When logging dir dentries of a directory, we iterate over the subvolume
tree to find dir index keys on leaves modified in the current transaction.
This however is heavy on locking, since btrfs_search_forward() may often
keep locks on extent buffers for quite a while when walking the tree to
find a suitable leaf modified in the current transaction and with a key
not smaller than then the provided minimum key. That means it will block
other tasks trying to access the subvolume tree, which may be common fs
operations like creating, renaming, linking, unlinking, reflinking files,
etc.
A better solution is to iterate the log tree, since it's much smaller than
a subvolume tree and just use plain btrfs_search_slot() (or the wrapper
btrfs_for_each_slot()) and only contains dir index keys added in the
current transaction.
The following bonnie++ test on a non-debug kernel (with Debian's default
kernel config) on a 20G null block device, was used to measure the impact:
$ cat test.sh
#!/bin/bash
DEV=/dev/nullb0
MNT=/mnt/nullb0
NR_DIRECTORIES=20
NR_FILES=20480 # must be a multiple of 1024
DATASET_SIZE=$(( (8 * 1024 * 1024 * 1024) / 1048576 )) # 8 GiB as megabytes
DIRECTORY_SIZE=$(( DATASET_SIZE / NR_FILES ))
NR_FILES=$(( NR_FILES / 1024 ))
umount $DEV &> /dev/null
mkfs.btrfs -f $DEV
mount $DEV $MNT
bonnie++ -u root -d $MNT \
-n $NR_FILES:$DIRECTORY_SIZE:$DIRECTORY_SIZE:$NR_DIRECTORIES \
-r 0 -s $DATASET_SIZE -b
umount $MNT
Before patchset:
Version 2.00a ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Name:Size etc /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
debian0 8G 376k 99 1.1g 98 939m 92 1527k 99 3.2g 99 9060 256
Latency 24920us 207us 680ms 5594us 171us 2891us
Version 2.00a ------Sequential Create------ --------Random Create--------
debian0 -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
20/20 20480 96 +++++ +++ 20480 95 20480 99 +++++ +++ 20480 97
Latency 8708us 137us 5128us 6743us 60us 19712us
After patchset:
Version 2.00a ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Name:Size etc /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
debian0 8G 384k 99 1.2g 99 971m 91 1533k 99 3.3g 99 9180 309
Latency 24930us 125us 661ms 5587us 46us 2020us
Version 2.00a ------Sequential Create------ --------Random Create--------
debian0 -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
20/20 20480 90 +++++ +++ 20480 99 20480 99 +++++ +++ 20480 97
Latency 7030us 61us 1246us 4942us 56us 16855us
The patchset consists of this patch plus a previous one that has the
following subject:
"btrfs: avoid iterating over all indexes when logging directory"
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
5d3e4f1d51