[PATCH RFC 0/5] readmirror feature

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

RFC patch as of now, appreciate your comments. This patch set has
been tested.

Function call chain  __btrfs_map_block()->find_live_mirror() uses
thread pid to determine the %mirror_num when the mirror_num=0.
    
The pid based mirror_num extrapolation has following disadvantages
 . A large single-process read IO will read only from one disk.
 . In a worst scenario all processes read accessing the FS could have
       either odd or even pid, the read IO gets skewed.
 . There is no deterministic way of knowing/controlling which copy will
       be used for reading.
 . May see performance variations for a given set of multi process
       workload ran at different times.
    
So we need other types of readmirror policies.
    
This patch introduces a framework so that we can add more policies, and
converts the existing %pid into as a configurable parameter using the
property. And also provides a transient readmirror mount option, so that
this property can be applied for the read io during mount and for
readonly FS.

 For example:
   btrfs property set <mnt> readmirror pid
   btrfs property set <mnt> readmirror ""
   btrfs property set <mnt> readmirror devid<n>

   mount -o readmirror=pid <mnt>
   mount -o readmirror=devid<n> <mnt>

Please note:
 The property storage is an extented attributes of root inode
 (BTRFS_FS_TREE_OBJECTID), the other choice is a new ondisk KEY,
 which is open for comments.

 This patch uses the unused btrfs_device::type, and does not use the
 bitwise ops because as type is u64 and bitwise ops need u32, so we
 need 32bits of the btrfs_device::type. Which is a kind of messy.
 Its open for suggestion for any better way.

Anand Jain (5):
  btrfs: add inode pointer to prop_handler::validate()
  btrfs: add readmirror pid property
  btrfs: add readmirror devid property
  btrfs: add readmirror pid mount option
  btrfs: add readmirror devid mount option

 fs/btrfs/props.c   | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++---
 fs/btrfs/super.c   | 29 ++++++++++++++++
 fs/btrfs/volumes.c | 29 +++++++++++++++-
 fs/btrfs/volumes.h | 11 ++++++
 4 files changed, 163 insertions(+), 5 deletions(-)

Anand Jain (2):
  btrfs-progs: add helper to create xattr name
  btrfs-progs: add readmirror policy

 props.c | 75 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 68 insertions(+), 7 deletions(-)
-- 
1.8.3.1