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RAID Results

RAID results are referring to standard disk based systems whether they are standard RAID, SATA or other interfaces that map into the RAID framework. The major result from a TPC-C result is throughput, measured as transactions per second. RAID test throughput will be examined first.

RAID Throughput

As memory was decreased from 1 gigabyte first to 500 megabytes and then to 250 megabytes, the RAID system responded as expected with less overall throughput as more and more disk I/O was performed. This is shown in Figure 4.4.

This shows that as the disk plays a more active roll in the transaction base, the latency involved in disk operations causes a net reduction in throughput. Another measure is bytes per second, and that will be presented next.

RAID Bytes per Second

When bytes per second (BPS) for RAID are examined, the same decreasing profile can be seen. This decrease in BPS as memory is reduced is of course expected as the number of transactions decreases. This is shown in Figure 4.5.

This shows that as the RAID environment becomes more and more dependent on the disk I/O subsystem, the transaction rate and rate of data flow decrease dramatically on a per user basis. Essentially, for a given user load that exceeds the breakpoint this breaks down to a simple equation:

TPSf = TPSi * (RBPT/IOR)*Q

Where:

  • TPSf – Transactions per second final

  • TPSi – Transactions per second Initial (Peak with 100% caching)

  • RBPT – Required bytes per transaction per second

  • IOR – Sustained Bytes IO rate in bytes per second

  • Q – A constant that takes into account other latency factors which should be a constant for a given configuration.

The breakpoint is the point of inflection for a given TPS curve.

So for a RAID based system the transactions will never be more than what can be achieved in a fully cached situation assuming that the interconnect latency is much less than the overall disk latency.

RAID Average Transaction Time

The performance behavior is inverted when average transaction time is considered. As memory in a RAID based system is decreased, average transaction time increases. As more processes contend for limited disk I/O capability, the average transaction time must increase resulting in fewer transactions per second and lower bytes per second. This is shown in Figure 4.6.

RAID Average Response Time

The final results for the RAID tests show the average response time. As would be expected, as the memory decreases, forcing more dependence on disk, the response time increases, just as with average transaction times. This is shown in Figure 4.7.

As expected, all performance indicators showed poorer performance in a disk-based system as available cache memory decreased. This decrease in performance is caused directly by increasing disk contention thereby forcing high latency times and queuing to occur on the underlying disks.

The same tests were performed using solid-state disk technology and those results will be examined next.


The above book excerpt is from:

Oracle RAC & Tuning with Solid State Disk

Expert Secrets for High Performance Clustered Grid Computing

ISBN 0-9761573-5-7

Donald K. Burleson & Mike Ault

http://www.rampant-books.com/book_2005_2_rac_ssd_tuning.htm

  
 

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