[Qemu-devel] [PATCH] qemu-img: Do not truncate before preallocation

[Nir Soffer <nirsof@gmail.com>]

When using file system that does not support fallocate() (e.g. NFS <
4.2), truncating the file only when preallocation=OFF speeds up creating
raw file.

Here is example run, tested on Fedora 24 machine, creating raw file on
NFS version 3 server.

$ time ./qemu-img-master create -f raw -o preallocation=falloc mnt/test 1g
Formatting 'mnt/test', fmt=raw size=1073741824 preallocation=falloc

real	0m21.185s
user	0m0.022s
sys	0m0.574s

$ time ./qemu-img-fix create -f raw -o preallocation=falloc mnt/test 1g
Formatting 'mnt/test', fmt=raw size=1073741824 preallocation=falloc

real	0m11.601s
user	0m0.016s
sys	0m0.525s

$ time dd if=/dev/zero of=mnt/test bs=1M count=1024 oflag=direct
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 15.6627 s, 68.6 MB/s

real	0m16.104s
user	0m0.009s
sys	0m0.220s

Running with strace we can see that without this change we do one
pread() and one pwrite() for each block. With this change, we do only
one pwrite() per block.

$ strace ./qemu-img-master create -f raw -o preallocation=falloc mnt/test 8192
...
pread64(9, "\0", 1, 4095)               = 1
pwrite64(9, "\0", 1, 4095)              = 1
pread64(9, "\0", 1, 8191)               = 1
pwrite64(9, "\0", 1, 8191)              = 1

$ strace ./qemu-img-fix create -f raw -o preallocation=falloc mnt/test 8192
...
pwrite64(9, "\0", 1, 4095)              = 1
pwrite64(9, "\0", 1, 8191)              = 1

This happens because posix_fallocate is checking if each block is
allocated before writing a byte to the block, and when truncating the
file before preallocation, all blocks are unallocated.

Signed-off-by: Nir Soffer <nirsof@gmail.com>
---

I sent this a week ago:
http://lists.nongnu.org/archive/html/qemu-devel/2017-01/msg06123.html

Sending again with improved commit message.

 block/file-posix.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 2134e0e..442f080 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1591,12 +1591,6 @@ static int raw_create(const char *filename, QemuOpts *opts, Error **errp)
 #endif
     }
 
-    if (ftruncate(fd, total_size) != 0) {
-        result = -errno;
-        error_setg_errno(errp, -result, "Could not resize file");
-        goto out_close;
-    }
-
     switch (prealloc) {
 #ifdef CONFIG_POSIX_FALLOCATE
     case PREALLOC_MODE_FALLOC:
@@ -1636,6 +1630,10 @@ static int raw_create(const char *filename, QemuOpts *opts, Error **errp)
         break;
     }
     case PREALLOC_MODE_OFF:
+        if (ftruncate(fd, total_size) != 0) {
+            result = -errno;
+            error_setg_errno(errp, -result, "Could not resize file");
+        }
         break;
     default:
         result = -EINVAL;
@@ -1644,7 +1642,6 @@ static int raw_create(const char *filename, QemuOpts *opts, Error **errp)
         break;
     }
 
-out_close:
     if (qemu_close(fd) != 0 && result == 0) {
         result = -errno;
         error_setg_errno(errp, -result, "Could not close the new file");
-- 
2.9.3