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2-10 About Universal Volume Manager Operations
Hitachi Universal Storage Platform V/VM Universal Volume Manager User’s Guide
• Cache Mode (Enable or Disable)
Cache mode specifies whether the write data from the host to the external
storage system is propagated synchronously (Disable) or asynchronously
(Enable). By default, cache mode is set to Enable. All I/O to and from the
local storage system (either Enable or Disable) always uses cache. Write
operations are always backed up in duplex cache.
– If you select Enable, after receiving the data into the local storage
system cache memory, the local storage system signals the host that
an I/O operation has completed and then asynchronously destages the
data to the external storage system.
– If you select Disable, the local storage system signals the host that an
I/O operation has completed only after the local storage system has
synchronously written the data to the external storage system.
If you perform the Cache Residency Manager operation on the external
volume, which the Cache Mode is set to Disable, the bind mode of
Cache Residency Manager cannot be specified. For the Cache Residency
Manager operation, see the Cache Residency Manager User’s Guide.
Note: When you set the cache mode, note the following:
• Data that is not written by the host (for example, data written by
ShadowImage) is asynchronously destaged to the external storage
system regardless of the Cache Mode setting.
• If you set the emulation type for a mainframe system (such as 3390-
x), data that is written by a host using a command such as Format
Write is asynchronously destaged to the external storage system
regardless of the Cache Mode setting. Data that is written by a host
using a command such as Update Write is destaged to the external
storage system as configured in the Cache Mode setting.
• Inflow Control (Enable or Disable)
Inflow control specifies whether the writing operation to the cache memory
is stopped (Enable) or continued (Disable) when the writing operation to
the external volume is impossible. By default, inflow control is set to
Disable.
– If you select Enable, the writing operation to cache is stopped and the
I/O from the host is not accepted when the writing operation to the
external volume is impossible.
– If you select Disable, the I/O from the host during the retry operation
is written to the cache memory even after the writing operation to the
external volume is impossible. Once the writing operation to the
external volume becomes normal, all the data in the cache memory is
written to the external volume (all the data is destaged).