Description of problem:
bitrot signature process takes long time.
EC volume and replicated volume 

Version-Release number of selected component (if applicable):
3.8.3

How reproducible:
always

Steps to Reproduce:
EC volume with 4 + 1
1. write a 100MB file from fuse mount point
2. once write completes, check file stat and xattr in any of the brick

  File: ‘test54-bs10M-c10.nul’
  Size: 26214400        Blocks: 51200      IO Block: 4096   regular file
Device: 821h/2081d      Inode: 536874169   Links: 2
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2016-09-20 12:23:42.902886624 +0530 *
Modify: 2016-09-20 12:23:44.378886622 +0530
Change: 2016-09-20 12:23:44.378886622 +0530 *
 Birth: -


# file: test54-bs10M-c10.nul
trusted.bit-rot.version=0x020000000000000057daa7b50002e5b4
trusted.ec.config=0x0000080501000200
trusted.ec.size=0x0000000006400000
trusted.ec.version=0x00000000000003200000000000000320
trusted.gfid=0x54e018dd8c5a4bd79e0317729d8a57c5


3. after 1 hour, check file stat and xattr

  File: ‘test54-bs10M-c10.nul’
  Size: 26214400        Blocks: 51200      IO Block: 4096   regular file
Device: 821h/2081d      Inode: 536874169   Links: 2
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2016-09-20 12:25:47.510886425 +0530 *
Modify: 2016-09-20 12:23:44.378886622 +0530
Change: 2016-09-20 12:38:05.954885243 +0530 *
 Birth: -


# file: test54-bs10M-c10.nul
trusted.bit-rot.signature=0x010200000000000000394c345f0b0c63ee652627a62eed069244d35c4d5134e4f07d4eabb51afda47e
trusted.bit-rot.version=0x020000000000000057daa7b50002e5b4
trusted.ec.config=0x0000080501000200
trusted.ec.size=0x0000000006400000
trusted.ec.version=0x00000000000003200000000000000320
trusted.gfid=0x54e018dd8c5a4bd79e0317729d8a57c5

Actual results:
bitrot signature process takes more than 12 minutes to calculate hash and update xattr

Expected results:
normal read time for the file using md5sum

time md5sum test54-bs10M-c10.nul
bed3c0a4a1407f584989b4009e9ce33f  test54-bs10M-c10.nul

real    0m0.166s

As soon as file fd closes and completion of wait time of 120 sec, updating xattr with bitrot signature should happen within seconds for this file size.

Additional info:

even though file was not used by any process, file fd shows in /proc/brickpid/fd as soon as file fd disappears from /proc/brickpid/fd bitrot signature updates in file xattr immediately.