().

Note the attached mostly changes indenting,
with the significant change being just:

$ git diff -w --no-ext-diff HEAD~

diff --git a/src/copy.c b/src/copy.c
index 9182c16..5b6ffe3 100644
--- a/src/copy.c
+++ b/src/copy.c
@@ -334,7 +334,10 @@ extent_copy (int src_fd, int dest_fd, char *buf, size_t 
buf_size,
         {
           off_t ext_start = scan.ext_info[i].ext_logical;
           uint64_t ext_len = scan.ext_info[i].ext_length;
+          uint64_t hole_size = ext_start - last_ext_start - last_ext_len;

+          if (hole_size)
+            {
           if (lseek (src_fd, ext_start, SEEK_SET) < 0)
             {
               error (0, errno, _("cannot lseek %s"), quote (src_name));
@@ -356,11 +359,6 @@ extent_copy (int src_fd, int dest_fd, char *buf, size_t 
buf_size,
               /* When not inducing holes and when there is a hole between
                  the end of the previous extent and the beginning of the
                  current one, write zeros to the destination file.  */
-              if (last_ext_start + last_ext_len < ext_start)
-                {
-                  uint64_t hole_size = (ext_start
-                                        - last_ext_start
-                                        - last_ext_len);
                   if (! write_zeros (dest_fd, hole_size))
                     {
                       error (0, errno, _("%s: write failed"), quote 
(dst_name));

cp-fiemap-1.diff
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