capable of providing a fixed number of I/O. Disk manufacturers calculate this based on the rotational speed, average latency, and seek time. Table 2.1 shows examples of typical physical drive IOPS specifications for the most common drive types.
Table 2.1 Typical average I/O per second (per physical disk)
A storage device’s IOPS capability is calculated as an aggregate of the sum of disks that make up the device. For instance, when considering a JBOD configuration, three disks rotating at 10,000 RPMs provide the JBOD with a total of 375 IOPS. However, with the exception of RAID 0 (which is simply a set of disks aggregated together to create a larger storage device), all RAID set configurations are based on the fact that write operations result in multiple writes to the RAID set, in order to provide the targeted level of availability and performance.
In a RAID 5 disk set, for example, for each random write request, the storage controller is required to perform multiple disk operations, which has a significant impact on the raw IOPS calculation. Typically, that RAID 5 disk set requires four IOPS per write operation. In addition, RAID 6, which provides a higher level of protection through double fault tolerance, also provides a significantly worse I/O penalty of six operations per write. Therefore, as the architect of such a solution, you must also plan for any I/O penalty associated with the RAID type being used in the design.
Table 2.2 summarizes the read and write RAID penalties for the most common RAID levels. Notice that you don’t have to calculate parity for a read operation, and no penalty is associated with this type of I/O. The I/O penalty relates specifically to writes, and there is no negative performance or IOPS impact when calculating read operations. It is only when you have writes to disk that you will see the RAID penalty come into play in RAID calculations and formulas. This is true even though in a parity-based RAID-type write operation, reads are performed as part of that write. For instance, writes in a RAID 5 disk set, where data is being written with a size that is less than that of a single block, require the following actions to be performed:
Table 2.2 RAID I/O penalty impact
1. Read the old data block.
2. Read the old parity block.
3. Compare data in the old block with the newly arrived data. For every changed bit, change the corresponding bit in parity.
4. Write the new data block.
5. Write the new parity block.
As noted previously, a RAID 0 stripe has no write penalty associated with it because there is no parity to be calculated. In Table 2.2, a no RAID penalty is expressed as a 1.
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