Hi, the virtual machine images files I create and use are mostly sparse, so that not too much space on a filesystem with snapshots and on filesystems that are receive targets is used. But I noticed that with just starting up and shutting down a virtual machine, the difference between 2 nightly snapshots as generated by send -p is huge. It turns out that the cause is the noholes flag of the filesystem. I have set this flag to have a bit less metadata hopefully, but it turns out to have a very negative side effect.
The sparse part of a file that is changed between 2 snapshots is added to the difference between the 2 snapshots when doing send -p, in full 'no-sparse' size (as zeros I think, as the data compresses extremely well). So in case of a 50G VM that has roughly 8G of actual written filesystem data (a new OS install), I saw 40G of generated btrfs sendstream data by just a start/stop of the VM. I think this behavior is not correct: Sparse is read as zeros normally AFAIK, so if the file locations that are sparse are the same for a snapshotted file and its parent, there should not be a difference. The following sequence shows the problem; if the btrfstune command is skipped, streamsize is as expected. uname -r btrfs --version truncate -s 5G /holetest.img losetup /dev/loop0 /holetest.img mkfs.btrfs /dev/loop0 btrfstune -n /dev/loop0 btrfs-show-super /dev/loop0 | grep incompat_flags mount /dev/loop0 /mnt btrfs sub create /mnt/vol1 truncate -s 4G /mnt/vol1/test1 btrfs sub snap -r /mnt/vol1 /mnt/vol1.ro0 dd if=/dev/urandom of=/mnt/vol1/test1 bs=1M count=1 conv=notrunc sync btrfs sub snap -r /mnt/vol1 /mnt/vol1.ro1 dd if=/dev/urandom of=/mnt/vol1/test1 bs=1M count=1 conv=notrunc sync btrfs sub snap -r /mnt/vol1 /mnt/vol1.ro2 btrfs send -p /mnt/vol1.ro1 /mnt/vol1.ro2 > /sendsize.btrfs ls -al /sendsize.btrfs umount /mnt losetup -d /dev/loop0 rm /holetest.img /sendsize.btrfs => output # uname -r 4.7.0-rc4-kade # btrfs --version btrfs-progs v4.6.1 # truncate -s 5G /holetest.img # losetup /dev/loop0 /holetest.img # mkfs.btrfs /dev/loop0 btrfs-progs v4.5.3+20160516 See http://btrfs.wiki.kernel.org for more information. Performing full device TRIM (5.00GiB) ... Label: (null) UUID: 338e948c-ee8c-4b9f-926e-b6f8fe140ae6 Node size: 16384 Sector size: 4096 Filesystem size: 5.00GiB Block group profiles: Data: single 8.00MiB Metadata: DUP 264.00MiB System: DUP 12.00MiB SSD detected: no Incompat features: extref, skinny-metadata Number of devices: 1 Devices: ID SIZE PATH 1 5.00GiB /dev/loop0 # # btrfstune -n /dev/loop0 # # btrfs-show-super /dev/loop0 | grep incompat_flags incompat_flags 0x341 # mount /dev/loop0 /mnt # btrfs sub create /mnt/vol1 Create subvolume '/mnt/vol1' # truncate -s 4G /mnt/vol1/test1 # btrfs sub snap -r /mnt/vol1 /mnt/vol1.ro0 Create a readonly snapshot of '/mnt/vol1' in '/mnt/vol1.ro0' # dd if=/dev/urandom of=/mnt/vol1/test1 bs=1M count=1 conv=notrunc 1+0 records in 1+0 records out 1048576 bytes (1,0 MB, 1,0 MiB) copied, 0,0779673 s, 13,4 MB/s # sync # btrfs sub snap -r /mnt/vol1 /mnt/vol1.ro1 Create a readonly snapshot of '/mnt/vol1' in '/mnt/vol1.ro1' # dd if=/dev/urandom of=/mnt/vol1/test1 bs=1M count=1 conv=notrunc 1+0 records in 1+0 records out 1048576 bytes (1,0 MB, 1,0 MiB) copied, 0,0634306 s, 16,5 MB/s # sync # btrfs sub snap -r /mnt/vol1 /mnt/vol1.ro2 Create a readonly snapshot of '/mnt/vol1' in '/mnt/vol1.ro2' # btrfs send -p /mnt/vol1.ro1 /mnt/vol1.ro2 > /sendsize.btrfs At subvol /mnt/vol1.ro2 # ls -al /sendsize.btrfs -rw-r--r-- 1 root root 4298025877 jun 25 00:26 /sendsize.btrfs -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
