Disk strategies

With magnetic disks, there are two strategies to get good OLTP or mail server performance. The "traditional way" is to combine a number of 15000RPM SAS "spindles", all working in parallel. The more "rebellious way" or "Google way" is to use a vast number of cheaper SATA drives. This last strategy is based on the observation that although SATA drives come with higher access times, you can buy more SATA spindles than SAS spindles for the same price. While Google opted for desktop drives, we worked with what we had in the lab: 16 enterprise 1TB Western Digital drives. Since these are one of the fastest 7200RPM drives that can be found on the market, it should give you a good idea what an array with lots of SATA drives can do compared to one with fewer fast spinning SAS drives.

SSDs add a new strategy: if space is not your primary problem, you can trade in storage space for huge amounts of random I/O operations per second, requiring fewer but far more expensive drives to obtain the same performance. SSDs offer superb read access times but slightly less impressive write access times.

As Anand has pointed out, a cheap SSD controller can really wreak havoc on writing performance, especially in a server environment where many requests are issued in parallel. EMC solved this with their high-end Enterprise Flash Disks, produced by STEC, which can store up to 400GB and come with a controller with excellent SRAM caches and a super capacitor. The super capacitor enables the controller to empty the relatively large DRAM caches and write the date to the flash storage in the event of a sudden power failure.

Intel went for the midrange market, and gave its controller less cache (16MB). The controller is still intelligent and powerful enough to crush the competition with the cheap JMicron JMF602-controllers. We check out the SLC version, the Intel X25-E SLC 32GB.

The newest Intel Solid State Disks with their access times of 0.075 ms and 0.15W power consumption could change the storage market for OLTP databases. However, the SLC drives have a few disadvantages compared to the best SAS drives out there:

• No dual ports
  
• The price per GB is 13 times higher

You can see the summary in the table below.

If you really need capacity, SATA or even SAS drives are probably the best choice. On the other hand, if you need spindles to get more I/O per second, it will be interesting to see how a number of SAS or SATA drives compares to the SLC drives. The most striking advantages of the Intel X25-E SLC drive are extremely low random access times, almost no power consumption at idle, low power consumption at full load, and high reliability.

(*) 5.5 ms = 3.5 ms seek time + 2 ms latency (rotation)
(**) 13 ms = 8.9 ms seek time + 4.1 ms latency (rotation)

Reliability testing is outside the scope of this document, but if only half of the Intel claims are true, the x25-E SLC drives will outlive the vast majority of magnetic disks. First is the 2 million MTBF specification, which is far better than the best SAS disks on the market (1.6 million hour MTBF). Intel also guarantees that if the X25-E performs 7000 8KB random access per second, consisting of 66% reads and 33% writes, the drive will continue to do so for 5 years! That is 2.9TB of written data per day, and it can sustain this for about 1800 days. That is simply breathtaking as no drive has to sustain that kind of IOPS 24 hours per day for such a long period.

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