Performance Tools for Network Workloads

In some scenarios, a JBOD configuration actually provides better performance than striped data layout schemes. For example, when serving multiple lengthy sequential streams, performance is best when a single disk services each stream. Also, workloads that are composed of small, random requests do not experience performance improvements when they are moved from a JBOD configuration to a striped data layout.

A JBOD configuration is susceptible to static and dynamic “hot spots” (frequently accessed ranges of disk blocks) that reduce available storage bandwidth due to the resulting load imbalance between the physical drives.

Any physical disk failure results in data loss in a JBOD configuration. However, the loss is limited to the failed drives. In some scenarios, a JBOD configuration provides a level of data isolation that can be interpreted as offering greater reliability than striped configurations.

For most workloads, a striped data layout provides better performance than a JBOD configuration if the stripe unit is appropriately selected based on server workload and storage hardware characteristics. The overall storage load is balanced across all physical drives.

This is the least expensive RAID configuration because all of the disk capacity is available for storing the single copy of data.

Because no capacity is allocated for redundant data, striping does not provide data recovery mechanisms such as those provided in the other resiliency schemes. Also, the loss of any disk results in data loss on a larger scale than a JBOD configuration because the entire file system or raw volume spread across n physical disks is disrupted; every nth block of data in the file system is missing.

Mirroring often has worse bandwidth and latency for write operations when compared to striping or JBOD. This is because data from each write request must be written to a pair of physical disks. Request latency is based on the slowest of the two (or more) write operations that are necessary to update all copies of the updated data blocks. In more complex implementations, write latencies may be reduced by write logging or battery-backed write caching, or by relaxing the requirement for dual write completions before returning the I/O completion notification.

Mirroring has the potential to provide faster read operations than striping because it can (with a sufficiently intelligent controller) read from the least busy physical disk of the mirrored pair, or the disk that will experience the shortest mechanical positioning delays.

Mirroring is the most expensive resiliency scheme in terms of physical disks because half (or more) of the disk capacity stores redundant data copies. A mirrored array can survive the loss of any single physical disk. In larger configurations, it can survive multiple disk failures if the failures do not involve all the disks of a specific mirrored disk set.

Mirroring has greater power requirements than a non-mirrored storage configuration. It doubles the number of disks; therefore, it doubles the required amount of idle power. Also, mirroring performs duplicate write operations that require twice the power of non-mirrored write operations.

In the simplest implementations, mirroring is the fastest of the resiliency schemes in terms of recovery time after a physical disk failure. Only a single disk (the other part of the broken mirror pair) must participate in bringing up the replacement drive. The second disk is typically still available to service data requests throughout the rebuilding process. In more complex implementations, multiple drives may participate in the recovery phase to help spread out the load for the duration of the rebuild.

The cost and power characteristics are similar to those of mirroring.

For read requests, RAID 5 has characteristics that resemble those of striping. However, small RAID 5 writes are much slower than those of other resiliency schemes because each parity block that corresponds to the modified data block(s) must also be updated. This process requires three additional disk requests in the simplest implementation, regardless of the size of the array. Each small write requires two reads (old data and old parity) and two writes (new data and new parity). Because multiple physical disk requests are generated for every logical write, bandwidth is reduced by up to 75 percent.

RAID 5 arrays provide data recovery capabilities because data can be reconstructed from the parity. Such arrays can survive the loss of any one physical disk, as opposed to mirroring, which can survive the loss of multiple disks if the mirrored pair (or triplet) is not lost.

RAID 5 requires additional time to recover from a lost physical disk compared to mirroring because the data and parity from the failed disk can be re-created only by reading all the other disks in their entirety. In a basic implementation, performance during the rebuilding period is severely reduced due to the rebuilding traffic and because the reads and writes that target the data that was stored on the failed disk must read all the disks (an entire “stripe”) to re-create the missing data. More complex implementations incorporating multiple arrays may take advantage of more parallelism from other disks to help speed up recovery time.

RAID 5 is more cost efficient than mirroring because it requires only an additional single disk per array, instead of double (or more) the total number of disks in an array.

Power guidelines: RAID 5 might consume more or less energy than a mirrored configuration, depending on the number of drives in the array, the characteristics of the drives, and the characteristics of the workload. RAID 5 might use less energy if it uses significantly fewer drives. The additional disk adds to the required amount of idle power as compared to a JBOD array, but it requires less additional idle power versus a full mirrored set of drives. However, RAID 5 requires four accesses for every random write request (in the basic implementation) to read the old data, read the old parity, compute the new parity, write the new data, and write the new parity.

This means that the power needed beyond idle to perform the write operations is up to four times that of a JBOD configuration or two times that of a mirrored configuration.

As far as power and performance, the same general statements can be made for RAID 6 that were made for RAID 5, but to a larger magnitude.