From: Nir Soffer <nsof...@redhat.com> When using file system that does not support fallocate(), posix_fallocate() fallback to emulation mode. In this mode, when preallocating blocks before file end, posix_preallocate is calling one pread() and one pwrite() per block. But when preallocation blocks after file end, it calls only one pwrite per block.
Truncating the file only when preallocation=OFF speeds up creating raw file in this situation. Here are example run with without and with this change, tested on Fedora 25 VM, creating a raw image on NFS version 3 mount over 1G nic: $ time ./qemu-img create -f raw -o preallocation=falloc mnt/test 1g Formatting 'mnt/test', fmt=raw size=1073741824 preallocation=falloc real 0m17.083s user 0m0.020s sys 0m0.404s $ rm mnt/test $ time ./qemu-img create -f raw -o preallocation=falloc mnt/test 1g Formatting 'mnt/test', fmt=raw size=1073741824 preallocation=falloc real 0m12.372s user 0m0.020s sys 0m0.376s $ strace ./qemu-img-up 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 Signed-off-by: Nir Soffer <nir...@gmail.com> --- 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 28b47d9..d7f6129 100644 --- a/block/file-posix.c +++ b/block/file-posix.c @@ -1588,12 +1588,6 @@ 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: @@ -1633,6 +1627,10 @@ 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; @@ -1641,7 +1639,6 @@ 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