Disk Stripes vs. Failures to Tolerate

Number of Disk Stripes Per Object

This defines the number of physical disks across which each replica of a storage object is striped. This impacts read and write operations as below:

Since all writes go to SSD (write buffer), the value of an increased stripe width may or may not improve performance. This is because there is no guarantee that the new stripe will use a different SSD; the new stripe may be placed on a HDD in the same disk group and thus the new strip will use the same SSD. The only occasion where an increased stripe width could add value is when there are many writes to destage from SSD to HDD. In this case, having a stripe could improve destage performance.

From a read perspective, an increased stripe width will help when you are experiencing many read cache misses. If you takes the example of a virtual machine consuming 2,000 read operations per second and experiencing a hit rate of 90%, then there are 200 read operations that need to be serviced from HDD. In this case, a single HDD which can provide 150 IOPs is not be able to service all of those read operations, so an increase in stripe width would help on this occasion to meet the virtual machine I/O requirements.

In general, the default stripe width of 1 should meet most, if not all virtual machine workloads. Stripe width is a capability that should only change when write destaging or read cache misses are identified as a performance constraint.

Number of Failures To Tolerate

This is the number of concurrent host, network or disk failures that may occur in the cluster and still ensuring the availability of the object. It specifies that configurations must contain at least “Number of Failures To Tolerate” replicas and may also contain an additional number of witnesses to ensure that the object’s data are available (maintain quorum). Witness disks provide a quorum when failures occur in the cluster or a decision has to be made when a split-brain situation arises.

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