Review: Enterprise solid state drives
Solid state drives (SSDs) have become ubiquitous in portable devices such as tablets and small laptops due to their shock resistance, low power usage and speed, but have not penetrated deeply in the enterprise space due to their cost, which runs as much as 70x the cost of hard drives, and a tendency in early SSDs to have limited numbers of write cycles, resulting in drives that could wear out in a few years. However, the wear issue has been largely addressed, and SSD performance is so much higher than even high-performance hard drives that it can have a major impact in applications that are limited by storage performance, such as databases. As prices continue to drop and performance continues to increase, administrators with applications that are limited by storage bottlenecks should investigate SSDs.
There are issues with SSDs, however. The major limiting factor currently is in write performance with long cycles of intensive writes. The issue is that each block or cell in the drive must be written as a whole – if you want to change one bit in a block or cell, you must store the contents of the block in memory, erase the block and write the entire block over again. This is not an issue during a short series of writes, but once the whole disk has been filled, performance can decline dramatically, or response times can vary by orders of magnitude during extended write cycles. Manufacturers have addressed this by modifying controllers to improve garbage collection and over-subscribing (putting more capacity in the drive than advertised). Both of these modifications add more cost to the drives, so enterprise drives can cost quite a bit more than standard consumer models.
One way to leverage SSD speed is to use the drives as cache between the server and storage, either utilizing a controller like the Adaptec MaxIQ or with SAN systems such as the Compellent Storage Center that use SSDs as a tier 0 and automatically move the most-used files to the SSD storage.
Another issue is using SSDs in RAIDs. First, SSDs are more reliable than hard drives, so the need for redundancy is much less. Second, since SSDs still have limited numbers of writes, and tend to wear out at around the same times, the potential for an entire set of drives in a RAID set to wear out at near the same time could be disastrous. Finally, the performance of SSDs is so fast that the RAID controller can be a limiting factor – in this test, using two SSDs in a RAID 0 volume with a RAID controller improved by only a small margin, not nearly the doubling of performance that would be typical with standard hard drives.
To test the overall performance of enterprise-class SSDs, I tested five drives, the Intel SSDSA2SH064G1GC 2.5” 64GB, LSI WarpDrive SLP-300 (SSS-6200) PCI-E 300GB, OWC Mercury Extreme Pro RE OWCSSDMXRE400 2.5” 400GB, Plextor PX-128M2S-02 2.5” 128GB and Pliant Lightning LS 300S 3.5” 300GB. The prices for these drives range from about $250 for the Plextor to about $8,400 for the LSI WarpDrive. In general, the prices reflect the performance of the drives, both in overall transfer rates in megabytes per second (MBps), input/output operations per second (IOps), and in the ability to sustain writes for extended periods without performance drops or increases in average transaction times. The table below shows IOps and MBps for each drive. The long and short test cycles show the impact of extended write cycles on each drive. Of note are the IOps and MBps for the LSI WarpDrive, which is a PCI-Express card rather than a SATA/SAS drive as the rest are. This gives it a much higher speed connection, which results in very high performance – about four times the next fastest drive. You can also see that write performance declines during an extended write cycle, however, which was not the case with the OWC and Pliant drives. With all the drives you can see that the average and maximum response times and average and maximum transaction times increase substantially during the same extended write cycles. This is an indication that the drives are struggling to complete writes quickly enough to maintain performance.
Note that the manufacturer’s specifications for these drives will show much higher throughputs and IOps, but the numbers in the table reflect a real-world test using a mix of reads and writes for the short test, and a write-intensive test over several hours in the long test, designed to stress the SSDs, rather than tests optimized to show the maximum throughput possible.
Intel X25 SSDSA2SH064G1GC
2.5” 64GB 3GB/sec $650
The Intel X25 series is generally intended for consumer applications rather than enterprise applications. It represents a high-performance consumer drive, which costs a good deal more than the Plextor drive, but delivers a higher level of performance as well.<?p>
While it is not optimized for write-intensive applications, which can be seen in the large increases between average and maximum response times, it could still improve performance substantially in read-intensive applications, for instance relatively static databases.
LSI WarpDrive SLP-300 (SSS-6200)
PCI-Express 8x 300GB $8,400
The LSI WarpDrive SLP-300 (SSS-6200) is very different from the other drives in this test. Rather than a SAS/SATA connection, it plugs into an 8x PCI-Express slot to use the higher-speed PCI bus rather than the relatively limited SATA or SAS connection. There are two issues with using the PCI-E interface – first, the number of slots in many servers is limited to one to four, which limits the total capacity, since each drive requires one slot. Also getting a server to boot from the WarpDrive is supposed to be possible, but I was unable to get this to work with either Windows 2008R2 or Windows 7.
The WarpDrive was able to achieve about four times the IOps of the next-fastest drive, and half-again the throughput of any other drive. The average and maximum response times were also much lower than the other drives, which can make a big difference in database applications. While the WarpDrive’s performance did drop during the write-intensive test, its performance was still better than any other drive by a large margin. With a cost of $8,400 for 300GB, the drive is expensive, but its speed and throughput make it an excellent choice for eliminating bottlenecks in database applications.
OWC Mercury Extreme Pro RE OWCSSDMXRE400
2.5” 400GB 3GB/sec $1,499
The OWC Mercury Extreme Pro series of drives offers very high performance for the price, with performance about 70% of the Pliant drive at less than a third of the cost. The OWCs were so fast that they overran the RAID controller I used to test with, showing little performance improvement in a two-drive RAID 0 configuration over a single drive. The OWCs also showed little degradation in performance during write-intensive operations.
2.5” 128GB 6GB/sec $250
The Plextor PX-128M2S-02 is very inexpensive for an SSD, at $250 for a 128GB, 6GB/sec drive. While its performance looks low in comparison to the other drives in this test, it still outperforms a standard hard drive by a substantial margin, at a relatively low price. It is capable of excellent performance in read-intensive applications, beating a standard hard drive by a factor of 10x in a read-intensive test. At a very low price per GB, the PX-128M2S-02 is ideal for testing SSDs in read-intensive environments.
Pliant Lightning LS 300S
3.5” 300GB 3GB/sec $6,899
Of the drives connected by SATA or SAS, the Pliant was the clear performance winner, with very high throughput and very low response times, ideal for database applications. It not only showed no performance degradation during the write-intensive test, it actually performed better in that test than in the standard mix test. While the drives are relatively expensive, they offer a degree of expandability not possible with the LSI WarpDrive, which requires an 8x PCI-Express slot for each drive, and the Pliant can also be used to boot a system which I was not able to get the LSI WarpDrive to do. The Pliant Lightning is an excellent choice for databases that are performance-limited by storage performance, and are much cheaper than the other usual alternative, increasing server memory by enough to keep indices in memory.
300GB SATA 7200 $100
300GB SAS 15k $185
SSD 256GB low $700
SSD 300GB high $7000
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