Acer SmartRAID V Bedienungsanleitung PDF herunterladen (Seite 176)

7-8

Theory of Operation

HEAVILY LOADED SYSTEM

Without Hardware Cache

Application programs write to OS cache:

Bus

activity:

OS cache flushes to disk (~11ms per write)

Idle

time

Idle

time

HEAVILY LOADED SYSTEM

With Hardware Cache

Application

programs

write to

OS cache

System Bus

activity

OS cache flushes

to SmartRAID

(1ms per write)

Idle

time

Idle

time

SmartRAID

Bus

SmartRAID

elevator-sorted writes to disk

(1-5ms per write)

As an example, consider an operating system with 4MB of dirty data in its

cache buffers. With a 4KB record size and an average disk I/O time of

11ms per record, this flush operation would take 11 seconds. However, with

controller cache installed the entire flush operation from the software

cache would take approximately 1 second instead of 11 seconds. The

SmartRAID V controller can then flush its cache to disk in elevator-sorted

order using its separate I/O bus, concurrently with other system activity.

Elevator Sorting

SmartRAID V cache flush operations occur much faster than from the

operating system cache, because all data in the controller cache is kept in

elevator sorted order. Data is written to disk in order of increasing block

address, minimizing both physical disk seeks and rotational latency. This

feature, called Elevator Sorted Write-back, eliminates much of the head

thrashing commonly associated with disk-intensive operating systems like

Novell NetWare, Windows NT and UNIX. By elevator sorting the sectors in

cache, the average write access time to the drive may be reduced from the

normal 11ms (seek + rotation) time, to between 1 and 5ms.

A good analogy is an elevator in a multistory building. Consider how much

slower the elevator would be if it went to the requested floors in the same

sequence that the buttons were pressed, instead of in floor number

sequence! An additional benefit of elevator sorting is that multiple short

disk write operations can often be coalesced within the controller cache

into one large write operation, significantly reducing peripheral bus

overhead.

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