A migration is the Master choosing to move a Tablet from TabletServerX
to TabletServerY.
Balancing is the process of moving a Tablet, again from one TabletServer
to another, per the configured LoadBalancer implementation to meet some
Tablet distribution.
In this case, there is a migration for one Tablet. If the Balancer were
to run now, the current state of Tablets it would operate on would be
invalidate because the current distribution of Tablets doesn't match the
desired distribution.
A good example would be a system with 10 tablets on one tabletserver. A
second tabletserver is started, but has no tablets yet. The balancer
would not want to rebalance [10, 0], so it would wait until the
migrations have been processed.
As long as balancing is happening (you don't constantly see such a
warning message), you can safely ignore it. Sometimes a server will take
a little time to process the migration due to load. However, we have had
bugs in the past where the Master gets stuck thinking there are
outstanding migrations and never runs the balancer. This causes lots of
problems after some time. If you are running into this, just restart the
master when you see this happen and consider updating to a newer version
of Accumulo (I'm not aware of any current bugs with this issue).
- Josh
mohit.kaushik wrote:
I am getting the following error usually in my master log file..
2015-09-17 10:22:55,798 [balancer.DefaultLoadBalancer] WARN : Not
balancing due to 1 outstanding migrations.
2015-09-17 10:22:55,798 [balancer.DefaultLoadBalancer] WARN : Not
balancing due to 1 outstanding migrations.
2015-09-17 10:22:55,798 [balancer.DefaultLoadBalancer] WARN : Not
balancing due to 1 outstanding migrations.
2015-09-17 10:23:00,823 [master.EventCoordinator] INFO : Migrating 1
more tablets, 1 total
2015-09-17 10:23:01,034 [master.EventCoordinator] INFO : [Normal
Tablets]: 1 tablets unloaded
2015-09-17 10:23:01,071 [master.EventCoordinator] INFO : tablet
7n;EsBihar_1234|modi_1234< was unloaded from orkash1:9997
2015-09-17 10:23:01,272 [master.Master] INFO : Assigning 1 tablets
2015-09-17 10:23:01,352 [master.EventCoordinator] INFO : [Normal
Tablets]: 1 tablets are UNASSIGNED
2015-09-17 10:23:01,352 [master.EventCoordinator] INFO : [Normal
Tablets]: 474 tablets are HOSTED
2015-09-17 10:23:01,381 [master.EventCoordinator] INFO : tablet
7n;EsBihar_1234|modi_1234< was loaded on orkash2:9997
2015-09-17 10:23:01,606 [master.EventCoordinator] INFO : [Normal
Tablets]: 475 tablets are HOSTED
Can anybody explain, what are the outstanding migrations in Accumulo and
how do they effect balancing?
Thanks
Mohit Kaushik//
