Am 03.02.2017 um 20:50 hat Nir Soffer geschrieben: > 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 <nir...@gmail.com>
Thanks, applied to the block branch. I'm not completely sure if doing an ftruncate() first couldn't improve PREALLOC_MODE_FULL somewhat in some cases, but I agree that the patch should still result in correct images. KEvin
