The biggest advantage of Solid State Drives is lack of moving parts as compared to traditional hard drives. This allows the drives to survive longer, and have faster read and write times. It makes them ideal for an enterprise environment where performance and reliability are expected. The SSD drives use less than a third of power compared to SAS or SATA, and promise twice the life expectancy. Power consumption alone should save you money in the long run.
Let us review one particular server that was upgraded from SAS to SSD drives. Although SSD drives are more expensive, the cost can be offset by higher productivity, stability, and faster load times. The best uses for SSD drives are for applications that require a lot of reads and writes to the disk, such as a MySQL database, as well as a low latency. You will also benefit by having SSD for disk-based caching. An SSD drive for NFS caching with Nginx can be used for static content delivery, significantly improving load times on your server.
Having a slow hard drive increases CPU load and can lock up the server during high traffic:
Even the network speed picked up because the LAMP stack was able to serve more requests:
What makes a server a great candidate for SSD upgrade is its current Inputs/Outputs Per Second rate (IOPS). For most SATA drives the upper limit is 150 IOPS, and for SAS it is 200 IOPS. If your server is constantly going above 200 IOPS you should consider upgrading to SSD drives.
Lets review another server with 2x 73GB 2.5” SAS drives in RAID1 :
Reviewing the Disk I/O graphs :
Since current IOPS rates are 167 reads/second and 1035 writes/second, the overall IOPS rate far exceeds the SAS drive capacity.
We can further troubleshoot the cause of high IOPS by using iotop :
In this particular case, solving the issue of why the Qmgr was writing so much to disk was the right course of action. It was due to deferred messages being logged by Postfix.
After purging Postfix queue, all of the deferred messaged stopped, which solved the high IO issue:
The IOPS rate was down significantly:
With iotop confirming that the issue has been resolved:
The biggest deterrent to getting SSD drives is price. Currently a new Seagate Savvio 10K.3 300GB SAS drive costs approximately $190, and Seagate Cheetah 15K.7 ST3450857SS 450GB drive costs approximately $220. While consumer grade SSD drives like Crucial m4 CT256M4SSD2 256GB cost $200, and Crucial m4 CT512M4SSD1 512GB cost $400. We would stick to comparing enterprise grade drives, since there is a hidden bonus for going enterprise – power consumption.
Enterprise grade SSD drives like Intel 520 480GB SSDSC2CW480A310 cost $500, and Samsung 840 MZ-7PD512BW 512GB drives cost $600.
This places the initial costs at $0.48/GB – $0.63/GB for SAS drives, $0.78/GB for consumer grade SSD drives, and $1.04/GB – $1.17/GB for enterprise SSD drives.
Before we dive into mathematics of how SSD drives save you money, here is a map of average prices for kWh of electricity (in cents):
The higher the price of electricity, the quicker will SSD drives break even as compared to SAS drives.
Comparing SAS to SSD drive in terms of power consumption, we have 4.8 kWh/day with SAS, and 0.06 kWh/day for SSD drive. This in turn comes out to 1752 kWh/year with SAS, and 21.9 kWh/year with SSD drive. You would be saving $207/year with SSD drives on power consumption alone.
(For reference: http://www.storageperformance.org/spc-1ce_results/Seagate/e00002_Seagate_Savvio-10K3/e00002_Seagate_Savvio-10K3_SPC1CE-executive-summary.pdf and http://www.storagereview.com/ocz_vertex_4_ssd_review Where Annual energy use in kWh = Nominal Power * 24 * 0.365. The annual cost is calculated at average of $0.12/kWh.)
So when would an SSD drive reach a break-even point? If you purchased 2x 300GB 10,000 RPM drives for your server, you would be paying $380 for drives, and $420/year for electricity. If you purchased 2x 450GB 15,000 RPM SAS drives you would be paying $440 for drives, and over $420/year for electricity.
Meanwhile, with 2x 480GB Intel 520 you would pay $1000 for drives. If you went with 2x 512GB Samsung 840s, you would pay $1200 for drives, and as as for power consumption, there is a bonus. Intel 520 has 0.85W power consumption rate, would use 7.446 kWh/year, and cost $0.89352 per year. Samsung 840 Pro has 0.068W power consumption rate, would use 0.5956 kWh/year and cost $0.0713 per year. For dual drives, this is still below $2/year in electrical charges.
Given all of these data points, even with dual Samsung 840 Pro SSD drives at $1200, as compared to dual SAS drives at $440, you are faced with a difference of $760 in initial cost for 2x 500GB drives. This $760 price saving is quickly eroded away by $420/year electrical bill, which after less than 2 years starts to cost you more money if you went with SAS drives. In the long run, SSD drives save you money, improve efficiency, and last longer.