[PATCH RFC 00/10] btrfs: new performance-based chunk allocation using device roles

[Anand Jain <anand.jain@oracle.com>]

In host hardware, devices can have different speeds. Generally, faster
devices come with lesser capacity while slower devices come with larger
capacity. A typical configuration would expect that:

 - A filesystem's read/write performance is evenly distributed on average
 across the entire filesystem. This is not achievable with the current
 allocation method because chunks are allocated based only on device free
 space.

 - Typically, faster devices are assigned to metadata chunk allocations
 while slower devices are assigned to data chunk allocations.

Introducing Device Roles:

 Here I define 5 device roles in a specific order for metadata and in the
 reverse order for data: metadata_only, metadata, none, data, data_only.
 One or more devices may have the same role.

 The metadata and data roles indicate preference but not exclusivity for
 that role, whereas data_only and metadata_only are exclusive roles.

Introducing Role-then-Space allocation method:

 Metadata allocation can happen on devices with the roles metadata_only,
 metadata, none, and data in that order. If multiple devices share a role,
 they are arranged based on device free space.

 Similarly, data allocation can happen on devices with the roles data_only,
 data, none, and metadata in that order. If multiple devices share a role,
 they are arranged based on device free space.

Finding device speed automatically:

 Measuring device read/write latency for the allocaiton is not good idea,
 as the historical readings and may be misleading, as they could include
 iostat data from periods with issues that have since been fixed. Testing
 to determine relative latency and arranging in ascending order for metadata
 and descending for data is possible, but is better handled by an external
 tool that can still set device roles.

On-Disk Format changes:

 The following items are defined but are unused on-disk format:

	btrfs_dev_item::
	 __le64 type; // unused
	 __le64 start_offset; // unused
	 __le32 dev_group; // unused
	 __u8 seek_speed; // unused
	 __u8 bandwidth; // unused

 The device roles is using the dev_item::type 8-bit field to store each
 device's role.

Anand Jain (10):
  btrfs: fix thresh scope in should_alloc_chunk()
  btrfs: refactor should_alloc_chunk() arg type
  btrfs: introduce btrfs_split_sysfs_arg() for argument parsing
  btrfs: introduce device allocation method
  btrfs: sysfs: show device allocation method
  btrfs: skip device sorting when only one device is present
  btrfs: refactor chunk allocation device handling to use list_head
  btrfs: introduce explicit device roles for block groups
  btrfs: introduce ROLE_THEN_SPACE device allocation method
  btrfs: pass device roles through device add ioctl

 fs/btrfs/block-group.c |  11 +-
 fs/btrfs/ioctl.c       |  12 +-
 fs/btrfs/sysfs.c       | 130 ++++++++++++++++++++--
 fs/btrfs/volumes.c     | 242 +++++++++++++++++++++++++++++++++--------
 fs/btrfs/volumes.h     |  35 +++++-
 5 files changed, 366 insertions(+), 64 deletions(-)

-- 
2.49.0