[PATCH] btrfs: Enhance btrfs chunk allocation algorithm to reduce ENOSPC caused by unbalanced data/mixed

When btrfs allocate a chunk, it will try to alloc up to 1G for data and
256M for metadata, or 10% of all the writeable space if there is enough
space for the stripe on device.

However, when we run out of space, this allocation may cause unbalanced
chunk allocation.
For example, there are only 1G unallocated space, and request for
allocate DATA chunk is sent, and all the space will be allocated as data
chunk, making later metadata chunk alloc request unable to handle, which
will cause ENOSPC.
This is the one of the common complains from end users about why ENOSPC
happens but there is still available space.

This patch will try not to alloc chunk which is more than half of the
unallocated space, making the last space more balanced at a small cost
of more fragmented chunk at the last 1G.

Some easy example:
Preallocate 17.5G on a 20G empty btrfs fs:
[Before]
 # btrfs fi show /mnt/test
Label: none  uuid: da8741b1-5d47-4245-9e94-bfccea34e91e
	Total devices 1 FS bytes used 17.50GiB
	devid    1 size 20.00GiB used 20.00GiB path /dev/sdb
All space is allocated. No space later metadata space.

[After]
 # btrfs fi show /mnt/test
Label: none  uuid: e6935aeb-a232-4140-84f9-80aab1f23d56
	Total devices 1 FS bytes used 17.50GiB
	devid    1 size 20.00GiB used 19.77GiB path /dev/sdb
About 230M is still available for later metadata allocation.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
---
 fs/btrfs/volumes.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)