On 2019/10/22 下午5:47, Tobias Reinhard wrote: > Hi, > > > I noticed that if you punch a hole in the middle of a file the available > filesystem space seems not to increase. > > Kernel is 5.2.11 > > To reproduce: > > ->mkfs.btrfs /dev/loop1 -f > > btrfs-progs v4.15.1 > See http://btrfs.wiki.kernel.org for more information. > > Detected a SSD, turning off metadata duplication. Mkfs with -m dup if > you want to force metadata duplication. > Label: (null) > UUID: 415e925a-588a-4b8f-bdc7-c30a4a0f5587 > Node size: 16384 > Sector size: 4096 > Filesystem size: 1.00GiB > Block group profiles: > Data: single 8.00MiB > Metadata: single 8.00MiB > System: single 4.00MiB > SSD detected: yes > Incompat features: extref, skinny-metadata > Number of devices: 1 > Devices: > ID SIZE PATH > 1 1.00GiB /dev/loop1 > > ->mount /dev/loop1 /srv/btrtest2 > > ->for i in $(seq 1 20); do dd if=/dev/urandom of=test$i bs=16M count=4 ; > sync ; fallocate -p -o 4096 -l 67100672 test$i && sync ; done > > this failed from the 16th file on because of no space left
Btrfs doesn't free the space until all space of a data extent get freed. In your case, your hole punch is [4k, 64M-4K), thus the 64M extent still has 4K being used. So the data extent won't be freed until you free the last 4K. > > ->df -T . > Filesystem Type 1K-blocks Used Available Use% Mounted on > /dev/loop1 btrfs 1048576 935856 2272 100% /srv/btrtest2 > > ->btrfs fi du . > Total Exclusive Set shared Filename > 8.00KiB 8.00KiB - ./test1 > 8.00KiB 8.00KiB - ./test2 > 8.00KiB 8.00KiB - ./test3 > 8.00KiB 8.00KiB - ./test4 > 8.00KiB 8.00KiB - ./test5 > 8.00KiB 8.00KiB - ./test6 > 8.00KiB 8.00KiB - ./test7 > 8.00KiB 8.00KiB - ./test8 > 8.00KiB 8.00KiB - ./test9 > 8.00KiB 8.00KiB - ./test10 > 8.00KiB 8.00KiB - ./test11 > 8.00KiB 8.00KiB - ./test12 > 8.00KiB 8.00KiB - ./test13 > 8.00KiB 8.00KiB - ./test14 > 8.00KiB 8.00KiB - ./test15 > 4.00KiB 4.00KiB - ./test16 > 4.00KiB 4.00KiB - ./test17 > 4.00KiB 4.00KiB - ./test18 > 4.00KiB 4.00KiB - ./test19 > 4.00KiB 4.00KiB - ./test20 > 140.00KiB 140.00KiB 0.00B . > > When doing this on XFS or EXT4 it works as expected: > > Filesystem Type 1K-blocks Used Available Use% Mounted on > /dev/loop1 ext4 999320 2764 927744 1% /srv/btrtest > /dev/loop2 xfs 1038336 40456 997880 4% /srv/xfstest > > How to i reclaim the space on BTRFS? Defrag does not seem to help. Rewrite the remaining 4K. Then the new write 4K will be cowed into a new 4K extent, the old large 64M extent gets fully freed and free space. Thanks, Qu > > Best regards > > > Tobias >
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