Acer SmartRAID V Uživatelský manuál stáhnout pdf (Strana 179)

7-11

Theory of Operation

Pre-Fetch

The SmartRAID V pre-fetch algorithm analyses disk access patterns to

detect whether the host is reading data that it previously read. By using

records stored in virtual cache, the caching algorithm determines the

boundaries of the contiguous block of previously read data surrounding

the current host read request, then reads that data block into cache.

Predictive Caching in Multi-user Systems

Disk read-ahead is effective in a multi-user environment when a large

number of read-ahead sectors from many different areas of the disk are

cached. This is because multi-user and networking operating systems

contain highly fragmented files and time-slice I/O requests from many

different users or tasks. Data that has been cached by a read-ahead

operation may not be accessed until after other queued commands are

processed and the same user or task has another chance to access the

disk.

In addition, patterns of sequential I/O are harder to detect in large multi-

user systems because they are typically interleaved with I/O from many

users. This is why the read-ahead FIFO buffer on a drive (which can store

only a limited number of data segments) provides acceptable performance

in a single-user system, but must be augmented by the larger cache on the

SmartRAID V controller in a system with multiple users.

AUTOMATIC READ-AHEAD

IS PERFORMED BY DRIVE

Read-Ahead dataRead data

Read-Ahead data is stored

in FIFO buffer on SCSI drive.

Command

terminates

here

READ-AHEAD SEGMENTS

ARE CACHED IN BACKGROUND

40MB/s Peripheral Bus

Read

Ahead

FIFOs

HARDWARE

DISK

CACHE

Stores

thousands of

Read-Ahead

segments

DRIVE DRIVE DRIVE DRIVE

1 2 ... 174 175 176 177 178 179 180 181 182 183 184 ... 255 256

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Acer SmartRAID V Uživatelský manuál Strana 179

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