Cache Control; Caching Write Data; Prefetch Operation - Seagate ST400FM0323 Product Manual

4.4

Cache control

All default cache mode parameter values (Mode Page 08h) for standard OEM versions of this drive family are given in Section 11.3.2.
4.4.1

Caching write data

Write caching is a write operation by the drive that makes use of a drive buffer storage area where the data to be written to the
medium is stored while the drive performs the WRITE command.
If the number of write data logical blocks exceed the size of the segment being written into, when the end of the segment is reached,
the data is written into the beginning of the same cache segment, overwriting the data that was written there at the beginning of
the operation; however, the drive does not overwrite data that has not yet been written to the medium.
If write caching is enabled (WCE=1), then the drive may return Good status on a WRITE command after the data has been transferred
into the cache, but before the data has been written to the medium. If an error occurs while writing the data to the medium, and
Good status has already been returned, a deferred error will be generated.
Data that has not been written to the medium is protected by a back up power source which provides the ability of the data to be
written to non-volatile medium in the event of an unexpected power loss.
The SYNCHRONIZE CACHE command may be used to force the drive to write all cached write data to the medium. Upon completion
of a SYNCHRONIZE CACHE command, all data received from previous WRITE commands will have been written to the medium.
Section 11.3.2 shows the mode default settings for the drive.
4.4.2

Prefetch operation

If the Prefetch feature is enabled, data in contiguous logical blocks on the medium immediately beyond that which was requested by
a Read command are retrieved and stored in the buffer for immediate transfer from the buffer to the host on subsequent Read
commands that request those logical blocks (this is true even if cache operation is disabled). Though the prefetch operation uses the
buffer as a cache, finding the requested data in the buffer is a prefetch hit, not a cache operation hit.
To enable Prefetch, use Mode Select page 08h, byte 12, bit 5 (Disable Read Ahead - DRA bit). DRA bit = 0 enables prefetch.
The drive does not use the Max Prefetch field (bytes 8 and 9) or the Prefetch Ceiling field (bytes 10 and 11).
When prefetch (read look-ahead) is enabled (enabled by DRA = 0), the drive enables prefetch of contiguous blocks from the medium
when it senses that a prefetch hit will likely occur. The drive disables prefetch when it decides that a prefetch hit is not likely to occur.
Seagate 1200.2 SAS SSD Product Manual, Rev. A 20