Acer SmartRAID V Manuel d'utilisateur télécharger pdf (Page 192)

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Theory of Operation

Command Queuing

O messaging allows the controller to queue multiple commands from the

host and execute them out of order for greater efficiency. When a

command is complete, the controller transfers status information to host

memory. This status message contains information indicating which

command was just completed and enables commands to be executed in a

different order than they were issued by the host. This allows commands

to be queued and rearranged so that they are executed in a more efficient

order for the particular peripheral configuration.

SmartRAID V controllers also support Tagged Command Queuing on the

peripheral bus. This feature allows the controller to transfer multiple

commands to a single device if that device can accept queued commands.

Auto Request Sense

When a SCSI error occurs, the controller automatically issues a Request

Sense command and returns the data to the host in the message reply

frame. Data returned by this command is stored in the controller’s Event

Log. You can use Storage Manager to view the Event Log.

Byte/Word Alignment

Data to be read or written by the SmartRAID V controllers need not be

aligned to word or long-word boundaries in system memory. If the data

starts on an odd memory address, the necessary bus control signals will

be generated by the SmartRAID V to access 8-bit, 16-bit, 24-bit and 32-bit

data as required. Data blocks of any byte length are correctly handled by

SmartRAID V.

Scatter/Gather

SmartRAID V controllers support Scatter/Gather. This is a method of

providing multiple memory addresses for data transfer in one host

command. This ability greatly increases performance in environments

such as UNIX, Novell NetWare and Windows NT. Scatter/Gather is

necessary because when using virtual memory addressing schemes,

system memory that may appear contiguous to the user can actually be

fragmented into many widely scattered physical address locations.

Because of this, when accessing a large amount of contiguous data from a

peripheral device, it is often necessary to split the transfer into many

different locations in system memory.

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Acer SmartRAID V Manuel d'utilisateur Page 192