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Dell PowerEdge Skrócona instrukcja obsługi

Storage

calculations; and support for an Intelligent

I/O (I2O) message unit helps maximize

performance. The controller is powered

by a state-of-the-art LSI 1078 ROC con-

troller, which includes both instruction

and data cache to help provide an addi-

tional performance boost. This solid hard-

ware foundation is designed to simplify

integration of new RAID firmware features

and performance increases in future

release updates.

caching policies

Flexible caching policies in PERC 6 control-

lers allow administrators to tune cache writ-

ing schemes to help optimize performance

or maximize data protection, including

choosing between write-through caching

and write-back caching. With write-through

caching enabled, the controller transfers a

block of data directly to the disk, and does

not send an acknowledgment of data trans-

fer completion to the host system until the

data is committed to the disk subsystem.

With write-back caching enabled, the con-

troller sends a data transfer completion

signal to the host when the controller cache

has received all data in a transaction, and

the controller then writes the cached data

to the storage device when system activity

is low or when the write buffer approaches

capacity; the cached data is not written to

the storage device immediately.

Write-through caching typically pro-

vides a data security advantage over

write-back caching, while write-back

caching typically provides a performance

advantage. The risk of using write-back

As enterprise data requirements con-

tinue to increase, deploying powerful,

easy-to-manage RAID solutions can

become essential for organizations

seeking to simplify their IT environ-

ments while increasing controller and

storage reliability and server uptime."

DELL POWER SOLUTIONS | May 2008

caching is that the cached data can be

lost if a power failure occurs before the

data is written to the storage device. This

risk can be mitigated by using the battery

backup units on PERC 6 controllers.

read and I/o policies

Read policies determine the type of read

option for a logical drive and can be set

to normal (the default setting), read-

ahead, or adaptive. The normal setting

specifies that the controller reads only the

requested data and does not read ahead

for the current logical drive. The read-

ahead setting specifies that the controller

reads sequentially ahead of requested data

and stores the additional data in cache

memory, anticipating that the data will be

needed soon; this policy typically supplies

sequential data faster than the normal (no-

read-ahead) setting, but is not as effective

when accessing random data. The adaptive

setting specifies that the controller begins

using read-ahead if the two most recent

disk accesses occurred in sequential sec-

tors. If all read requests are random, the

algorithm reverts to the normal setting;

however, requests are still evaluated for

possible sequential operation.

I/O policies are used to enable read

buffering in cache memory and apply to

reads on a specific logical drive. This set-

ting does not affect the read-ahead cache.

The cached I/O setting specifies that the

controller buffers reads in cache memory,

while the direct I/O setting specifies that

reads and writes are not buffered in cache

memory. This setting does not override

the read policy settings; data is trans-

ferred to cache and the host concurrently.

If the same data block is read again, it

comes from cache memory.

Software features

Enabling disk caching can have a signifi-

cant impact on system performance, but

the potential for data loss increases if

power to the disk drives is lost while data

written to the disk is still in the cache.

Native command queuing (NCQ) for SATA

and tagged command queuing (TCQ) for

SAS hard drives helps increase perfor-

mance by allowing individual hard disks

to optimize the order in which read and

write commands are executed.

SImplIfIed management

PERC 6 controllers integrate technologies

and applications designed to simplify

management for IT administrators. For

example, SAS disk connectivity capabili-

ties have made it increasingly common to

connect a large number of disks to a PERC

controller, and PERC 6 controllers include

features that can automatically track disk

order and organization through disk roam-

ing. PERC 6 controllers also support

moving physical disks from one cable con-

nection or backplane slot to another on

the same controller; the controllers can

automatically recognize the relocated

physical disks and logically place them in

the appropriate virtual disks that are part

of the disk group.

Other advanced online capabilities

allow administrators to migrate disks from

one controller to another. The Advanced

Import feature enables administrators to

import, or migrate, disk arrays created

by a different PERC controller while the

server remains online. The controllers can

even import partial, or degraded, arrays

and provide array preview capabilities

before import to further enhance data

serviceability.

Dell PERC 6 controllers utilize Smart

Cache Technology that offers administra-

tors pre-tuned cache ratios for read and

write operations that automate cache