Bug 1270123

+++ This bug was initially created as a clone of Bug #1240577 +++

Description of problem:
=======================
When  a file is being continuously modified or written to, data base locks are being observed as below
[2015-07-07 12:46:00.025876] E [MSGID: 101106] [gfdb_sqlite3.c:694:gf_sqlite3_find_recently_chan
ged_files] 0-sqlite3: Failed preparing statment select GF_FILE_TB.GF_ID, (select group_concat( G
F_PID || ',' || FNAME || ',' || FPATH || ',' || W_DEL_FLAG ||',' || LINK_UPDATE , '::') from GF_
FLINK_TB where GF_FILE_TB.GF_ID = GF_FLINK_TB.GF_ID)  from GF_FILE_TB where ((GF_FILE_TB.W_SEC *
 1000000 + GF_FILE_TB.W_MSEC) >= ? ) OR ((GF_FILE_TB.W_READ_SEC * 1000000 + GF_FILE_TB.W_READ_MS
EC) >= ?) : database is locked



Due to this a file getting continuosly written is being demoted unncessarily


Version-Release number of selected component (if applicable):
==========================================================
[root@nchilaka-tier01 ~]# gluster --version
glusterfs 3.7.1 built on Jul  2 2015 21:01:51
Repository revision: git://git.gluster.com/glusterfs.git
Copyright (c) 2006-2011 Gluster Inc. <http://www.gluster.com>
GlusterFS comes with ABSOLUTELY NO WARRANTY.
You may redistribute copies of GlusterFS under the terms of the GNU General Public License.
[root@nchilaka-tier01 ~]# rpm -qa|grep gluster
gluster-nagios-common-0.2.0-1.el6rhs.noarch
vdsm-gluster-4.16.20-1.2.el6rhs.noarch
glusterfs-client-xlators-3.7.1-7.el6rhs.x86_64
glusterfs-server-3.7.1-7.el6rhs.x86_64
nfs-ganesha-gluster-2.2.0-3.el6rhs.x86_64
python-gluster-3.7.1-6.el6rhs.x86_64
glusterfs-3.7.1-7.el6rhs.x86_64
glusterfs-api-3.7.1-7.el6rhs.x86_64
glusterfs-cli-3.7.1-7.el6rhs.x86_64
glusterfs-geo-replication-3.7.1-7.el6rhs.x86_64
glusterfs-rdma-3.7.1-7.el6rhs.x86_64
gluster-nagios-addons-0.2.4-2.el6rhs.x86_64
glusterfs-libs-3.7.1-7.el6rhs.x86_64
glusterfs-fuse-3.7.1-7.el6rhs.x86_64
glusterfs-ganesha-3.7.1-7.el6rhs.x86_64






Steps to Reproduce:
==================
1.create a tiered volume
2.set the tier vol options 
3.now create a file and keep appending lines in a loop as below 
  `for in in {0..1000000};do echo "hello world" >>file1 ;done`
4. Check the tier.log and it can be seen that database lock messages would be thrown



sos report server rhsqe-repo.lab.eng.blr.redhat.com:/home/repo/sosreports/bug.1240569

--- Additional comment from Joseph Elwin Fernandes on 2015-07-07 05:58:29 EDT ---

The reason this happens is in WAL mode opening a new db connection is an expensive operation as it will try to a lock on the WAL file(even though for a short time). The migration process opens a new connection per brick per promotion/demotion cycle. This is bad scheme.

Solution:
1) Create a new connection only in tier_init per brick and have the connection alive and use it for every promotion/demotion 
2) Enable pooling (Pooling=True in the connection string) when the connection is established, there isn't any locking of WAL file, because existing connections are reused internal to sqlite.

http://dev.yorhel.nl/doc/sqlaccess

--- Additional comment from Vijay Bellur on 2015-09-19 12:09:28 EDT ---

REVIEW: http://review.gluster.org/12191 (tier/ctr: Solution for db locks for tier migrator and ctr using sqlite version less than 3.7 i.e rhel 6.7) posted (#2) for review on master by Joseph Fernandes

--- Additional comment from Vijay Bellur on 2015-09-19 14:04:56 EDT ---

REVIEW: http://review.gluster.org/12191 (tier/ctr: Solution for db locks for tier migrator and ctr using sqlite version less than 3.7 i.e rhel 6.7) posted (#3) for review on master by Joseph Fernandes