Seagate ST1000VX000 SV35 Series PATA Product Manual - Page 45
Word 95 Stream Minimum Request Size
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4.1.5 Streaming feature set support The Streaming feature set is an optional feature set that allows a host to request delivery of data from a contiguous logical block address range within an allotted time. This places a higher priority on time to access the data than the integrity of the data. Streaming feature set commands only support 48-bit, LBA-mode-only addressing. Seagate implements the following Streaming commands: • CONFIGURE STREAM • READ STREAM DMA EXT • WRITE STREAM DMA EXT Command Completion Time Out (CCTO) errors are not deferrable on DMA operations, as they require some DMA engine clean up on the host side. Therefore, Seagate does not anticipate supporting any configurations where a CCTO needs to hide from reporting as an error, and simply show up in a Stream Error log. Since the Stream Error logs were intended to be used to report deferred errors such as this, Seagate has not implemented them. 4.1.5.1 Identify Device command These drives will set word 84 bit 4 (Streaming Feature Set Supported) and word 87 bit 4 (Configure Stream command was issued) when a non-zero default time limit has been set for either reading or writing. If the host reconfigures both defaults to 0, the bit will clear. Word 95 (Stream Minimum Request Size) World 95 contains the number of sectors that provide optimum performance in a streaming environment. This number shall be a power of two, with a minimum of eight sectors (4096 bytes). The starting LBA value for each streaming command should be evenly divisible by this request size. Words 96, 97, and 104 Unsupported. Words 99:98 These words define the fixed unit of time that is used only in the Command Completion Time Limit (CCTL) that is passed in streaming commands. The unit of time for this parameter is microseconds. For example, a value of 50000 indicates 50 ms. 4.1.5.2 Streaming commands Streaming commands are defined to be time-critical data transfers rather than the standard data-integrity-critical commands. Each command shall be completed within the time specified in the CONFIGURE STREAM command or in the streaming command itself to ensure the stream requirements of the AV-type application. The drive may execute background tasks as long as the READ STREAM and WRITE STREAM command execution time limits are still met. The host may use the CONFIGURE STREAM command to define the default Command Completion Time Limit (CCTL) for reads and writes independently--this assists the device in setting up its caching for best performance. If the host does not use a CONFIGURE STREAM command, the device shall use the CCTL specified in each streaming command, and the time limit is effective for one time only. If the CCTL is not set by a CONFIGURE STREAM command, the operation of a streaming command with a zero CCTL is device vendor specific. "Stream ID" as noted in the ATA specification is not used by the Seagate implementation. SV35 Series Product Manual, Rev. B 39