On Tue, Oct 18, 2016 at 02:14:47PM +0200, Marcin Mirosław wrote:

W dniu 09.09.2016 o 11:00, Kent Overstreet pisze:

quoted

On Fri, Sep 09, 2016 at 09:52:56AM +0200, Marcin Mirosław wrote:

quoted

I'm using defaults from bcache format, knobs don't have description
aboutwneh I should change some options or when I should don't touch it.
On this, particular filesystem btree_node_size=128k according to sysfs.

Yeah, documentation needs work. Next time you format maybe try 256k, I'd like to
know if that helps.

Hi!

# bcache format --help
bcache format - create a new bcache filesystem on one or more devices
Usage: bcache format [OPTION]... <devices>

Options:
  -b, --block=size
      --btree_node=size       Btree node size, default 256k
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ it's not true

It is if your bucket size is big enough - btree node size can't be bigger than
bucket size.

# bcache format  /dev/mapper/system10-bcache
/dev/mapper/system10-bcache contains a bcache filesystem
Proceed anyway? (y,n) y
External UUID:                  1a064a62-fb61-42c8-8f0e-68961ad37d4c
Internal UUID:                  c2802bef-fbc4-414a-9fb0-e071943582c8
Label:
Version:                        6
Block_size:                     512
Btree node size:                128.0K
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


I see another problem, I observed it due to long mount time.
I'm creating many dirs:
# for x in {0..31}; do eatmydata \
mkdir -p /mnt/test/a/${x}/{0..255}/{0..255}; done

# find /mnt/test|wc -l
2105378

df -h shows:
/dev/mapper/system10-bcache          9,8G  421M  9,4G   5% /mnt/test

next I removing all those dirs. Umount, mount:
[ 6172.131784] bcache (dm-12): starting mark and sweep:
[ 6189.113714] bcache (dm-12): mark and sweep done
[ 6189.113979] bcache (dm-12): starting journal replay:
[ 6189.114201] bcache (dm-12): journal replay done, 129 keys in 88
entries, seq 28579
[ 6189.114214] bcache (dm-12): journal replay done
[ 6189.114214] bcache (dm-12): starting fs gc:
[ 6189.118244] bcache (dm-12): fs gc done
[ 6189.118246] bcache (dm-12): starting fsck:
[ 6189.119220] bcache (dm-12): fsck done

So mount time is still long, even with empty fileystem.
df shows:
/dev/mapper/system10-bcache  9,8G  421M  9,4G   5% /mnt/test

# find /mnt/test|wc -l
1

It looks that creating and removing dirs doesn't clean some internal
structures.

The issue is that right now btree node coalescing is only run as a batch pass
when mark and sweep GC runs (it has nothing to do with GC, it just runs at the
same time in the current code). At some point we need to come up with a good way
of triggering it as needed.

Try triggering a gc, and then check mount time:

echo 1 > /sys/fs/bcache/<uuid>/internal/trigger_gc