Administrator Guide

• The Disabled option turns off read-ahead cache. This is useful if the host is triggering read ahead for what are random accesses. This
can happen if the host breaks up the random I/O into two smaller reads, triggering read ahead.
About thin provisioning
Thin provisioning is a virtual storage feature that allows a system administrator to overcommit physical storage resources. This allows the
host system to operate as though it has more storage available than is actually allocated to it. When physical resources fill up, the
administrator can add physical storage by adding additional disk groups on demand.
Paging is required to eliminate the lack of flexibility associated with linear mapping. Linear mapping limits the ability to easily expand the
physical storage behind the thin-provisioned volume. Paged mapping allows physical resources to be disparate and noncontiguous, making
it much easier to add storage on the fly.
For example, contrast the methods for creating a volume for Microsoft Exchange Server data:
• Typically, administrators create a storage-side volume for Exchange and map that volume with an assigned Logical Unit Number
(LUN) to hosts, and then create a Microsoft Windows volume for that LUN. Each volume has a fixed size. There are ways to increase
the size of a storage-side volume and its associated Windows volume, but they are often cumbersome. The administrator must make a
trade-off between initial disk costs and a volume size that provides capacity for future growth.
• With thin provisioning, the administrator can create a very large volume, up to the maximum size allowed by Windows. The
administrator can begin with only a small number of disks, and add more as physical storage needs grow. The process of expanding the
Windows volume is eliminated.
NOTE: For a thin-provisioned volume mapped to a host, when data is deleted from the volume not all of the pages, or
space associated with that data will be deallocated, or released. This is especially true for smaller files. To deallocate the
pages in Windows, select the mapped volume and do either of the following:
• Perform a quick format.
• View its properties, select the Tools tab, and under Defragmentation, click Optimize.
About automated tiered storage
Automated Tiered Storage is a virtual storage feature that automatically moves data residing in one class of disks to a more appropriate
class of disks based on data access patterns, with no manual configuration necessary.
• Frequently accessed data can move to disks with higher performance.
• Infrequently accessed data can move to disks with lower performance and lower costs.
Each virtual disk group, depending on the type of disks it uses, is automatically assigned to one of the following tiers:
• Performance – This highest tier uses SSDs, which provide the best performance but also the highest cost. For more information on
SSDs, see About SSDs on page 18.
• Standard – This middle tier uses enterprise-class spinning SAS disks, which provide good performance with mid-level cost and
capacity.
• Archive – This lowest tier uses midline spinning SAS disks, which provide the lowest performance with the lowest cost and highest
capacity.
When the status of a disk group in the Performance Tier becomes critical (CRIT), the system will automatically drain data from that disk
group to disk groups using spinning disks in other tiers providing that they can contain the data on the degraded disk group. This occurs
because similar wear across the SSDs is likely, so more failures may be imminent.
If a system only has one class of disk, no tiering occurs. However, automated tiered storage rebalancing happens when adding or removing
a disk group in a different tier.
NOTE: Tiers are automatically set up within a single virtual pool, but tiers do not span virtual pools.
Volume tier affinity
Volume tier affinity is a setting that enables a storage administrator to define QoS (Quality of Service) preferences for volumes in a
storage environment.
The three volume tier affinity settings are:
• No Affinity – This setting uses the highest available performing tiers first and only uses the Archive tier when space is exhausted in the
other tiers. Volume data moves into higher performing tiers based on the frequency of access and available space in the tiers.
Getting started
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