Chapter 3. IBM System Storage DS3500 Storage System planning tasks
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Because a DS3500 Storage System has its own RAID arrays and logical volumes, we do not
work with real physical disks in the system. Functions, such as intra-disk allocation, write
scheduling, and write-verify policies, do not help much, and it is hard to determine the
performance benefits when using them. They must only be used after additional testing, and it
is not unusual that trying to use these functions will lead to worse results.
On the other hand, we must not forget about the important inter-disk allocation policy.
Inter-disk allocation policy
The inter-disk allocation policy is used to specify the number of disks and how the logical
partitions (LPs) are placed on specified physical volumes, which is also referred to as
range
of physical volumes
in the
smitty mklv
screen:
With an inter-disk allocation policy of minimum, LPs are placed on the first PV until it is full,
then on the second PV, and so on.
With an inter-disk allocation policy of maximum, the first LP is placed on the first PV listed,
the second LP is placed on the second PV listed and so on, in a round robin fashion.
By setting the inter-physical volume allocation policy to maximum, you also ensure that the
reads and writes are shared among PVs, and in systems such as a DS3500 Storage System,
also among controllers and communication paths.
If systems are using only one big volume, it is owned by one controller, and all the traffic goes
through one path only, which happens because of the static load balancing that DS3500
controllers use.
3.5.2 Planning for systems without LVM: Windows example
Today, the Microsoft Windows operating system does not have a powerful LVM such as
certain UNIX systems. Distributing the traffic among controllers in such an environment might
be a little bit harder. Actually, Windows systems have an integrated reduced version of Veritas
Volume Manager (also known as Veritas Foundation Suite) called Logical Disk Manager
(LDM), but it does not offer the same flexibility as regular LVM products. The integrated LDM
version in Windows is used for the creation and use of
dynamic disks
.
With Windows 2003 and 2008, there are two types of disks: basic disks and dynamic disks.
By default, when a Windows system is installed, the basic disk system is used.
Basic disks and basic volumes are the storage types most often used with Microsoft Windows
operating systems. A basic disk refers to a disk that contains basic volumes, such as primary
partitions and logical drives. A basic volume refers to a partition on a basic disk. For Windows
2003 and 2008, a primary partition on a basic disk can be extended using the
extend
command in the diskpart.exe utility.
Dynamic disks provide features that basic disks do not, such as the ability to create volumes
that span multiple disks (spanned and striped volumes), as well as the ability to create
software level fault tolerant volumes (mirrored and RAID 5 volumes). All volumes on dynamic
disks are known as
dynamic volume
s.
Best practice: For random I/O, the best practice is to create arrays of the same type and
size. For applications that do not spread I/Os equally across containers, create VGs
comprised of one logical drive from every array, use a maximum inter-disk allocation policy
for all LVs in the VG, and use a random disk order for each LV. Check that the ownership of
logical drives selected are spread evenly across DS3500 controllers. Applications that
spread their I/Os equally across containers, such as DB2®, use another layout.
Summary of Contents for DS3500
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