Hello,
We've had a 5-node C* cluster (version 1.1.0) running for several months.
Up until now we've mostly been writing data, but now we're starting to
service more read traffic. We're seeing far more disk I/O to service these
reads than I would have anticipated.
The CF being queried consists of chat messages. Each row represents a
conversation between two people. Each column represents a message. The
column key is composite, consisting of the message date and a few other
bits of information. The CF is using compression.
The query is looking for a maximum of 50 messages between two dates, in
reverse order. Usually the two dates used as endpoints are 30 days ago and
the current time. The query in Astyanax looks like this:
ColumnList<ConversationTextMessageKey> result =
keyspace.prepareQuery(CF_CONVERSATION_TEXT_MESSAGE)
.setConsistencyLevel(ConsistencyLevel.CL_QUORUM)
.getKey(conversationKey)
.withColumnRange(
textMessageSerializer.makeEndpoint(endDate,
Equality.LESS_THAN).toBytes(),
textMessageSerializer.makeEndpoint(startDate,
Equality.GREATER_THAN_EQUALS).toBytes(),
true,
maxMessages)
.execute()
.getResult();
We're currently servicing around 30 of these queries per second.
Here's what the cfstats for the CF look like:
Column Family: conversation_text_message
SSTable count: 15
Space used (live): 211762982685
Space used (total): 211762982685
Number of Keys (estimate): 330118528
Memtable Columns Count: 68063
Memtable Data Size: 53093938
Memtable Switch Count: 9743
Read Count: 4313344
Read Latency: 118.831 ms.
Write Count: 817876950
Write Latency: 0.023 ms.
Pending Tasks: 0
Bloom Filter False Postives: 6055
Bloom Filter False Ratio: 0.00260
Bloom Filter Space Used: 686266048
Compacted row minimum size: 87
Compacted row maximum size: 14530764
Compacted row mean size: 1186
On the C* nodes, iostat output like this is typical, and can spike to be
much worse:
avg-cpu: %user %nice %system %iowait %steal %idle
1.91 0.00 2.08 30.66 0.50 64.84
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
xvdap1 0.13 0.00 1.07 0 16
xvdb 474.20 13524.53 25.33 202868 380
xvdc 469.87 13455.73 30.40 201836 456
md0 972.13 26980.27 55.73 404704 836
Any thoughts on what could be causing read I/O to the disk from these
queries?
Much thanks!
-Jon
