Dell PowerVault MD3000i - Manual (Page 9)

Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices
December 2008 – Revision A01 Page 9
• To optimize for data transfer rate, multiply the number of physical data
disks by the segment size to equal the I/O size. However, there are
always exceptions. For small/medium I/Os care should be taken to avoid
splitting up the I/O in such a way that an even smaller I/O is sent to the
disk drives. Please note that data disks do not include parity or mirror
disks used in a RAID set.
For IOPS or transaction-oriented applications, the number of drives becomes
more significant because disk drive random I/O rates are relatively low. Select a
number of drives that matches the per virtual disk group I/O rate needed to
support the application. Make sure to account for the I/Os required to implement
the data protection of the selected RAID level. Make the segment size at least as
large as the typical application I/O size.
A segment size of 128K is a reasonable starting point for most applications. In
most applications, the greater the number of drives in a disk group, the better the
average performance. The drive count of an existing disk group can be increased
using the CLI.
4.3 Virtual Disk Location and Capacity
The location of virtual disks within a disk group, the number and location of virtual
disks allocated within a disk group, and the capacity of a virtual disk are
important factors to consider when optimizing an array for performance.
When using rotating storage medium, the capacity of a virtual disk and its
location within a disk-group greatly impact achieved performance. This is
primarily due to differences in angular velocity in the outside zones. The effect of
allocating the outermost edges of a rotational storage medium to increase
performance is termed short stroking of a drive. While it is beyond the scope of
this white paper to cover the technical details involved around short-stroking,
typically the outer third of a rotational medium is the fastest, while the inner most
zones are the slowest. Short stroking can easily be accomplished by creating a
disk group consisting of a single virtual disk, which is allocated less than a third
of the total capacity. The obvious downside to short-stroking a volume is the loss
of additional usable capacity. Thus, this performance gain must be weighed
directly against the capacity loss.
In addition to the performance gains of short-stroking, the effect of drive head
seek should be taken into account when carving up a disk group into virtual
disks. Virtual disks are aligned in series within a Disk Group with the first virtual
disk starting in the fastest outer regions and progressing inwards. When taking
this into account, a disk group should be designed with as few virtual disks as
possible.
Dell™ does not recommend using more than four virtual disks or repositories per
disk group for peak performance. Additionally, where performance is critical,
isolate virtual disks to separate disk groups as possible. When multiple high-