man2/fallocate.2: Document behavior with shared blocks

Note that FALLOC_FL_UNSHARE may use CoW to unshare blocks to
guarantee that a disk write won't fail with ENOSPC.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

diff --git a/man2/fallocate.2 b/man2/fallocate.2
index 54d634096aae2315f3b53941e85c74d22318649b..3ed14601d8dbc9c7fe20ad00583a9f8f64c82dc7 100644 (file)
--- a/man2/fallocate.2
+++ b/man2/fallocate.2
@@ -79,6 +79,16 @@ but the file size will not be changed even if
 is greater than the file size.
 Preallocating zeroed blocks beyond the end of the file in this manner
 is useful for optimizing append workloads.
+
+If the
+.B FALLOC_FL_UNSHARE
+flag is specified in
+.IR mode ,
+shared file data extents will be made private to the file to guarantee
+that a subsequent write will not fail due to lack of space.
+Typically, this will be done by performing a copy-on-write operation on
+all shared data in the file.
+This flag may not be supported by all filesystems.
 .PP
 Because allocation is done in block size chunks,
 .BR fallocate ()