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From Kanwar Sangha <kan...@mavenir.com>
Subject RE: High disk I/O during reads
Date Fri, 22 Mar 2013 14:38:17 GMT
Are your Keys spread across all SSTables ? That will cause every sstable read which will increase
the I/O.

What compaction are you using ?

From: zodiak@fifth-aeon.net [mailto:zodiak@fifth-aeon.net] On Behalf Of Jon Scarborough
Sent: 21 March 2013 23:00
To: user@cassandra.apache.org
Subject: High disk I/O during reads

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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