Acer SmartRAID V Bedienungsanleitung PDF herunterladen (Seite 177)

7-9

Theory of Operation

DISK WRITES

Without Elevator Sorting:

11ms average

disk write

DISK WRITES

With Elevator Sorting:

1-5ms average

disk write

Caching

SmartRAID V controllers can operate in write-back or write-through

mode and also use a predictive caching algorithm to optimize disk I/O for

the host system. This section describes the methods and rules used by

SmartRAID V controllers to cache data for host I/O operations.

Write-back and Write-through Algorithm

Write-back is a method of postponing data to be written to a peripheral

device, such as a disk drive, by temporarily saving the data in a cache. The

data can then be written at a later time when the device would otherwise

be idle. In Write-back mode, writes to disk are deferred until after

command completion. This type of caching generally provides better I/O

performance.

In addition, the data can be processed by techniques such as elevator

sorting, to increase write operation performance.

In Write-through mode, all data is written immediately to disk for each

write command before command complete status is returned to the host.

The data may also be held in cache for subsequent read requests.

NOTE When you first install a PM3755U2B controller with a BB4050

module, automatic low-battery write-through mode is disabled.

Use the Battery Backup Configuration dialog in Storage

Manager to set a threshold for when write-through mode

should be used if the battery backup capability drops below a

predetermined level.

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Acer SmartRAID V Bedienungsanleitung Seite 177