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From: Daniel Doubleday <daniel.doubleday@gmx.net>
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Subject: Monitoring bytes read per cf
Date: Thu, 12 May 2011 19:06:09 +0200
Message-Id: <F302C6BE-584A-496C-8B16-8635A1B5E718@gmx.net>
To: user@cassandra.apache.org
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Hi all

got a question for folks with some code insight again.

To be able to better understand where our IO load is coming from we want =
to monitor the number of bytes read from disc per cf. (we love stats)

What I have done is wrapping the FileDataInput in SSTableReader to sum =
the bytes read in CFS. This will only record data file access but that =
would be good enough for us.

It seems to work fine but maybe someone here knows that this is not a =
good idea ....

Cheers,
Daniel

Some code:

SSTableReader:
    private static final boolean KEEP_IO_STATISICS =3D =
Boolean.getBoolean("cassandra.keepIOStats");
    public FileDataInput getFileDataInput(DecoratedKey decoratedKey, int =
bufferSize)
    {
        long position =3D getPosition(decoratedKey, Operator.EQ);
        if (position < 0)
            return null;

        FileDataInput segment =3D dfile.getSegment(position, =
bufferSize);
        return (KEEP_IO_STATISICS) ? new =
MonitoringFileDataIInput(metadata, segment) : segment;=20
    }

with MonitoringFileDataIInput

public class MonitoringFileDataIInput implements FileDataInput, =
Closeable
{

    private final FileDataInput fileDataInput;
    private final ColumnFamilyStore columnFamilyStore;

    public MonitoringFileDataIInput(CFMetaData cfMetaData, FileDataInput =
fileDataInput)
    {
        columnFamilyStore =3D =
Table.open(cfMetaData.tableName).getColumnFamilyStore(cfMetaData.cfId);
        this.fileDataInput =3D fileDataInput;
    }

    @Override
    public boolean readBoolean() throws IOException
    {
        columnFamilyStore.addBytesRead(1);
        return fileDataInput.readBoolean();       =20
    }

    // ... etc

and ColumnFamilyStore
    private final AtomicLong bytesRead =3D new AtomicLong(0L);
   =20
    @Override // ColumnFamilyStoreMBean
    public long getBytesRead()
    {
        return bytesRead.get();
    }
   =20
    public void addBytesRead(int num)=20
    {
        bytesRead.addAndGet(num);
    }

 =20=