Disk preinstallation guidelines

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The mechanisms for defining and expanding logical volumes attempt to make the best possible default choices. However, satisfactory disk-I/O performance is much more likely if the installer of the system tailors the size and placement of the logical volumes to the expected data storage and workload requirements.

Recommendations are as follows:

  • If possible, the default volume group, rootvg, should consist only of the physical volume on which the system is initially installed. Define one or more other volume groups to control the other physical volumes in the system. This recommendation has system management, as well as performance, advantages.
  • If a volume group consists of more than one physical volume, you may gain performance by:
    • Initially defining the volume group with a single physical volume.
    • Defining a logical volume within the new volume group. This definition causes the allocation of the volume group's journal logical volume on the first physical volume.
    • Adding the remaining physical volumes to the volume group.
    • Defining the high-activity file systems on the newly added physical volumes.
    • Defining only very-low-activity file systems, if any, on the physical volume containing the journal logical volume. This affects performance only if I/O would cause journaled file system (JFS) log transactions.

      This approach separates journaled I/O activity from the high-activity data I/O, increasing the probability of overlap. This technique can have an especially significant effect on NFS server performance, because both data and journal writes must be complete before NFS signals I/O complete for a write operation.

  • At the earliest opportunity, define or expand the logical volumes to their maximum expected sizes. To maximize the probability that performance-critical logical volumes will be contiguous and in the desired location, define or expand them first.
  • High-usage logical volumes should occupy parts of multiple disk drives. If the RANGE of physical volumes option on the Add a Logical Volume screen of the SMIT program (fast path: smitty mklv) is set to maximum, the new logical volume will be divided among the physical volumes of the volume group (or the set of physical volumes explicitly listed).
  • If the system has drives of different types (or you are trying to decide which drives to order), consider the following guidelines:
    • Place large files that are normally accessed sequentially on the fastest available disk drive.
    • If you expect frequent sequential accesses to large files on the fastest disk drives, limit the number of disk drivers per disk adapter.
    • When possible, attach drives with critical, high-volume performance requirements to a high speed adapter. These adapters have features, such as back-to-back write capability, that are not available on other disk adapters.
    • On the smaller disk drives, logical volumes that will hold large, frequently accessed sequential files should be allocated in the outer_edge of the physical volume. These disks have more blocks per track in their outer sections, which improves sequential performance.
    • On the original SCSI bus, the highest-numbered drives (those with the numerically largest SCSI addresses, as set on the physical drives) have the highest priority. Subsequent specifications usually attempt to maintain compatibility with the original specification. Thus, the order from highest to lowest priority is as follows: 7-6-5-4-3-2-1-0-15-14-13-12-11-10-9-8.

      In most situations this effect is not noticeable, but large sequential file operations have been known to exclude low-numbered drives from access to the bus. You should probably configure the disk drives holding the most response-time-critical data at the highest addresses on each SCSI bus.

      The lsdev -Cs scsi command reports on the current address assignments on each SCSI bus. For the original SCSI adapter, the SCSI address is the first number in the fourth pair of numbers in the output. In the following output example, one 400 GB disk is at SCSI address 4, another at address 5, the 8mm tape drive at address 1, and the CDROM drive is at address 3. 
      cd0    Available 10-80-00-3,0 SCSI Multimedia CD-ROM Drive
hdisk0 Available 10-80-00-4,0 16 Bit SCSI Disk Drive
hdisk1 Available 10-80-00-5,0 16 Bit SCSI Disk Drive
rmt0   Available 10-80-00-1,0 2.3 GB 8mm Tape Drive
    • Large files that are heavily used and are normally accessed randomly, such as databases, must be spread across two or more physical volumes.