Hi group,
Restarting a ES cluster triggers recovery which is long-lasting and load
expensive. I am searching for a way to reduce the runtime and load of a
restart. I read someone executes daily rolling restarts of his large ES
cluster to ensure the primary and replica shards are 100% indentical,
meaning they will be fast recoverable. But that sounds like a hack and not
something you should happy with as SRE. And its impact on ES performance
may be acceptable on a large cluster, but not on our 3 node cluster.
How I believe shard recovery works: if ES spots differences between a
primary and its replica shard(s). It will rebuild the replica shard(s) as
an exact copy of the primary shard. Rebuiding results in lots of network
traffic and disk I/O.
We have a 3-node ES 1.0.1 cluster with 3k primary shards and 3k replica
shards. During a recent restart (to reduce heapsize to 31G to get
CompressedOops back) the recovery of the 1st node took the longest time (~6
hours). Recovery and the 2nd less (~2 hours) and the 3rd is quick (<1
hour). I believe recovery becomes faster after each node, because each
recovery ends with more replica shards as exact copies of their primary.
I tried force-merging with an expensive max_num_segments=1, but the metrics
segments.count + segments.memory of same shards still differ between pri +
rep. No luck. For the curious few I have included the before + after
results below.
Any ideas?
Regards,
Renzo
BEFORE:
idx shard prirep docs store segments.count
segments.memory
logstash-pro-oracle-2014.04.24 0 p 1072 485592 8
14615
logstash-pro-oracle-2014.04.24 0 r 1072 449022 1
11958
logstash-pro-oracle-2014.04.24 1 p 1095 493774 7
14336
logstash-pro-oracle-2014.04.24 1 r 1095 459966 1
11988
logstash-pro-oracle-2014.04.24 2 p 1039 452078 5
13158
logstash-pro-oracle-2014.04.24 2 r 1039 458513 6
13480
logstash-pro-oracle-2014.04.24 3 p 1094 492753 8
14574
logstash-pro-oracle-2014.04.24 3 r 1094 483347 6
13850
logstash-pro-oracle-2014.04.24 4 p 1099 494740 8
14645
logstash-pro-oracle-2014.04.24 4 r 1099 488953 7
14251
AFTER:
idx shard prirep docs store segments.count
segments.memory
logstash-pro-oracle-2014.04.24 0 p 1072 449358 1
11958
logstash-pro-oracle-2014.04.24 0 r 1072 448884 1
11958
logstash-pro-oracle-2014.04.24 1 p 1095 460391 1
11980
logstash-pro-oracle-2014.04.24 1 r 1095 459918 1
11988 <-- rep is 8 bigger than its pri
logstash-pro-oracle-2014.04.24 2 p 1039 431341 1
11580
logstash-pro-oracle-2014.04.24 2 r 1039 431695 1
11572 <-- rep is 8 smaller than its pri
logstash-pro-oracle-2014.04.24 3 p 1094 457135 1
11907
logstash-pro-oracle-2014.04.24 3 r 1094 457970 1
11907
logstash-pro-oracle-2014.04.24 4 p 1099 457640 1
11957
logstash-pro-oracle-2014.04.24 4 r 1099 457165 1
11957
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