IBM System Storage DS3000, Руководство по программированию

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Chapter 10. Using the Performance Read Cache premium
feature
Note: The Performance Read Cache premium feature is available only on the DS3500 storage subsystem
controller and the DCS3700 storage subsystem with Performance Module Controllers controller.
The Performance Read Cache premium feature provides a way to improve read-only performance.
Performance Read Cache is a set of Solid-State Disk (SSD) drives that you logically group together in
your storage subsystem to implement a read cache for end-user volumes. Performance Read Cache is a
secondary cache for use with the primary cache in the controller DRAM. In controller cache, the data is
stored in DRAM after a host read. In Performance Read Cache, the data is copied from user-specified
base volumes and stored on two internal RAID volumes (one per controller) that are automatically
created when you create an Performance Read Cache. These volumes are used for internal cache
processing purposes. These volumes are not accessible or displayed in the user interface. However, these
two volumes do count against the total number of volumes allowed in the storage subsystem.
Note: The Performance Read Cache feature is available only when the Performance Read Cache premium
feature is enabled, an Performance Read Cache entity is created, and each base volume for which you
want to use the Performance Read Cache has the Performance Read Cache attributes enabled.
Using high performance SSDs to cache the data from the base volumes improves the application I/O
performance and response times, and delivers sustained performance improvement across different
workloads, especially for high-IOP workloads. Simple volume I/O mechanisms are used to move data to
and from the Performance Read Cache. After data is cached and stored on the SSDs, subsequent reads of
that data are performed on the Performance Read Cache, thereby eliminating the need to access the base
volume.
These are the characteristics of a workload that benefits from using Performance Read Cache:
v
Performance is limited by hard disk drive IOPs.
v There are a high percentage of reads relative to writes.
v
A large number of reads that are repeat reads to the same or adjacent areas of the drive.
v The capacity of the data that is repeatedly accessed is smaller than the cache capacity. The more
volumes being cached, the higher the probability that the capacity of the data accessed exceeds the
capacity of the cache.
The Performance Read Cache premium feature moves data from the hard drives to SSDs following a host
read or write so that a subsequent host read of the same logical block addressing (LBA) can be read
directly from SSD with a much lower response time than rereading from hard drives.
When you create an Performance Read Cache, you choose which I/O type (file system, database, or web
server) that most closely matches the applications and volumes that will use the Performance Read
Cache. In addition, you specify the capacity of the Performance Read Cache from a list of possible
candidates consisting of different counts of SSD drives. You also have the option to enable Performance
Read Cache on all eligible volumes that are currently mapped to hosts. Lastly, after you create the
Performance Read Cache, you can enable or disable it on existing volumes or as part of a new volume
creation.
Performance Read Cache operations
The following list includes common tasks that you can perform with the Performance Read Cache
feature.
v
Create
© Copyright IBM Corp. 2008, 2012
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