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923546d3c3
[MINOR PROBLEM]
When adding a new btrfs device, the corresponding DEV_STATS item creation
can only triggered by a mount cycle if there is no other error
triggered:
# mkfs.btrfs -f $dev1 $mnt
# mount $dev1 $mnt
# btrfs dev add $dev2 $mnt
# sync
# btrfs ins dump-tree -t dev $dev1
device tree key (DEV_TREE ROOT_ITEM 0)
leaf 30588928 items 6 free space 15853 generation 9 owner DEV_TREE
item 0 key (DEV_STATS PERSISTENT_ITEM 1) itemoff 16243 itemsize 40 <<<
persistent item objectid DEV_STATS offset 1
device stats
write_errs 0 read_errs 0 flush_errs 0 corruption_errs 0 generation 0
item 1 key (1 DEV_EXTENT 13631488) itemoff 16195 itemsize 48
Only after a mount cycle and a new transaction, the DEV_STATS for devid
2 can show up:
# umount $mnt
# mount $dev1 $mnt
# touch $mnt
# sync
# btrfs ins dump-tree -t dev $dev1
device tree key (DEV_TREE ROOT_ITEM 0)
leaf 30605312 items 7 free space 15788 generation 10 owner DEV_TREE
item 0 key (DEV_STATS PERSISTENT_ITEM 1) itemoff 16243 itemsize 40
persistent item objectid DEV_STATS offset 1
device stats
write_errs 0 read_errs 0 flush_errs 0 corruption_errs 0 generation 0
item 1 key (DEV_STATS PERSISTENT_ITEM 2) itemoff 16203 itemsize 40
persistent item objectid DEV_STATS offset 2
device stats
write_errs 0 read_errs 0 flush_errs 0 corruption_errs 0 generation 0
[CAUSE]
Btrfs only updates the DEV_STATS item when the device->dev_stats_ccnt
counter is not 0.
This is to reduce COW for the device tree. However that dev_stats_ccnt is
only increased at the following call sites:
- btrfs_dev_stat_inc()
This happens when some IO error happened.
- btrfs_dev_stat_read_and_reset()
This happens for GET_DEV_STATS ioctl with BTRFS_DEV_STATS_RESET flag.
- btrfs_dev_stat_set()
This happens inside btrfs_device_init_dev_stats().
So when a new device is added, its dev_stats_ccnt is just initialized to
0, and btrfs won't create nor update the corresponding DEV_STATS item at
all.
[ENHANCEMENT]
When a new device is added, also increase the dev_stats_ccnt by one.
This includes both device add ioctl and dev-replace.
This will force btrfs to create a new DEV_STATS item or update the
existing one with the correct values.
This not only makes the DEV_STATS creation early, but also prevents
old DEV_STATS left from older kernels to cause false alerts for the
newly added device.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
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