Use Linux as a SAN Provider
Another feature often run with iSCSI is multipathing. This allows Linux to use multiple networks at once to access the iSCSI target. It usually is run on separate physical networks, so in the event that one fails, the other still will be up and the initiator will not experience loss of a volume or a system crash. Multipathing can be set up in two ways, either active/passive or active/active. Active/active generally is the preferred way, as it can be set up not only for redundancy, but also for load balancing. Like Fibre Channel, multipath assigns World Wide Identifiers (WWIDs) to devices. These are guaranteed to be unique and unchanging. When one of the paths is removed, the other one continues to function. The initiator may experience slower response time, but it will continue to function. Re-integrating the second path allows the system to return to its normal state.
When working with local disks, people often turn to Linux's software RAID or LVM systems to provide redundancy, growth and snapshotting. Because SAN volumes show up as block devices, it is possible to use these tools on them as well. Use them with care though. Setting up RAID 5 across three iSCSI volumes causes a great deal of network traffic and almost never gives you the results you're expecting. Although, if you have enough bandwidth available and you aren't doing many writes, a RAID 1 setup across multiple iSCSI volumes may not be completely out of the question. If one of these volumes drops, rebuilding may be an expensive process. Be careful about how much bandwidth you allocate to rebuilding the array if you're in a production environment. Note that this could be used at the same time as multipathing in order to increase your bandwidth.
To set up RAID 1 over iSCSI, first load the RAID 1 module:
After partitioning your first disk, /dev/sdb, copy the partition table to your second disk, /dev/sdc. Remember to set the partition type to Linux RAID autodetect:
sfdisk -d /dev/sdb | sfdisk /dev/sdc
Assuming you set up only one partition, use the mdadm command to create the RAID group:
mdadm --create /dev/md0 --level=1 --raid-disks=2 /dev/sdb1 /dev/sdc1
After that, cat the /etc/mdstat file to watch the state of the synchronization of the iSCSI volumes. This also is a good time to measure your network throughput to see if it will stand up under production conditions.
Running a SAN on Linux is an excellent way to bring up a shared environment in a reasonable amount of time using commodity parts. Spending a few thousand dollars to create a multiterabyte array is a small budget when many commercial arrays easily can extend into the tens to hundreds of thousands of dollars. In addition, you gain flexibility. Linux allows you to manipulate the underlying technologies in ways most of the commercial arrays do not. If you're looking for a more-polished solution, the Openfiler Project provides a nice layout and GUI to navigate. It's worth noting that many commercial solutions run a Linux kernel under their shell, so unless you specifically need features or support that isn't available with standard Linux tools, there's little reason to look to commercial vendors for a SAN solution.
Michael Nugent has spent a good deal of his time designing large-scale solutions to fit into tiny budgets, leveraging Linux to fulfill roles that typically would be filled by large commercial appliances. Recently, Michael has been working to design large, private clouds for SaaS environments in the financial industry. When not building systems, he likes sailing, scuba diving and hanging out with his cat, MIDI. Michael can be reached at firstname.lastname@example.org.
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