User guide

1–14 Introduction
Hitachi Unified Storage VM Block Module Hardware User Guide
Cache capacity
The cache capacity is determined by the RAID level, the number of drives
installed in the system, and whether Dynamic Provisioning (DP)/Dynamic
Tiering (DT)/Dynamic Cache Residency (DCR)/Universal Volume Manager
(UVM) are applied/not applied.The recommended data cache capacity per
Cache Logical Partition (CLPR) = (CLPR capacity) - ( DCR Extent setting
capacity per CLPR)When CLPR is not applied to DP/DT/DCR, install the
recommended data cache capacity shown in the following table.
Table 1-4: Recommended data cache capacity when DP, DT, DCR and
UVM are not being used
Memory operation
The HUS VM places all read and write data in the cache. The amount of fast-
write data in cache is dynamically managed by the cache control algorithms
to provide the optimum amount of read and write cache, depending on the
workload read and write I/O characteristics.
Data Protection
The HUS VM is designed so that it cannot lose data or configuration
information from the cache if the power fails. The cache is protected from
data loss up for up to ten minutes by the cache destage batteries while the
data is copied to the 80 GB cache SSD (flash memory) on the cache boards.
This is explained in detail in Battery backup operations on page 5-6.
Total logical capacity of
External volumes +
Internal volumes per CLPR
Recommended data
cache capacity per CLPR
Less than 2,900GB 15GB or more
less than 2,900 G 15 GB or more
2,900 GB or more 16 GB or more
11,500 GB or more 22 GB or more
14,400 GB or more 24 GB or more
100,000 GB or more 30 GB or more
128,000 GB or more 32 GB or more
182,000 GB or more 40 GB or more
218,000 GB or more 48 GB or more
254,000 GB or more 56 GB or more
290,000 GB or more 64 GB or more
326,000 GB or more 72 GB or more