Write Cache; Cache Operation - Fujitsu MHW2040AC Product Manual

Operations

6.5 Write Cache

Write Cache is the function for reducing the command processing time by
separating command control to disk media from write control to disk media.
When Write Cache is permitted, the write command can be keep receiving as long
as the space available for data transfers remains free on the data buffer. Because
of this function, command processing appears to be completed swiftly from the
viewpoint of the host. It improves system throughput.
The disk drive has a drive-specific function. When the temperature becomes about
-20°C or lower, the function disables the Write cache function, writes data into
user areas, and automatically performs the data verify operation (Read check) for
those areas. When writing data and verifying the area are completed during this
operation, the busy state is canceled. (When the temperature is about -20°C or
higher, the disk drive operates depending on whether the SET FEATURES
command caching function is enabled or disabled, and only the normal write
operation is performed.)

6.5.1 Cache operation

(1) Command that are targets of caching
The Commands that are targets of caching are as follows:
WRITE SECTOR (S)
•
WRITE MULTIPLE
•
WRITE DMA
•
WRITE SECTOR (S) EXT
•
WRITE MULTIPLE EXT
•
WRITE DMA EXT
•
However, the caching operation is not performed when the caching function is
prohibited by the SET FEATURES command.
(2) Invalidation of cached data
If an error occurs during writing onto media, write processing is repeated up to as
many times as specified for retry processing. If retry fails for a sector because the
retry limit is reached, automatic alternate sector processing is executed for the
sector. If the automatic alternate sector processing fails, the data in the sector for
which automatic alternate sector processing failed is invalidated without being
guaranteed.
If data remains in sectors following a sector for which automatic alternate sector
processing failed, the data is invalidated without being guaranteed.
Moreover, when the command (clause 6.4.2(3)) is accepted and HOST CRC Error
is generated, the caching data is invalidated.
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