About Cache Residency Manager
Overview of Cache Residency Manager
Cache Residency Manager enables you to retain frequently accessed data in the storage system's cache memory so that it is immediately available to hosts.
Using Cache Residency Manager, you can increase the data access speed for specific data by enabling read and write I/Os to be performed at the higher front-end access speeds. You can use Cache Residency Manager for both open-systems and mainframe data.
When Cache Residency Manager is used, total storage system cache capacity must be increased to avoid data access performance degradation for non-cache-resident data. The maximum allowable Cache Residency Manager cache area is configured when the cache is installed, so you must plan carefully for Cache Residency Manager operations and work with your Hitachi Vantara representative to calculate the required amount of cache memory for your configuration and requirements.
All write I/Os to Cache Residency Manager data are duplex writes, guaranteeing full data integrity. The Cache Residency Manager data remains fixed in cache until you manually delete it. Deletion of Cache Residency Manager cache areas destages any write data to the affected volumes.
Cache Residency Manager provides the following functions:
- Priority cache mode
- Bind cache mode
- Prestaging data in cache
Priority mode (read data in cache)
In priority mode the Cache Residency Manager extents are used to hold read data for specific extents on volumes. Write data is write duplexed in cache other than Cache Residency Manager cache, and the data is destaged to the drives when drive utilization is low.
The required total cache capacity for priority mode (normal mode) is:
standard cache + Cache Residency Manager cache + additional cache
The next table specifies the standard cache capacity requirements for priority mode operations. Meeting these requirements is important for preventing performance degradation. For more information about calculating cache size for priority mode, see Estimating cache size for Cache Residency Manager.
Settings of priority mode | Standard cache capacity |
Specified number of cache areas is 8,192 or less and the specified capacity is 128 GB or less | 16 GB |
Specified number of cache areas exceeds 8,192 or the specified capacity exceeds 128 GB | 32 GB |
1 GB = 1,073,741,824 bytes | |
Bind mode (read and write data in cache)
In bind mode the Cache Residency Manager extents are used to hold read and write data for specific extents on volumes, so both read and write speeds are improved. Data written to the Cache Residency Manager bind area is not destaged to the drives. To ensure data integrity, write data is duplexed in the Cache Residency Manager cache area, which consumes a significant amount of the Cache Residency Manager cache.
Bind mode provides the following advantages over priority mode:
- The accessibility of read data is the same as Cache Residency Manager priority mode.
- Write operations do not have to wait for available cache segments.
- There is no back-end contention caused by destaging data.
The required total cache capacity for bind mode is:
standard cache + Cache Residency Manager cache
Cache Residency Manager bind data that has write attributes is normally not destaged. However, the data is destaged to disk in the following cases:
- When the cache residency area is released.
- During cache blockage that is caused by certain maintenance operations (for example, cache upgrades) or by cache failure.
- When the storage system is powered off.
- When the volume is deleted from Cache Residency Manager bind mode.
The next table specifies the cache requirements for bind mode operations. Meeting these requirements is important for preventing performance degradation. For more information about calculating cache size for bind mode, see Estimating cache size for Cache Residency Manager.
|
System Type |
RAID Level or Volume Type |
Capacity Specifications |
Cache Residency Cache Requirement |
|
Open systems |
RAID 5 (3390) or RAID 6 |
Slot capacity: 256 KB Cache segment capacity: 16.5 KB Cache segments needed per slot: 48 (slot capacity / cache segment capacity) |
3 times the space required for user data: 1 slot = 3 × 256 KB = 768 KB = 48 cache segments |
|
RAID 1, or external volumes |
Slot capacity: 256 KB Cache segment capacity: 16.5 KB Cache segments needed per slot: 32 (slot capacity / cache segment capacity) |
2 times the space required for user data: 1 slot = 2 × 256 KB = 512 KB = 32 cache segments | |
|
Mainframe (for example, 3390-3, 3390-9) |
RAID 5 mainframe or RAID 6 |
Slot capacity: 66 KB Cache segment capacity: 16.5 KB Cache segments needed per slot: 12 (slot capacity / cache segment capacity) Note: Even though a mainframe track is 56 KB, because cache is divided into 16.5 KB segments, it requires 4 segments. |
3 times the space required for user data: 1 slot = 3 × 66 KB = 198 KB = 12 cache segments |
|
RAID 1 mainframe, or external volumes |
Slot capacity: 66 KB Cache segment capacity: 16.5 KB Cache segments needed per slot: 8 (slot capacity / cache segment capacity) |
2 times the space required for user data: 1 slot = 2 × 66 KB = 132 KB = 8 cache segments |
Prestaging data in cache
Normally, the Cache Residency Manager data is staged into the Cache Residency cache at the first access from the host. From the second access, cache hit can be done. By using the Cache Residency Manager prestage function, the cache hit is available from the first host access, further improving data access performance. Prestaging can be used for both priority mode and bind mode operations.
Prestaging occurs under any of the following circumstances:
- When prestaging is performed using Cache Residency Manager.
- When the storage system is powered on.
- When cache maintenance is performed.
Cache Residency Manager system specifications
|
Item |
Specification | |
|
Open system |
Mainframe system | |
|
Supported device emulation types |
OPEN-V OPEN-3, 8, 9, E, K, L |
3390-1, 2, 3, 9, A, L, M 3390-3A, 3B, 3C 3390-9A, 9B, 9C 3390-LA, LB, LC 3390-MA, MB, MC |
|
Supported volume types |
Virtual LUN |
Virtual LVI |
|
Unit of cache area allocation |
For OPEN-V, at least 512 LBAs: Equivalent to 256 KB For other than OPEN-V, at least 96 LBAs: Equivalent to 48 KB |
At least one cache slot (or track): Equivalent to 66 KB. Up to 1 LDEV. |
|
Number of cache areas |
Per storage system: 16,384 Per LDEV: 4,096 | |
Allocate the cache residency cache to an LDEV for each logical cylinder or head. Cache Residency Manager recognizes the logical block address (LBA) in the unit of 512 blocks for OPEN-V, or in the unit of 96 blocks for other emulation types.
When you configure cache residency, specify whether to enable the Cache Residency mode (priority mode or bind mode), and the prestage function.
Requirements for using performance functions
- Performance management products: The following software products are required for performance management operations.
- Hitachi Performance Monitor
- Hitachi Server Priority Manager
- Cache Residency Manager
- Hitachi Virtual Partition Manager
- Access: Administrator access for
Hitachi Device Manager - Storage Navigator or write access for the performance management software products is required to perform operations. Users without Administrator access or write access can only view the performance management information and settings. You need the specific administrator roles for each function:
- Performance Monitor: Storage Administrator (Performance Management)
- Server Priority Manager, Cache Residency Manager, Virtual Partition Manager: Storage Administrator (System Resource Management)
- License keys: The license keys for the performance management software products must be installed. For details about installing license keys, see the System Administrator Guide.
- Java: Java is required for Server Priority Manager and Cache Residency Manager. For details about installing Java and configuring Device Manager - Storage Navigator, see the System Administrator Guide.
- Hitachi Device Manager - Storage Navigator secondary windows: Secondary windows must be enabled before you use Server Priority Manager or Cache Residency Manager. Java and some settings of Hitachi Device Manager - Storage Navigator are required for the Hitachi Device Manager - Storage Navigator secondary window. For details, see the System Administrator Guide.
- Cache memory for Cache Residency Manager: Use of Cache Residency Manager might require additional cache memory in your storage system. For details, see Estimating cache size for Cache Residency Manager.
- Cache memory for Virtual Partition Manager: Use of Virtual Partition Manager might require additional cache memory in your storage system.