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Database Patterns

In the Database pattern the HDD is processing a stream of requests to read and write 8KB random-address data blocks. The ratio of read to write requests is changing from 0% to 100% with a step of 10% throughout the test while the request queue depth varies from 1 to 256.

You can click this link to view the tabled results.

We will build diagrams for request queue depths of 1, 16 and 256.

The Seagate comes out the winner at the shortest queue depth, just as we might have expected, considering its response time results. The Hitachi uses deferred writing to overtake the leader in the right part of the diagram, though. The WD is not that good at deferred writing although has the same amount of cache (64 megabytes). It may take a different approach to using the cache, though, like creating fewer, but larger cache lines.

As the requests queue gets longer, the Seagate enjoys an even larger advantage at high percentages of reads. The WD also performs well at such loads, competing with the Hitachi. The latter goes ahead at high percentages of writes, though, leaving the WD far behind.

Winding up this part of our tests, we will build diagrams showing the performance of each HDD at five different request queue depths.

The Hitachi has rather aggressive firmware algorithms but enables its deferred writing only at high percentages of writes or at very long queue depths.

The Seagate Barracuda XT makes us recall server-optimized HDDs, also from Seagate themselves. They usually have such smoothly rising graphs. So, Seagate have come up with efficient server-optimized firmware.

The WD disk shows what we can call a special character. Other WD disks behave like that, too. Take note that the HDD slows down at certain percentages of writes when the request queue is long. It seems to allocate more cache memory for reordering read requests at the expense of deferred writing when the queue is long.

 
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