IBM DS8000 manual Managing multiple paths, Limited number of Scsi devices

After creating the device files you may have to change their owner, group, and file permission settings to be able to use them. Often, the easiest way to do this is by duplicating the settings of existing device files, as shown in Example A-11.Be aware that after this sequence of commands, all special device files for SCSI disks have the same permissions. If an application requires different settings for certain disks, you have to correct them afterwards.

Example: A-11 Duplicating the permissions of special device files

knox:~ # ls -l				/dev/sda	/dev/sda1
rw-rw----			1	root	disk	8,	0 2003-03-14	14:07	/dev/sda
rw-rw----			1	root	disk	8,	1 2003-03-14	14:07	/dev/sda1
knox:~	#	chmod		660 /dev/sd*
knox:~	#	chown		root:disk /dev/sda*

Managing multiple paths

If you assign a DS8000 volume to a Linux system through more than one path, it will see the same volume more than once. It will also assign more than one special device file to it. To utilize the path redundancy and increased I/O bandwidth, you need an additional layer in the Linux disk subsystem to recombine the multiple disks seen by the system into one, to manage the paths and to balance the load across them.

The IBM multipathing solution for DS8000 attachment to Linux on Intel IA-32 and IA-64 architectures, IBM pSeries and iSeries is the IBM Subsystem Device Driver (SDD) (see 15.2, “Subsystem Device Driver” on page 324). SDD for Linux is available in the Linux RPM package format for all supported distributions from the SDD download site. It is proprietary and binary only. It only works with certain kernel versions with which it was tested. The README file on the SDD for Linux download page contains a list of the supported kernels.

The version of the Linux Logical Volume Manager that comes with all current Linux distributions does not support its physical volumes being placed on SDD vpath devices.

SDD is not available for Linux on zSeries. SUSE Linux Enterprise Server 8 for zSeries comes with built-in multipathing provided by a patched Logical Volume Manager. Today there is no multipathing support for Redhat Enterprise Linux for zSeries.

Limited number of SCSI devices

Due to the design of the Linux SCSI I/O subsystem in the Linux Kernel version 2.4, the number of SCSI disk devices is limited to 256. Attaching devices through more than one path reduces this number. If, for example, all disks were attached through 4 paths, only up to 64 disks could be used.

Important: The latest update to the SUSE Linux Enterprise Server 8, Service Pack 3, uses a more dynamic method of assigning major numbers and allows the attachment of up to 2304 SCSI devices.

SCSI device assignment changes

Linux assigns special device files to SCSI disks in the order they are discovered by the system. Adding or removing disks can change this assignment. This can cause serious problems if the system configuration is based on special device names (for example, a file system that is mounted using the /dev/sda1 device name). You can avoid some of them by using:

￿Disk Labels instead of device names in /etc/fstab

￿LVM Logical Volumes instead of /dev/sd.. devices for file systems

360DS8000 Series: Concepts and Architecture