SSD versus Enterprise SAS and SATA disksby Johan De Gelas on March 20, 2009 2:00 AM EST
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- IT Computing
For our performance testing we used a 3.3GHz (120W TDP) Core 2 X5470; we admit to being a bit paranoid and we wanted the CPU to have plenty of processing power in reserve. In the case of purely storage related tasks, the CPU never achieved more than 15% CPU load with software RAID. Only SysBench was capable of pushing it up to 80%, but if we want to measure the power consumption of our SC-836TQ storage enclosure the SysBench value is unrealistic. In most cases, the server will run the database and perform the transactions. The storage enclosure attached to the server will perform only the I/O processing. Therefore we measure the power consumption of our storage enclosure using IOMeter, and we use a more sensible (80W) 2.5GHz Core 2 E5420 CPU. High performance enclosures (such as those of EMC) also use Xeons to perform the I/O processing.
The SC-836TQ uses one Ablecom PWS-902-1R 900W 75A power supply, one Xeon E5420 "Harpertown", 4x2GB 667MHz FB-DIMM, and one Adaptec 5085 RAID controller. "Full Load" means that the storage enclosure is performing the IOMeter Random Read/Write tests. The difference between sequential reads and random writes is only a few watts (with both SSD and SAS).
|Drive Power Consumption|
|8 x SSD X25-E||257||275||6||24||0.75||3|
|4 x SSD X25-E||254||269||3||18||0.75||4.5|
|8 x SAS (Seagate)||383||404||132||153||16.5||19.125|
|4 x SAS (Seagate)||316||328||65||77||16.25||19.25|
|No disks at all
(One system disk)
While the Intel SLC X25-E consumes almost nothing in idle (0.06W), the reality is that the drive is attached to a RAID controller. That RAID controller consumes a little bit of energy to keep the connection to the idle drive alive. Still, the fact that eight SLC drives need 129W less power than eight SAS drives while offering 3 to 13 times better OLTP performance is a small revolution in storage land.
Let us do a small thought experiment. Assume that you have a 100GB database that is performance limited. Our SysBench benchmark showed that eight SLC X25-E drives perform at least three times (up to 13 times) better than ten 15000RPM SAS drives. You need at least 30 SAS drives to achieve the same performance as the SSDs. We'll ignore the fact that you would probably need another enclosure for the 30 drives and simply look at the costs associated with an eight SLC SSD setup versus a 30 drive 15000RPM SAS setup.
We base our KWh price on the US Department of Energy numbers which states that on average 1 KWh costs a little more than 10 cents; the real price is probably a bit higher, but that's close enough. It is important to note that we add 50% more power to account for the costs of air conditioning for removing the heat that the disks generate. We assume that the drives are working eight hours under full load and 16 under light load.
|Power per drive||1.5||17.375||16 hours idle, 8 hours full load|
|KWh per drive (3 years)||38.88||450.36||360 days, 24 hours|
|Number of drives||8||30||Based on SysBench performance measurements|
|Total KWh for disks||311.04||13510.8|
|Cooling (50%)||155.52||6755.4||to remove heat from array|
|Total KWh in datacenter||466.56||20266.2||disks power + cooling|
|Price per KW||$0.10||$0.10|
|Total Power costs (3 years)||$46.656||$2026.62|
|TCA||$6400||$6000||Eight 64GB SLC drives at $800
Thirty 15000RPM SAS drives at $200
If you use six drives for the RAID 10 data LUN (two drives for the logs), you need the 64GB SLC drives. That is why we use those in this calculation. Note that our calculation is somewhat biased in favor of the SAS drives: the SLC drives probably run at idle much more than the SAS drives, and it is very likely that even 30 SAS drives won't be able to keep with our eight SSDs. Even with the bias, the conclusion is crystal clear: if you are not space limited but you are performance limited, SSDs are definitely a better deal and will save you quite a bit of money as they lower the TCO.