doc: clarify dd sparse detection is by *output* block

The wording of the dd --help text suggests that output will be skipped
for sparse *input* blocks (i.e. that NUL-checking is done on input
blocks) while the code actually checks/skips all-NUL *output* blocks.[1]

* src/dd.c (usage): Update the --help text to clarify the above.
* tests/dd/sparse.sh: Ensure sparseness is controlled with obs.

[1]: https://superuser.com/a/1136358

diff --git a/src/dd.c b/src/dd.c
index 2888b8e33ce9993970fb733f8f0cc076d1463351..ef0d07ac395faa0a2e606dd9b7dcc7ca6fc1e173 100644 (file)
--- a/src/dd.c
+++ b/src/dd.c
@@ -601,7 +601,7 @@ Each CONV symbol may be:\n\
   unblock   replace trailing spaces in cbs-size records with newline\n\
   lcase     change upper case to lower case\n\
   ucase     change lower case to upper case\n\
-  sparse    try to seek rather than write the output for NUL input blocks\n\
+  sparse    try to seek rather than write all-NUL output blocks\n\
   swab      swap every pair of input bytes\n\
   sync      pad every input block with NULs to ibs-size; when used\n\
             with block or unblock, pad with spaces rather than NULs\n\

diff --git a/tests/dd/sparse.sh b/tests/dd/sparse.sh
index b6ec7d78c375e5112cbcff2f4d321ed0f7801e27..272e8d24acf8b6a4960d036ed043bb80845185dc 100755 (executable)
--- a/tests/dd/sparse.sh
+++ b/tests/dd/sparse.sh
@@ -60,9 +60,9 @@ kb_alloc() { du -k "$1"|cut -f1; }
 # after its creation.
 if test $(kb_alloc file.in) -gt 3000; then
 
-  # Ensure NUL blocks smaller than the block size are not made sparse.
+  # Ensure NUL blocks smaller than the *output* block size are not made sparse.
   # Here, with a 2MiB block size, dd's conv=sparse must *not* introduce a hole.
-  dd if=file.in of=file.out bs=2M conv=sparse || fail=1
+  dd if=file.in of=file.out ibs=1M obs=2M conv=sparse || fail=1
 
   # Intermittently BTRFS returns 0 allocation for file.out unless synced
   sync file.out || framework_failure_
@@ -75,8 +75,8 @@ if test $(kb_alloc file.in) -gt 3000; then
   rm -f file.out
   truncate --size=3M file.out
 
-  # Ensure that this 1MiB string of NULs *is* converted to a hole.
-  dd if=file.in of=file.out bs=1M conv=sparse,notrunc
+  # Ensure that this 1MiB *output* block of NULs *is* converted to a hole.
+  dd if=file.in of=file.out ibs=2M obs=1M conv=sparse,notrunc
   test $(kb_alloc file.out) -lt 2500 || fail=1
 
 fi