saintstack commented on a change in pull request #2665:
URL: https://github.com/apache/hbase/pull/2665#discussion_r524806902
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File path: src/main/asciidoc/_chapters/architecture.adoc
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@@ -2865,26 +2865,51 @@ The first mechanism is store file refresher which is
introduced in HBase-1.0+. S
For turning this feature on, you should configure
`hbase.regionserver.storefile.refresh.period` to a non-zero value. See
Configuration section below.
-==== Asnyc WAL replication
-The second mechanism for propagation of writes to secondaries is done via
“Async WAL Replication” feature and is only available in HBase-1.1+. This works
similarly to HBase’s multi-datacenter replication, but instead the data from a
region is replicated to the secondary regions. Each secondary replica always
receives and observes the writes in the same order that the primary region
committed them. In some sense, this design can be thought of as “in-cluster
replication”, where instead of replicating to a different datacenter, the data
goes to secondary regions to keep secondary region’s in-memory state up to
date. The data files are shared between the primary region and the other
replicas, so that there is no extra storage overhead. However, the secondary
regions will have recent non-flushed data in their memstores, which increases
the memory overhead. The primary region writes flush, compaction, and bulk load
events to its WAL as well, which are also replicated through w
al replication to secondaries. When they observe the flush/compaction or bulk
load event, the secondary regions replay the event to pick up the new files and
drop the old ones.
+[[async.wal.replication]]
+==== Async WAL replication
+The second mechanism for propagation of writes to secondaries is done via the
“Async WAL Replication” feature. It is only available in HBase-1.1+. This works
similarly to HBase’s multi-datacenter replication, but instead the data from a
region is replicated to the secondary regions. Each secondary replica always
receives and observes the writes in the same order that the primary region
committed them. In some sense, this design can be thought of as “in-cluster
replication”, where instead of replicating to a different datacenter, the data
goes to secondary regions to keep secondary region’s in-memory state up to
date. The data files are shared between the primary region and the other
replicas, so that there is no extra storage overhead. However, the secondary
regions will have recent non-flushed data in their memstores, which increases
the memory overhead. The primary region writes flush, compaction, and bulk load
events to its WAL as well, which are also replicated throu
gh wal replication to secondaries. When they observe the flush/compaction or
bulk load event, the secondary regions replay the event to pick up the new
files and drop the old ones.
Committing writes in the same order as in primary ensures that the secondaries
won’t diverge from the primary regions data, but since the log replication is
asynchronous, the data might still be stale in secondary regions. Since this
feature works as a replication endpoint, the performance and latency
characteristics is expected to be similar to inter-cluster replication.
Async WAL Replication is *disabled* by default. You can enable this feature by
setting `hbase.region.replica.replication.enabled` to `true`.
-Asyn WAL Replication feature will add a new replication peer named
`region_replica_replication` as a replication peer when you create a table with
region replication > 1 for the first time. Once enabled, if you want to disable
this feature, you need to do two actions:
+The Async WAL Replication feature will add a new replication peer named
`region_replica_replication` as a replication peer when you createa table with
region replication > 1 for the first time. Once enabled, if you want to disable
this feature, you need to do two actions:
* Set configuration property `hbase.region.replica.replication.enabled` to
false in `hbase-site.xml` (see Configuration section below)
* Disable the replication peer named `region_replica_replication` in the
cluster using hbase shell or `Admin` class:
[source,bourne]
----
hbase> disable_peer 'region_replica_replication'
----
+Async WAL Replication and the `hbase:meta` table is a little more involved and
gets its own section below; see <<async.wal.replication.meta>>
+
=== Store File TTL
In both of the write propagation approaches mentioned above, store files of
the primary will be opened in secondaries independent of the primary region. So
for files that the primary compacted away, the secondaries might still be
referring to these files for reading. Both features are using HFileLinks to
refer to files, but there is no protection (yet) for guaranteeing that the file
will not be deleted prematurely. Thus, as a guard, you should set the
configuration property `hbase.master.hfilecleaner.ttl` to a larger value, such
as 1 hour to guarantee that you will not receive IOExceptions for requests
going to replicas.
+[[async.wal.replication.meta]]
=== Region replication for META table’s region
-Currently, Async WAL Replication is not done for the META table’s WAL. The
meta table’s secondary replicas still refreshes themselves from the persistent
store files. Hence the `hbase.regionserver.meta.storefile.refresh.period` needs
to be set to a certain non-zero value for refreshing the meta store files. Note
that this configuration is configured differently than
-`hbase.regionserver.storefile.refresh.period`.
+Up until hbase-2.4.0, Async WAL Replication did not work for the META table’s
WAL. The meta table’s secondary replicas refreshed themselves from the
persistent store files every `hbase.regionserver.meta.storefile.refresh.period`,
+(a non-zero value). Note how the META replication period is distinct from the
user-space `hbase.regionserver.storefile.refresh.period` value.
+
+Async WAL replication for META is a new feature in 2.4.0 still under active
development. Use with caution.
+Set `hbase.region.replica.replication.catalog.enabled` to enable async WAL
Replication for META region replicas.
+Its off by default.
+
+Regards the META replicas count, up to hbase-2.4.0, you would set the special
property 'hbase.meta.replica.count'.
+Now you can alter the META table as you would a user-space table (if
`hbase.meta.replica.count` is set, it
+will take precedent over what is set for replica count in the META table
updating META replica count to
+match).
+
+==== Load Balancing META table load ====
+
+hbase-2.4.0 adds a new client-side `LoadBalance` mode. When enabled
client-side, clients will try to read META replicas first before falling back
on the primary. Before this,
+the lookup mode -- now named `HedgedRead` -- had clients read the primary and
if no response after a configurable amount of time had elapsed, it would start
up reads against the replicas.
+The new 'LoadBalance' mode helps alleviate hotspotting on the META table
distributing the META read load.
Review comment:
Oh. Yes. Let me fix.
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