IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

74 IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

Example 3-11 LUN allocation from Extent Pool

dscli> lsextpool -l

Date/Time: 26 July 2005 6:33:42 IBM DSCLI Version: 5.0.5.6 DS: IBM.1750-1301234

Name ID stgtype rankgrp status availstor (2^30B) %allocated available reserved numvols numranks

====================================================================================================

ITSO_P0 P0 fb 0 exceeded 773 0 773 0 0 1

dscli> mkfbvol -extpool p0 -cap 50 -name ITSO_50GB_#h 0001

Date/Time: 26 July 2005 6:35:12 IBM DSCLI Version: 5.0.5.6 DS: IBM.1750-1301234

CMUC00025I mkfbvol: FB volume 0001 successfully created.

dscli> lsextpool -l

Date/Time: 26 July 2005 6:35:23 IBM DSCLI Version: 5.0.5.6 DS: IBM.1750-1301234

Name ID stgtype rankgrp status availstor (2^30B) %allocated available reserved numvols numranks

====================================================================================================

ITSO_P0 P0 fb 0 exceeded 723 6 723 0 1 1

dscli>

3.5.8 Preferred paths

See also 2.10.4, “Preferred Path” on page 49.

In the DS6000, host ports have a fixed assignment to a server (or controller card). In other

words, all data traffic that uses its preferred path to a server avoids having to cross through

the DS6000 inter-server connection to the other server. There is a small performance penalty

if data from a logical volume managed by one server is accessed from a port that is located

on the other server. The request for the logical volume and the data would have to be

transferred across the bridge interface that connects both servers. These transfers add some

latency to the response time. Furthermore, this interface carries other communication traffic

between the servers, such as being used to mirror the persistent memory and for other

inter-server communication. It could become a bottleneck if too many normal I/O requests

also run across it, although it is a high bandwidth, low latency, PCI-X connection.

Open systems hosts should ensure that they use multipath management software such as

IBM’s Multipath Subsystem Device Driver (SDD) that recognizes this preferred path usage

and can preferentially direct I/O requests to the preferred path.

When assigning host ports for open systems usage, always consider preferred pathing,

because the use of non-preferred paths will have a performance impact on your DS6000.

z/OS users already have this preferred path management capability inherent in the z/OS

operating system.

3.6 Performance and sizing considerations

To determine the most optimal DS6000 layout, the I/O performance requirements of the

different servers and applications should be well understood before you start designing the

disk subsystem configuration. These requirements will play a large part in dictating both the

physical and logical configuration of the disk subsystem.

3.6.1 Workload characteristics

The answers to questions like how many host connections do I need?, and how much cache

do I need?

and the like, always depend on the workload requirements (such as, how many

I/Os per second per server, how many I/Os per second per gigabyte of storage, and so forth).

Attention: It is important to balance the allocation of your logical volumes across both

storage servers (server0 and server1) to reduce the potential for I/O imbalance.