Seagate Cheetah ST3146854, User Manual

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14 Cheetah 15K.4 SCSI Product Manual, Rev. D
requested LBs from the cache to the host in accordance with the disconnect/reconnect specification men-
tioned above.
3. If the prefetch feature is enabled, refer to Section 4.5.2 for operation from this point.
Case B - A Read command requests data, the first LB of which is not in any segment of the cache:
1. The drive fetches the requested LBs from the disc and transfers them into a segment, and from there to
the host in accordance with the disconnect/reconnect specification referred to in case A.
2. If the prefetch feature is enabled, refer to Section 4.5.2 for operation from this point.
Each buffer segment is actually a self-contained circular storage (wrap-around occurs), the length of which is
an integer number sectors. The wrap-around capability of the individual segments greatly enhances the
buffer’s overall performance as a cache storage, allowing a wide range of user selectable configurations, which
includes their use in the prefetch operation (if enabled), even when cache operation is disabled (see Section
4.5.2). The number of segments is set dynamically by the drive and cannot normally be set by the host (it can
be set using the PM bit as described in Section 4.5.3). The size in Kbytes of each segment is not reported by
the Mode Sense command page 08h, bytes 14 and 15. The value 0XFFFF is always reported. If a size specifi-
cation is sent by the host in a Mode Select command (bytes 14 and 15) no new segment size is set up by the
drive, and if the STRICT bit in Mode page 00h (byte 2, bit 1) is set to one, the drive responds as it does for any
attempt to change unchangeable parameters (see SCSI Interface Product Manual).
4.5.1 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 in one or more segments while the drive performs the write command.
If read caching is enabled (RCD=0), then data written to the medium is retained in the cache to be made avail-
able for future read cache hits. The same buffer space and segmentation is used as set up for read functions.
The buffer segmentation scheme is set up or changed independently, having nothing to do with the state of
RCD. When a write command is issued, if RCD=0, the cache is first checked to see if any logical blocks that
are to be written are already stored in the cache from a previous read or write command. If there are, the
respective cache segments are cleared. The new data is cached for subsequent Read commands.
If the number of write data logical blocks exceeds 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.
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.
Tables 19 show Mode default settings for the drives.
4.5.2 Prefetch operation
If the Prefetch feature is enabled, data in contiguous logical blocks on the disc immediately beyond that which
was requested by a Read command can be 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. Prefetch is enabled using Mode Select page 08h, byte 12,
bit 5 (Disable Read Ahead - DRA bit). DRA bit = 0 enables prefetch. Since data that is prefetched replaces