Articles: Storage

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Disk Response Time

For 10 minutes IOMeter is sending a stream of requests to read and write 512-byte data blocks with a request queue of 1. The total of requests processed by each SSD is much larger than its cache, so we get a sustained response time that doesn’t depend on the SSD’s buffer size.

Well, once again we have to admit in our response time tests that today’s SSDs are just perfect in this respect. Modern flash memory is highly responsive at reading whereas advanced controllers with effective algorithms, TRIM instruction and a pool of spare memory blocks have helped improve the response time at writing. The IBIS 240 GB even has a better response time at writing than at reading! Well, the numbers are really so small that we shouldn’t compare them directly considering the inaccuracies of our statistical measurement method.

Anyway, we want to note one thing that stands out in the diagram: the RevoDrive products with the reduced number of chips, including the RevoDrive x2, are somewhat slower than their full-capacity counterparts. The JBOD-based model is downright poor, by the way. We suspect the SiI3124 controller has something to do with its high read response time compared to the same SSD in its normal operation mode.

Random Read & Write Patterns

Now we will see how the performance of the SSDs in random read and write modes depends on the size of the requested data block.

We cannot expect very different results at random-address reading. SSDs usually all behave in a similar manner here, and they do so today, but we can note a couple of exceptions. The JBOD sample and the 50GB RevoDrive have the worst results. The reduction of memory channels by half (by disabling one of the two SF1200 chips in the former and by halving the amount of the latter’s memory chips) has a very negative effect of their performance. It looks like an SSD must have 2x8 or 2x6 memory access channels in order to show maximum performance. Adding even more channels is not that rewarding.

You can see a characteristic performance peak on 4KB data blocks, which is the size of a data page in SSDs, and data are actually written into the memory cells in such pages. We’ve got one obvious loser here: the cut-down sample with JBOD. If you are looking for maximum performance, the 240GB RevoDrive should please you. It even outperforms the same-capacity IBIS model with its four flash controllers.

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