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authorNir Soffer <nirsof@gmail.com>2017-02-17 02:51:27 +0200
committerKevin Wolf <kwolf@redhat.com>2017-02-24 16:09:23 +0100
commitc6ccc2c5e6a58a847f51075abfef20e212c1a3e5 (patch)
treede31e895999cfacd8fee6aba614fb28af93e3330 /block
parent5a1dad9d5a5777c4883de78d9f35bd8141175798 (diff)
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qemu-img: Improve documentation for PREALLOC_MODE_FALLOC
Now that we are truncating the file in both PREALLOC_MODE_FULL and PREALLOC_MODE_OFF, not truncating in PREALLOC_MODE_FALLOC looks odd. Add a comment explaining why we do not truncate in this case. Signed-off-by: Nir Soffer <nirsof@gmail.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Diffstat (limited to 'block')
-rw-r--r--block/file-posix.c7
1 files changed, 6 insertions, 1 deletions
diff --git a/block/file-posix.c b/block/file-posix.c
index d24e34b..4de1abd 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1594,9 +1594,14 @@ static int raw_create(const char *filename, QemuOpts *opts, Error **errp)
switch (prealloc) {
#ifdef CONFIG_POSIX_FALLOCATE
case PREALLOC_MODE_FALLOC:
- /* posix_fallocate() doesn't set errno. */
+ /*
+ * Truncating before posix_fallocate() makes it about twice slower on
+ * file systems that do not support fallocate(), trying to check if a
+ * block is allocated before allocating it, so don't do that here.
+ */
result = -posix_fallocate(fd, 0, total_size);
if (result != 0) {
+ /* posix_fallocate() doesn't set errno. */
error_setg_errno(errp, -result,
"Could not preallocate data for the new file");
}