[sheepdog] [sheepdog/sheepdog] 796c88: sheep: update ledger objects in an asynchronous ma...

[Hitoshi Mitake]

  Branch: refs/heads/stable-0.9
  Home:   https://github.com/sheepdog/sheepdog
  Commit: 796c88df19e32a6a83a6ba54f2b17b03e166b3d2
      https://github.com/sheepdog/sheepdog/commit/796c88df19e32a6a83a6ba54f2b17b03e166b3d2
  Author: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
  Date:   2015-11-19 (Thu, 19 Nov 2015)

  Changed paths:
    M sheep/gateway.c

  Log Message:
  -----------
  sheep: update ledger objects in an asynchronous manner

This patch let gateway update ledger objects in an asynchronous
manner. This change improves performance of COW and
dereferencing. Especially vdi deletion will be improved.

Of course this change introduces a chance of inconsistency of
metadata, but but sheepdog is a distributed file system without
metadata journaling. Therefore inconsistency of metadata introduced by
machine crash or network error, etc must be solved by methods like
fsck (dog vdi check, etc). For this purpose, the next patch enhances
VID GC for collecting garbage objects which belong to removed VDIs.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>