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From Jon Scarborough <...@fifth-aeon.net>
Subject High disk I/O during reads
Date Fri, 22 Mar 2013 03:59:43 GMT
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

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