Hi all,
I discovered that making a file sparse with "fallocate -d filename"
fails on the last block of a file, because - usually being partial - the
system call only zeroes that part instead of deallocating the block. See
man fallocate(2) - section "Deallocating file space".
The expected call is punching the whole block beyond eof, which doesn't
change the file length because of flag FALLOC_FL_KEEP_SIZE.
The following commands illustrate this (all on Debian buster with a real
existing file).
Behaviour with fallocate from system: file gets fragmented
user:~/dev/util-linux$ cp /usr/lib/locale/C.UTF-8/LC_CTYPE .
user:~/dev/util-linux$ /usr/sbin/filefrag -e ./LC_CTYPE
Filesystem type is: 9123683e
File size of ./LC_CTYPE is 200752 (50 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected:
flags:
0: 0.. 49: 22403653.. 22403702: 50:
last,eof
./LC_CTYPE: 1 extent found
user:~/dev/util-linux$ fallocate -v -d ./LC_CTYPE
./LC_CTYPE: 48 B (48 bytes) converted to sparse holes.
user:~/dev/util-linux$ /usr/sbin/filefrag -e ./LC_CTYPE
Filesystem type is: 9123683e
File size of ./LC_CTYPE is 200752 (50 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected:
flags:
0: 0.. 48: 22403653.. 22403701: 49:
1: 49.. 49: 22385666.. 22385666: 1: 22403702:
last,eof
./LC_CTYPE: 2 extents found
user:~/dev/util-linux$ du -s ./LC_CTYPE
200 ./LC_CTYPE
Behaviour with fixed fallocate: file becomes sparse
user:~/dev/util-linux$ cp /usr/lib/locale/C.UTF-8/LC_CTYPE .
user:~/dev/util-linux$ ./fallocate -v -d ./LC_CTYPE
./LC_CTYPE: 48 B (48 bytes) converted to sparse holes.
user:~/dev/util-linux$ /usr/sbin/filefrag -e ./LC_CTYPE
Filesystem type is: 9123683e
File size of ./LC_CTYPE is 200752 (50 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected:
flags:
0: 0.. 48: 22346286.. 22346334: 49:
last
./LC_CTYPE: 1 extent found
user:~/dev/util-linux$ du -s ./LC_CTYPE
196 ./LC_CTYPE
Please consider the attached patch. I also tested with other files
containing real holes (instead of a notch) that function is preserved.
Kind regards,
Gero
>From 7b3e0a3921ca7c84fa3913b2bb916be938dfe04a Mon Sep 17 00:00:00 2001
From: Gero Treuner <gero@xxxxxx>
Date: Thu, 10 Sep 2020 21:43:03 +0200
Subject: [PATCH] fallocate: fix --dig-holes at end of files
---
sys-utils/fallocate.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/sys-utils/fallocate.c b/sys-utils/fallocate.c
index 014b94624..ba97092fc 100644
--- a/sys-utils/fallocate.c
+++ b/sys-utils/fallocate.c
@@ -264,8 +264,11 @@ static void dig_holes(int fd, off_t file_off, off_t len)
off += rsz;
}
if (hole_sz) {
+ off_t alloc_sz = hole_sz;
+ if (off >= end)
+ alloc_sz += st.st_blksize; /* meet block boundary */
xfallocate(fd, FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE,
- hole_start, hole_sz);
+ hole_start, alloc_sz);
ct += hole_sz;
}
file_off = off;
--
2.20.1
