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Disk Workload Characteristics

How can you determine what type of disk workload your server is experiencing so that you can appropriately distribute the disk activities across multiple RAID levels or standalone disks? Performance Monitor provides two counters (% Disk Read Time and % Disk Write Time) under the LogicalDisk object that let you identify disk subsystem workload characteristics. The % Disk Read Time counter measures the percentage of elapsed time that the selected disk is busy servicing read requests, and the % Disk Write Time counter measures the percentage of elapsed time that the selected disk is busy servicing write requests.

Using these counters, you can determine how much time your disk spends writing and reading data. These counters provide a high-level view of the type of disk activity you must plan for. Use this information with the information in Table 1 to select the RAID levels that provide the best performance for your environment. For example, if the value for % Disk Read Time is 10 percent and the value for % Disk Write Time is 90 percent, consider either RAID 1, RAID 0, or a standalone disk (for the latter option, remember that you forfeit any disk fault tolerance for improved performance). As Table 1 shows, RAID 5 write performance is lower than other RAID levels because every RAID 5 write incurs four disk operations: read the data, read the parity data (compare the two using the CPU), write the data, and write the parity data. Conversely, if the value for % Disk Read Time is 80 percent and the value for % Disk Write Time is 20 percent, RAID 5 is a good choice.

This performance guide shows the relative performance ratings when comparing the various RAID options using a sector/stripe size of 128KB. Use this guide when selecting the appropriate performance level that match your server's disk I/O characteristics.

Table 1

Relative Performance of RAID Levels

Raid Level

Random Read

Random Write

Sequential Read

Sequential Write

Stripe

(RAID 0)

1st

1st

1st

1st

Mirror

(RAID 1)

4th

3rd

4th

3rd

Stripe w/parity

(RAID 5)

1st

4th

1st

4th

Mirrored Stripe Set

(RAID 10)

1st

2nd

1st

2nd

Many enterprise networks have a mixed read and write environment with some RAID devices experiencing much higher workloads than others. In these instances, you need to load-balance your disk devices for optimal performance.

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