HP StorageWorks P2000 HP StorageWorks P2000 G3 MSA System SMU Reference Guide - Page 26

You can change the amount of data read in advance after two back-to-back reads are made. Increasing the read-ahead cache size can greatly improve performance for multiple sequential read streams; however, increasing read-ahead size will likely decrease random read performance. • The Default option works well for most applications: it sets one chunk for the first access in a sequential read and one stripe for all subsequent accesses. The size of the chunk is based on the chunk size used when you created the vdisk (the default is 64 KB). Non-RAID and RAID-1 vdisks are considered to have a stripe size of 64 KB. • Specific size options let you select an amount of data for all accesses. • The Maximum option lets the controller dynamically calculate the maximum read-ahead cache size for the volume. For example, if a single volume exists, this setting enables the controller to use nearly half the memory for read-ahead cache. Only use Maximum when disk latencies must be absorbed by cache. For example, for read-intensive applications, you will want data that is most often read to be in cache so that the response to the read request is very fast; otherwise, the controller has to locate which disks the data is on, move it up to cache, and then send it to the host. Do not use Maximum if more than two volumes are owned by the controller on which the read-ahead setting is being made. If there are more than two volumes, there is contention on the cache as to which volume's read data should be held and which has the priority; each volume constantly overwrites the other volume's data in cache, which could result in taking a lot of the controller's processing power. • 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. You can also change the optimization mode. • The standard read-ahead caching mode works well for typical applications where accesses are a combination of sequential and random; this method is the default. For example, use this mode for transaction-based and database update applications that write small files in random order. • For an application that is strictly sequential and requires extremely low latency, you can use Super Sequential mode. This mode makes more room for read-ahead data by allowing the controller to discard cache contents that have been accessed by the host. For example, use this mode for video playback and multimedia post-production video- and audio-editing applications that read and write large files in sequential order. Related topics • Changing a volume's cache settings on page 54 • Changing system cache settings on page 49 • Viewing information about a volume on page 90 About managing remote systems You can add a management object to obtain information from a remote storage system. This allows a local system to track remote systems by their network-port IP addresses and cache their login credentials - the user name and password for a manage-level user on that system. The IP address can then be used in commands that need to interact with the remote system. After a remote system has been added, you can check the connectivity of host ports in the local system to host ports in that remote system. A port in the local system can only link to ports with the same host interface, such as Fibre Channel (FC), in a remote system. Communication between local and remote systems is an essential part of the remote replication feature. Related topics • Adding a remote system on page 52 • Removing a remote system on page 52 • Viewing information about a remote system on page 100 • Checking links to a remote system on page 83 • About the Remote Snap replication feature on page 101 26 Getting started