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defined service level.
Based on our example in Table 6.15 , we could set an SLA for Gold at 800 IOPS per
TB, Silver at 200 IOPS per TB, and Wood at 20 IOPS per TB. We have rounded down
to take a conservative approach and ensure the SLA can always be met.
Table 6.15 IOPS per TB Based on Example 30% Read Workload at 5000 IOPS
Tip
It is possible to achieve higher IOPS per disk by using only a small portion (say,
25%) of the disk's total capacity. This is known as short stroking or partial
stroking a disk. This is because when you use the first part of a spinning disk, the
rotational latency is a lot lower, as the outside of the disk platters are spinning
faster than the inside, and you cover more sectors in less time. See
http://searchsolidstatestorage.techtarget.com/definition/Short-Stroking .
The Economics of RAID Performance
You have seen how performance requirements can drive storage design, and how many
spinning disks are required when using different RAID levels to meet performance
requirements. In our example, we used a SQL Server requiring 5,000 IOPS and 2.5TB
capacity. Now we will look at the economics of different RAID choices and using solid
state disks (SSDs) or enterprise flash disks (EFDs) instead of spinning disks.
From Table 6.15 , in order to meet a 30% read 5,000 IOPS requirement and a 2.5TB
capacity requirement, the Gold storage policy is the most cost effective. It would use
half the number of disks to deliver the performance required, and more than covers the
capacity requirements. It would be half the cost of the Silver storage policy for this
workload. Now let's take a look at how this might change if EFDs were used instead of
spinning disks.
Table 6.16 shows the effective read and write IOPS after accounting for RAID penalties
associated with using EFD disks with an assumed 5,000 IOPS per disk.
 
 
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