Administrator Guide

● The Adaptive option works well for most applications: it enables adaptive read-ahead, which allows the controller to
dynamically calculate the optimum read-ahead size for the current workload.
● The Stripe option sets the read-ahead size to one stripe. The controllers treat NRAID and RAID-1 disk groups internally as if
they have a stripe size of 512 KB, even though they are not striped.
● Specific size options let you select an amount of data for all accesses.
● 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.
Pool overcommit behavior
The standard behavior for thin provisioning depends on whether the pool overcommit feature is disabled or enabled.
● When the overcommit feature is disabled, the host does not lose read or write access to the pool volumes when the pool
reaches or exceeds the high threshold value.
● When the overcommit feature is enabled, the storage system sends the data protect sense key Add, Sense: Space
allocation failed write protect to the host when the pool reaches or exceeds the high threshold value. If the
host is rebooted after the pool reaches or exceeds the high threshold value, the host loses read and write access to the pool
volumes. The only way to regain read and write access to the pool volumes is to add more storage to the pool.
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 19.
● 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.
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Getting started