IBM 000-8697 Manual de usuario descargar pdf (Pagina 80)

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1-46 IBM Informix OnLine Database Server Administrator’s Guide

OnLine Disk Space Allocation

Criticaltablesanddatabasesshould bemirrored.Therootdbspaceshouldbe

mirrored. Mirroring is speciﬁed by chunk. Locate the primary and the

mirrored chunk on different disks. Ideally, different controllers should

handle the disks.

You can placeatablewithhigh I/O activityon a diskdevice that isdedicated

to its use. When disk drives have different performance levels, you can put

the tables with the most use on the fastest drives. Separate disk devices

reducecompetition fordisk access when joins are formedbetween twohigh-

demand tables.

To reduce contention between programs using the same table, you can

attempt to spread the table across multiple devices. To do this, locate a

tblspace in a dbspace that includes multiple chunks, each of which are

located on different disks. Although you have no control over how the table

data is spread across the chunks, this layout might result in multiple disk

access arms for one table.

To minimize disk head movement, place the most frequently accessed data

as close to the middle partitions of the disk as possible. (When a disk device

is partitioned, the innermost partitions have cylinder numbers that are

nearestthemiddleoftherangeofcylindernumbersandgenerallyexperience

thefastest access time.)Placetheleast frequently used dataon the outermost

partitions. This overall strategy minimizes disk head movement.

When two or more large, growing tables share a dbspace, their new extents

can be interleaved. This interleaving creates gaps between the extents of any

one table. Performance might suffer if disk seeks must span more than one

extent. Work with the table owners to optimize the table extent sizes, or

consider placing the tables in separate dbspaces.

Boththelogicallogﬁlesandthephysicallogareextremelyactiveandshould

begivenpriorityindiskplacement.Bothshouldbeonthefastestdevices and

on the most central disk cylinders.

Theinitialconﬁgurationautomaticallyplacesthephysical and logical logs in

the initial chunkofthe root dbspace. Sincetherootdbspace also isextremely

active, you can place the root dbspace on the most central disk cylinder and

createotherdbspacesforuserdatabasetables.Anotherstrategyistoimprove

performance by physically separating the logs and placing them in separate

dbspacesondiskdevicesthatarenotsharedbyactivetables.Forinstructions

on how to change the location of the logical and physical log after initial-

ization, refer to page 3-31 and page 3-107, respectively.

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