1-46 IBM Informix OnLine Database Server Administrator’s Guide
OnLine Disk Space Allocation
Criticaltables and databases should be mirrored. The root dbspace should be
mirrored. Mirroring is specified by chunk. Locate the primary and the
mirrored chunk on different disks. Ideally, different controllers should
handle the disks.
Youcan place a table with high I/O activity on a disk device that is dedicated
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 for disk access when joins are formed between two high-
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
locatedon 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
asclose to the middle partitions of the disk as possible. (When a disk device
is partitioned, the innermost partitions have cylinder numbers that are
nearestthe middle of the range of cylinder numbers and generally experience
thefastest access time.) Place the least frequently used data on the outermost
partitions. This overall strategy minimizes disk head movement.
When two or more large, growing tables share a dbspace, their new extents
canbe 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.
Boththe logical log files and the physical log are extremely active and should
begiven priority in disk placement. Both should be on the fastest devices and
on the most central disk cylinders.
Theinitial configuration automatically places the physical and logical logs in
theinitial chunk of the root dbspace. Since the root dbspace also is extremely
active,you can place the root dbspace on the most central disk cylinder and
createother dbspaces for user database tables. Another strategy is to improve
performanceby physically separating the logs and placing them in separate
dbspaceson disk devices that are not shared by active tables. For instructions
on how to change the location of the logical and physical log after initial-
ization, refer topage 3-31 and page 3-107, respectively.