[HBASE-6389] Modify the conditions to ensure that Master waits for sufficient number of Region Servers before starting region assignments

• Type: Bug
• Status: Closed
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: 0.94.0, 0.95.2
• Fix Version/s: 0.94.3, 0.95.0
• Component/s: master
• Hadoop Flags: Incompatible change, Reviewed

Release Note

Reverts the cluster startup behavior to pre 0.94.0.

With this, Master will wait until "hbase.master.wait.on.regionservers.mintostart" number of Region Servers have registered with it before it starts region assignment. The default value of this setting is 1.

In large clusters with thousands of regions you may want to increase this to a higher number which is sufficient to handle the task of opening those region in parallel.

If left to the default, at times, the Master could assign all regions to a single Region Server which will result in slow startup and in worst case could OOM the Region Server (some time resulting in META inconsistency).

Here is how it works now (from the javadoc):

We wait until one of these condition is met:
 - the master is stopped
 - the 'hbase.master.wait.on.regionservers.maxtostart' number of region servers is reached
 - the 'hbase.master.wait.on.regionservers.mintostart' is reached AND there have been no new region server in for 'hbase.master.wait.on.regionservers.interval' time AND the 'hbase.master.wait.on.regionservers.timeout' is reached

• Assignee: Aditya Kishore
• Reporter: Aditya Kishore
• Created: 13/Jul/12 01:05
• Updated: 26/Feb/13 08:20
• Resolved: 03/Nov/12 22:56

Description

Continuing from HBASE-6375.

It seems I was mistaken in my assumption that changing the value of "hbase.master.wait.on.regionservers.mintostart" to a sufficient number (from default of 1) can help prevent assignment of all regions to one (or a small number of) region server(s).

While this was the case in 0.90.x and 0.92.x, the behavior has changed in 0.94.0 onwards to address HBASE-4993.

From 0.94.0 onwards, Master will proceed immediately after the timeout has lapsed, even if "hbase.master.wait.on.regionservers.mintostart" has not reached.

Reading the current conditions of waitForRegionServers() clarifies it

ServerManager.java (trunk rev:1360470)

....
581	  /**
582	   * Wait for the region servers to report in.
583	   * We will wait until one of this condition is met:
584	   *  - the master is stopped
585	   *  - the 'hbase.master.wait.on.regionservers.timeout' is reached
586	   *  - the 'hbase.master.wait.on.regionservers.maxtostart' number of
587	   *    region servers is reached
588	   *  - the 'hbase.master.wait.on.regionservers.mintostart' is reached AND
589	   *   there have been no new region server in for
590	   *      'hbase.master.wait.on.regionservers.interval' time
591	   *
592	   * @throws InterruptedException
593	   */
594	  public void waitForRegionServers(MonitoredTask status)
595	  throws InterruptedException {
....
....
612	    while (
613	      !this.master.isStopped() &&
614	        slept < timeout &&
615	        count < maxToStart &&
616	        (lastCountChange+interval > now || count < minToStart)
617	      ){
....

So with the current conditions, the wait will end as soon as timeout is reached even lesser number of RS have checked-in with the Master and the master will proceed with the region assignment among these RSes alone.

As mentioned in HBASE-4993, and I concur, this could have disastrous effect in large cluster especially now that MSLAB is turned on.

To enforce the required quorum as specified by "hbase.master.wait.on.regionservers.mintostart" irrespective of timeout, these conditions need to be modified as following

ServerManager.java

..
  /**
   * Wait for the region servers to report in.
   * We will wait until one of this condition is met:
   *  - the master is stopped
   *  - the 'hbase.master.wait.on.regionservers.maxtostart' number of
   *    region servers is reached
   *  - the 'hbase.master.wait.on.regionservers.mintostart' is reached AND
   *   there have been no new region server in for
   *      'hbase.master.wait.on.regionservers.interval' time AND
   *   the 'hbase.master.wait.on.regionservers.timeout' is reached
   *
   * @throws InterruptedException
   */
  public void waitForRegionServers(MonitoredTask status)
..
..
    int minToStart = this.master.getConfiguration().
    getInt("hbase.master.wait.on.regionservers.mintostart", 1);
    int maxToStart = this.master.getConfiguration().
    getInt("hbase.master.wait.on.regionservers.maxtostart", Integer.MAX_VALUE);
    if (maxToStart < minToStart) {
      maxToStart = minToStart;
    }
..
..
    while (
      !this.master.isStopped() &&
        count < maxToStart &&
        (lastCountChange+interval > now || timeout > slept || count < minToStart)
      ){
..