Red Hat / CentOS Linux 4: Setup Device Mapper Multipathing

Posted on in Categories CentOS, data center, fedora linux, File system, Hardware, High performance computing, Howto, kernel, Linux, Linux Scalability, Linux Virtualization, Networking, RedHat/Fedora Linux, Tips last updated July 6, 2008

Multipath I/O is a fault-tolerance and performance enhancement technique whereby there is more than one physical path between the CPU in a computer system and its mass storage devices through the buses, controllers, switches, and bridge devices connecting them.

A simple example would be a SCSI disk connected to two SCSI controllers on the same computer or a disk connected to two Fibre Channel ports. Should one controller, port or switch fail, the operating system can route I/O through the remaining controller transparently to the application, with no changes visible to the applications, other than perhaps incremental latency.

This is useful for:

  1. Dynamic load balancing
  2. Traffic shaping
  3. Automatic path management
  4. Dynamic reconfiguration

Linux device-mapper

In the Linux kernel, the device-mapper serves as a generic framework to map one block device onto another. It forms the foundation of LVM2 and EVMS, software RAIDs, dm-crypt disk encryption, and offers additional features such as file-system snapshots.

Device-mapper works by processing data passed in from a virtual block device, that it itself provides, and then passing the resultant data on to another block device.

How do I setup device-mapper multipathing in CentOS / RHEL 4 update 2 or above?

Open /etc/multipath.conf file, enter:
# vi /etc/multipath.conf
Make sure following line exists and commented out:

devnode_blacklist {
        devnode "*"
}

Make sure default_path_grouping_policy option in the defaults section set to failover. Here is my sample config:

defaults {
       multipath_tool  "/sbin/multipath -v0"
       udev_dir        /dev
       polling_interval 10
       default_selector        "round-robin 0"
       default_path_grouping_policy    failover
       default_getuid_callout  "/sbin/scsi_id -g -u -s /block/%n"
       default_prio_callout    "/bin/true"
       default_features        "0"
       rr_min_io              100
       failback                immediate
}

Save and close the file. Type the following command to load drivers:
# modprobe dm-multipath
# modprobe dm-round-robin

Start the service, enter:
# /etc/init.dmultipathd start
multipath is used to detect multiple paths to devices for fail-over or performance reasons and coalesces them:
# multipath -v2
Turn on service:
# /sbin/chkconfig multipathd on
Finally, create device maps from partition tables:
# kpartx -a /dev/mapper/mpath#
You need to use fdisk on the underlying disks such as /dev/sdc.

References:

  • man page kpartx,multipath, udev, dmsetup and hotplug

How To Avoid Sudden Outburst Of Backup Shell Script or Program Disk I/O on Linux

Posted on in Categories High performance computing, kernel, Linux, Storage, Troubleshooting, Tuning last updated June 3, 2008

A sudden outburst of violent disk I/O activity can bring down your email or web server. Usually, a web, mysql, or mail server serving millions and millions pages (requests) per months are prone to this kind of problem. Backup activity can increase current system load too. To avoid this kind of sudden outburst problem, run your script with scheduling class and priority. Linux comes with various utilities to manage this kind of madness.