Working with Read Cache Settings

The Read Cache unit policy lets you enable either Basic Read Caching or Intelligent Read Caching. You can also disable the Read Cache unit policy, if preferred.

By default, Read Cache is set to Intelligent. You can change it to Basic or to Disabled, as appropriate.

About the Read Cache Basic and Intelligent Modes

Basic Mode is used to store read cache data locally on the controller that has recently been written to media and is likely to be frequently accessed. This improves read access times for applications such as a database that can take advantage of storage caching. The read cache may be disabled without reducing performance for applications that are write intensive, or that rarely read back data recently written.

Intelligent Mode enables the Intelligent Read Prefetch (IRP) feature.

The Intelligent Read Prefetch feature includes a typical read-ahead caching method which is used to proactively retrieve data from media and store it locally on the controller with the anticipation that it may be requested by the host. For example, the host may read blocks 1, 2, and 3. With the read-ahead caching included in IRP, the controller will also retrieve and hold in its cache blocks 4, 5, and 6 in anticipation of getting those command requests from the host. By loading a larger set of data into the cache, chances are improved that another request can be filled by data that is already in the cache. This can be helpful with applications that are sequential in nature, such as video on demand, video surveillance playback, and restoring from a disk-to-disk backup. Performance benefits of read-ahead are especially pronounced when the host queue depth is low. In addition, this read-ahead caching also improves sequential read performance when the unit is degraded.

The Intelligent Read Prefetch (IRP) feature also includes an intelligent and adaptive stream management layer to improve performance at higher queue depth in multiple read only or mixed read/write stream environments. The performance improvements should be seen for most RAID types regardless of the unit’s operational state (normal, degraded, and so forth).

Note that if Intelligent Mode is enabled, the features in Basic mode are also enabled.

When to Use Each Read Cache Setting

The following table provides some recommendations for when to use each Read Cache setting.

Recommendations for Using Each Read Cache Setting
Use this Read Cache Setting	For this reason	Example applications
Intelligent	Sequential applications, with a low host command queue depth	Video on Demand, Video Surveillance Playback Disk-to-Disk Backup Restores, File Server
Basic	Frequent access to recently written data	Database
Disabled	Applications that produce a high queue depth or perform their own read- ahead can generate enough I/O to negate the benefits of controller read caching or read-ahead. This is especially true for applications that produce a lot of random I/O.	Online Transaction Processing (OLTP)

Changing the Read Cache Policy for Existing Units

The Read Cache policy is initially set when you create a unit. You can change it later from 3DM2.

To change the Read Cache unit policy In 3DM2

1	Choose Management >> Controller Settings from the menu bar in 3DM2.

2	In the Unit Policies section of the Controller Settings page, select the desired Read Cache setting.

Intelligent: Enables both the Basic and Intelligent Read Prefetch (IRP) caching features.

Basic: Enables the Basic read caching mode for the unit.

Disabled: Disables the Read Cache policy for the unit.

Show Read Cache policy on the Controller Settings page

After you change the selection, the page refreshes, and a message at the top confirms the change you have made.

Related Topics: Enabling and Disabling the Unit Write Cache; Enabling or Disabling Auto-Verify for a Unit; Setting Overwrite ECC (Continue on Source Error When Rebuilding); Enabling and Disabling Queuing for a Unit; Setting the StorSave Profile for a Unit; Rapid RAID Recovery