Selecting Drives - IBM System Storage DS3500 Introduction And Implementation Manual

Draft Document for Review March 28, 2011 12:24 pm

3.3.1 Selecting drives

The speed and the type of the drives used will impact the performance. Typically, the faster
the drive, the higher the performance. This increase in performance comes at a cost; the
faster drives typically cost more than the lower performance drives.
The DS3500 Storage System currently supports the following types of SAS drives for the two
different models of chassis as shown below in Table 3-2.
Table 3-2 DS3500 families HDD support
Drives Supported
SAS 15KRPM
SAS 15KRPM FDE
SAS 10KRPM
SAS 10KRPM FDE
Nearline SAS
7.5KRPM
Maximum drives
Storage system
capacity (max)
The Full Disk Encryption (FDE) drives; are drives with built-in disk encryption hardware that
prevents unauthorized access to the data on a drive that is physically removed from the
storage subsystem.
Best practice: Generally it is best to use the fastest drives available for best performance.
This can be critical to transaction based (high IOPS) workloads.
The speed of the drive is measured by the number or revolutions per minute (RPM). A 15K
drive rotates 15,000 times per minute. With higher speeds, the drives tend to be denser,
because a large diameter plate driving at such speeds is likely to wobble. With the faster
speeds, greater throughput is possible.
Seek time is the measure of how long it takes for the drive head to move to the correct sectors
on the drive to either read or write data. It is measured in thousands of a second (milliseconds
or ms). The faster the seek time, the quicker data can be read from or written to the drive. The
average seek time reduces when the speed of the drive increases. Typically, a 7.2K drive will
have an average seek time of around 9 ms, a 10K drive will have an average seek time of
around 5.5 ms, and a 15K drive will have an average seek time of around 3.5 ms.
Command queuing (or queue depth) allows for multiple commands to be outstanding to the
disk drive at the same time. The drives have a queue where outstanding commands can be
dynamically rescheduled or re-ordered, along with the necessary tracking mechanisms for
outstanding and completed portions of workload. The DS3500 provides a drive command
queue depth of four operations per disk. The
depth for all drives to 16.
Avoid using the SAS nearline drives for high IOPS operations. SAS nearline can, however, be
used for streaming and archiving applications. These are both very good uses for the slower
RPM drives, where high throughput rates are required, at a lower cost. If properly configured,
DS3512/EXP3512
300GB, 450GB,
600GB
600GB
None
None
1TB, 2TB
12/96
450 GB SAS / 1 TB
SATA
High Performance Tier
Chapter 3. IBM System Storage DS3500 Storage System planning tasks
7914DS3KPlanning_090710.fm
DS3524/EXP3524
73GB, 146GB
None
300GB
300GB
500GB
24/96
450 GB SAS / 1 TB
SATA
increases the queue
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