MSA 2040 SMU Reference Guide

System concepts 17

When you create a vdisk you can use the default chunk size or one that better suits your application. The chunk size

is the amount of contiguous data that is written to a disk before moving to the next disk. After a vdisk is created its

chunk size cannot be changed. For example, if the host is writing data in 16-KB transfers, that size would be a good

choice for random transfers because one host read would generate the read of exactly one disk in the volume. That

means if the requests are random-like, then the requests would be spread evenly over all of the disks, which is good

for performance. If you have 16-KB accesses from the host and a 64-KB block size, then some of the hosts accesses

would hit the same disk; each chunk contains four possible 16-KB groups of data that the host might want to read,

which is not an optimal solution. Alternatively, if the host accesses were 128 KB, then each host read would have to

access two disks in the vdisk. For random patterns, that ties up twice as many disks.

When you create a vdisk you can also create volumes within it. A volume is a logical subdivision of a vdisk, and can

be mapped to controller host ports for access by hosts. The storage system presents only volumes, not vdisks, to hosts.

You can create vdisks with or without volumes by using the Provisioning Wizard, or you can create vdisks manually.

TIP: Best practices for creating vdisks include:

• To maximize capacity, use disks of similar size.

• For greatest reliability, use disks of the same size and rotational speed.

• For storage configurations using many disks, create a few vdisks each containing many disks instead of many

vdisks each containing a few disks.

• To maximize capacity and disk usage (but not performance), you can create vdisks larger than 2 TB and divide

them into multiple volumes each having a capacity of 2 TB or less. This increases the usable capacity of storage

configurations by reducing the total number of parity disks required when using parity-protected RAID levels. This

differs from using a volume larger than 2 TB, which requires specific support by the host operating system, I/O

adapter, and application.

• For maximum use of a dual-controller system’s resources, each controller should own a similar number of vdisks.

• Set the chunk size to match the transfer block size of the host application.