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qemu-img: Improve documentation for PREALLOC_MODE_FALLOC

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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>
This commit is contained in:
Nir Soffer 2017-02-17 02:51:27 +02:00 committed by Kevin Wolf
parent 5a1dad9d5a
commit c6ccc2c5e6
1 changed files with 6 additions and 1 deletions
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7
block/file-posix.c
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@ -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");
}