[HBASE-22301] Consider rolling the WAL if the HDFS write pipeline is slow

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 3.0.0-alpha-1, 1.5.0, 2.3.0
• Component/s: wal
• Hadoop Flags: Reviewed

Release Note

This change adds new conditions for rolling the WAL for when syncs on the HDFS writer pipeline are perceived to be slow.

As before the configuration parameter hbase.regionserver.wal.slowsync.ms sets the slow sync warning threshold.

If we encounter hbase.regionserver.wal.slowsync.roll.threshold number of slow syncs (default 100) within the interval defined by hbase.regionserver.wal.slowsync.roll.interval.ms (default 1 minute), we will request a WAL roll.

Or, if the time for any sync exceeds the threshold set by hbase.regionserver.wal.roll.on.sync.ms (default 10 seconds) we will request a WAL roll immediately.

Operators can monitor how often these new thresholds result in a WAL roll by looking at newly added metrics to the WAL related metric group:
* slowSyncRollRequest - How many times a roll was requested due to sync too slow on the write pipeline.

Additionally, as a part of this change there are also additional metrics for existing reasons for a WAL roll:
* errorRollRequest - How many times a roll was requested due to I/O or other errors.
* sizeRollRequest - How many times a roll was requested due to file size roll threshold.

• Assignee: Andrew Kyle Purtell
• Reporter: Andrew Kyle Purtell
• Created: 23/Apr/19 23:43
• Updated: 22/Aug/19 23:36
• Resolved: 02/May/19 01:30

Description

Consider the case when a subset of the HDFS fleet is unhealthy but suffering a gray failure not an outright outage. HDFS operations, notably syncs, are abnormally slow on pipelines which include this subset of hosts. If the regionserver's WAL is backed by an impacted pipeline, all WAL handlers can be consumed waiting for acks from the datanodes in the pipeline (recall that some of them are sick). Imagine a write heavy application distributing load uniformly over the cluster at a fairly high rate. With the WAL subsystem slowed by HDFS level issues, all handlers can be blocked waiting to append to the WAL. Once all handlers are blocked, the application will experience backpressure. All (HBase) clients eventually have too many outstanding writes and block.

Because the application is distributing writes near uniformly in the keyspace, the probability any given service endpoint will dispatch a request to an impacted regionserver, even a single regionserver, approaches 1.0. So the probability that all service endpoints will be affected approaches 1.0.

In order to break the logjam, we need to remove the slow datanodes. Although there is HDFS level monitoring, mechanisms, and procedures for this, we should also attempt to take mitigating action at the HBase layer as soon as we find ourselves in trouble. It would be enough to remove the affected datanodes from the writer pipelines. A super simple strategy that can be effective is described below:

This is with branch-1 code. I think branch-2's async WAL can mitigate but still can be susceptible. branch-2 sync WAL is susceptible. 

We already roll the WAL writer if the pipeline suffers the failure of a datanode and the replication factor on the pipeline is too low. We should also consider how much time it took for the write pipeline to complete a sync the last time we measured it, or the max over the interval from now to the last time we checked. If the sync time exceeds a configured threshold, roll the log writer then too. Fortunately we don't need to know which datanode is making the WAL write pipeline slow, only that syncs on the pipeline are too slow and exceeding a threshold. This is enough information to know when to roll it. Once we roll it, we will get three new randomly selected datanodes. On most clusters the probability the new pipeline includes the slow datanode will be low. (And if for some reason it does end up with a problematic datanode again, we roll again.)

This is not a silver bullet but this can be a reasonably effective mitigation.

Provide a metric for tracking when log roll is requested (and for what reason).

Emit a log line at log roll time that includes datanode pipeline details for further debugging and analysis, similar to the existing slow FSHLog sync log line.

If we roll too many times within a short interval of time this probably means there is a widespread problem with the fleet and so our mitigation is not helping and may be exacerbating those problems or operator difficulties. Ensure log roll requests triggered by this new feature happen infrequently enough to not cause difficulties under either normal or abnormal conditions. A very simple strategy that could work well under both normal and abnormal conditions is to define a fairly lengthy interval, default 5 minutes, and then insure we do not roll more than once during this interval for this reason.

Issue Links

• causes: HBASE-22902 At regionserver start there's a request to roll the WAL
• is related to: HBASE-21806 add an option to roll WAL on very slow syncs