[PATCH 0/2] Policy to balance read across mirrored devices

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

In case of RAID1 and RAID10 devices are mirror-ed, a read IO can
pick any device for reading. This choice of picking a device for
reading should be configurable. In short not one policy would
satisfy all types of workload and configs.

So before we add more policies, this patch-set makes existing
$pid policy configurable from the mount option and adds a devid
based read_mirror device policy. And keeps $pid based policy as
the default option for now. So this mount option helps to try out
different read mirror load balances.

Further we can add more policies on top of it, for example..

  mount -o read_mirror_policy=pid (current, default) [1]
  mount -o read_mirror_policy=<devid> [2]
  mount -o read_mirror_policy=lba (illustration only) [3]
  mount -o read_mirror_policy=ssd (illustration only) [4]
  mount -o read_mirror_policy=io (illustration only) [5]
  mount -o read_mirror_policy=<heuristic1> (illustration only) [6]

[1]
 Current PID based read mirror balance.

[2]
 Set the devid of the device which should be used for read. That
 means all the normal read will go to that particular device only.
 This also helps testing and gives a better control for the test
 scripts including mount context reads.

[3]
 In case of SAN storage, some storage prefers that host access the
 LUN based on the LBA so that there won't be duplications of
 caching on the storage.

[4]
 In case of mix of SSD and HD we may want to use SSD as the primary
 read device.

[5]
 If storage caching is not the bottleneck but the transport layer
 is then read load should be tuned based on the IO load.

[6]
 Or a combination of any of above as per the priority.
 Timofey should consider to base his patch on top of this.
    Btrfs: enchanse raid1/10 balance heuristic

This patch set is on top of the preparatory patch set:
  [PATCH 0/2] Preparatory to add read_mirror mount option


Anand Jain (2):
  btrfs: add mount option read_mirror_policy
  btrfs: add read_mirror_policy parameter devid

 fs/btrfs/ctree.h   |  2 ++
 fs/btrfs/super.c   | 31 +++++++++++++++++++++++++++++++
 fs/btrfs/volumes.c | 18 +++++++++++++++++-
 fs/btrfs/volumes.h |  7 +++++++
 4 files changed, 57 insertions(+), 1 deletion(-)

-- 
2.7.0