Caching Write Data; Prefetch Operation; Advanced Caching (512E/4Kn Only) Operations - Seagate ST2400MM0129 Product Manual

4.5.1

Caching write data

Write caching in this section is the traditional SCSI write caching (WCE=1) where writes are not protected on power loss.
Note
Advanced Caching (512E/4Kn only) provides NVC-protected write caching when WCE=0 over a portion of the DRAM.
Refer to the SAS Interface Manual for more detail concerning the cache bits.
Note
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 read caching is enabled (RCD=0), then data written to the medium is retained in the cache to be made available 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 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.
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.
4.5.2

Prefetch operation

If the Prefetch feature is enabled, data in contiguous logical blocks on the disk 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 disk
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.
4.5.3

Advanced Caching (512E/4Kn only) operations

Read data that has been promoted into the Advanced read cache does not persist through a power cycle. Read data is retrieved from
the rotating media after a power cycle.
When WCE=0, Advanced provides NVC-protected write caching over the portion of the DRAM used to coalesce writes. Write data
only goes into NVC when there is an unexpected power loss to the drive. The NVC has 90-day data retention. When WCE=1, a
Advanced Caching drive will operate on writes like a standard drive–writes in cache are not protected by NVC and may be lost with
power loss.
Seagate Enterprise Performance 10K HDD v9 Product Manual, Rev. D (Draft 2) 18