IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

Chapter 7. Open systems servers - UNIX 233

7.8 Volume groups, logical volumes and file systems

Logical disks created on the DS6000 and presented to host systems are often referred to as

logical volumes. Unfortunately, this can cause confusion when we move over to the UNIX

operating system because here, too, we create logical volumes. The DS6000 logical volume

and the UNIX operating system logical volume are two separate things. So, in this section, we

will refer to the logical volumes created on the DS6000 as LUNs. And we will use operating

system LVM commands to create logical volumes using the available DS6000 LUNs.

In the following sections, we will explore the considerations associated with creating volume

groups, logical volumes, and file systems. We will use the AIX Logical Volume Manager (LVM)

to create examples, but the topics discussed are applicable to all UNIX platforms.

Remember the

recommended method discussed in 7.2.1, “I/O balanced across Extent Pools”

on page 191? We build upon that concept as we continue with the LVM configuration. So at

this point, the LUNs have already been created in the DS6000 and assigned to your host

system. The following sections explore what we need to do next.

LVM setup for AIX proceeds in the following order:

򐂰 Create the volume group.

򐂰 Create the logical volume.

򐂰 Create the file systems and file system logs.

7.8.1 Creating the volume group

A volume group is a logical collection of LUNs. The first question to consider is how many

volume groups (VGs) you need. Here are some volume group recommendations:

򐂰 Normally, one volume group for a system is adequate and provides the most flexibility for

data layout is the simplest to manage. The general recommendation is that when you

assign one LUN per Array to your host system, put all the LUNs in the same volume group.

Doing this is convenient later when creating logical volumes because you can just select

all the LUNs in the volume group to stripe across.

We had a lively debate about the above recommendation. The word

normally was a

compromise. There are situations where you should really make the above

recommendation a strict rule, like if you are planning to use traditional logical volume

striping. And there are circumstances where you are planning to use inter-disk logical

volumes and one LUN from every Rank resulted in way more storage than the application

server required and all the logical volumes have roughly the same I/O rates. The idea we

will say time and again is that, to get the best performance you have to do two things:

– Get all the spindles in the DS6000 working for you

– Distribute your I/O load evenly across all the spindles to avoid hot spots that will slow

you down and everybody else using storage on the hot spot.

Note: Note that the prealloc command for HP-UX and the lmktemp command for AIX have

a 2 GB size limitation. Those commands are not able to create a file greater than 2 GB in

size. If you want a file larger than 2 GB for a sequential read test, concatenate a couple of

2 GB files together.

Note: For AIX 5.2 and beyond, JFS2 and the 64 bit kernel are recommended. Note that the

nointegrity filesystem mount option is not supported in JFS2.