Controller Management
RC=ONOFF enables the Read Continuous (RC) option for Audio/Video streaming data; the system starts the data transfer for read operations after RCTIMEOUT is reached, even if the disks commands have not finished. Use this to reduce the latency for read operations in Audio/Visual environments where latency is more important than data integrity. This parameter is saved on a
WARNING! | This feature allows the system to return invalid data to the initiator. |
RCTIMEOUT=x Default setting: disables the Read Continuous option for Audio/Video streaming data.
Note: This parameter is saved on a
LUN=x sets the host command timeout for the Read Continuous option for Audio/Video streaming data. Set to 0 to disable the Read Continuous feature in the system. This value is in 100 millisecond increments. The range for 'x' is 0 to 255. The default is 8.
FAILCC=ON instructs the host ports to report a check condition for all SCSI commands when the unit is in a failed state. This command should only be used in AV environments when a check condition is required instead of taking the unit off the loop.
FAILCC=OFF This is the default setting. Host ports will NOT report a check condition for all SCSI commands when the unit is in a failed state.
3.3.3Locking LUN in Cache
Locking a LUN in data cache will keep all of the data for the LUN in the cache for faster access. Once a LUN is locked, the data that is gathered to service read and write commands will stay permanently in the cache. The controller will continue to fill up the cache until 50% of the total cache is filled with data from locked LUNs, while the other 50% of the cache is reserved to service I/O for unlocked LUNs.
Initial Cache
50% of Data Cache used to service Unlocked LUNs
50% of Data Cache used to service Locked LUNs
For example, when a host issues a read command for data from LUN 1 that has been locked in cache, the following will occur:
•The controller reads data from disks, locks data in cache, and sends data to host
•Any reads of the same data will be serviced from cache, which provides faster access than reading from disks.
Cache allocation after I/O completes
Unlocked LUN data | Unallocated | Data for | |
cache* | LUN 1 | ||
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*Unallocated cache can be used for unlocked LUNs’ or locked LUNs’ data. Once cache has been allocated to a locked LUN, however, it cannot be used by an unlocked LUN.
Once the size of the locked LUNs exceeds 50% of the total cache, the controller must create cache space to process a new I/O by removing older data from the locked portion of cache. The Least Recently Used (LRU) algorithm is used to determine which locked data to remove from cache.
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