From 3a20013fbb26d2a1bd11ef148eefdb1508783787 Mon Sep 17 00:00:00 2001 From: Nir Soffer Date: Tue, 27 Aug 2019 04:05:27 +0300 Subject: block: posix: Always allocate the first block When creating an image with preallocation "off" or "falloc", the first block of the image is typically not allocated. When using Gluster storage backed by XFS filesystem, reading this block using direct I/O succeeds regardless of request length, fooling alignment detection. In this case we fallback to a safe value (4096) instead of the optimal value (512), which may lead to unneeded data copying when aligning requests. Allocating the first block avoids the fallback. Since we allocate the first block even with preallocation=off, we no longer create images with zero disk size: $ ./qemu-img create -f raw test.raw 1g Formatting 'test.raw', fmt=raw size=1073741824 $ ls -lhs test.raw 4.0K -rw-r--r--. 1 nsoffer nsoffer 1.0G Aug 16 23:48 test.raw And converting the image requires additional cluster: $ ./qemu-img measure -f raw -O qcow2 test.raw required size: 458752 fully allocated size: 1074135040 When using format like vmdk with multiple files per image, we allocate one block per file: $ ./qemu-img create -f vmdk -o subformat=twoGbMaxExtentFlat test.vmdk 4g Formatting 'test.vmdk', fmt=vmdk size=4294967296 compat6=off hwversion=undefined subformat=twoGbMaxExtentFlat $ ls -lhs test*.vmdk 4.0K -rw-r--r--. 1 nsoffer nsoffer 2.0G Aug 27 03:23 test-f001.vmdk 4.0K -rw-r--r--. 1 nsoffer nsoffer 2.0G Aug 27 03:23 test-f002.vmdk 4.0K -rw-r--r--. 1 nsoffer nsoffer 353 Aug 27 03:23 test.vmdk I did quick performance test for copying disks with qemu-img convert to new raw target image to Gluster storage with sector size of 512 bytes: for i in $(seq 10); do rm -f dst.raw sleep 10 time ./qemu-img convert -f raw -O raw -t none -T none src.raw dst.raw done Here is a table comparing the total time spent: Type Before(s) After(s) Diff(%) --------------------------------------- real 530.028 469.123 -11.4 user 17.204 10.768 -37.4 sys 17.881 7.011 -60.7 We can see very clear improvement in CPU usage. Signed-off-by: Nir Soffer Message-id: 20190827010528.8818-2-nsoffer@redhat.com Reviewed-by: Max Reitz Signed-off-by: Max Reitz --- tests/qemu-iotests/175.out | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) (limited to 'tests/qemu-iotests/175.out') diff --git a/tests/qemu-iotests/175.out b/tests/qemu-iotests/175.out index 6d9a5ed..263e521 100644 --- a/tests/qemu-iotests/175.out +++ b/tests/qemu-iotests/175.out @@ -2,17 +2,17 @@ QA output created by 175 == creating image with default preallocation == Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 -size=1048576, nothing allocated +size=1048576, min allocation == creating image with preallocation off == Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 preallocation=off -size=1048576, nothing allocated +size=1048576, min allocation == creating image with preallocation full == Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 preallocation=full -size=1048576, everything allocated +size=1048576, max allocation == creating image with preallocation falloc == Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 preallocation=falloc -size=1048576, everything allocated +size=1048576, max allocation *** done -- cgit v1.1