December 5, 2009

Day 5 - Storage Availability: DRBD

This article was written by Jeff Hengesbach.

Storage and storage technologies have really moved into the limelight in recent years. Previously, advanced storage and storage concepts were hidden away in large corporate datacenters; they were complicated, proprietary, and costly. Most small and mid-sized organizations used or still use systems with local or direct attached hard drives. In the event of drive failure, RAID was relied upon as the sole savior to keep things running uninterrupted. In the event of array or system loss, restore from backups was the primary or only option. The drives were expensive and storage needs modest (arguably due to storage costs).

Move forward a few years to the now. Storage needs are, relatively speaking, monumental, and space is much less costly. Virtualization has come to the forefront, and you are now behind the proverbial technology curve if not taking advantage of its benefits. System densities have increased (blades, virtualization, etc) and now, more than ever, storage is a critical component of the infrastructure. Most of these density-increasing technologies that improve space, power consumption, cooling, management, disaster recovery, and availability, require carefully designed, reliable, and centralized storage. Reliable storage starts with good drives and controllers with appropriate RAID levels for the application as well as connectivity to that storage. What happens when a controller fails, or a multiple drive failure brings down a RAID set, or connectivity is lost? The proprietary solutions of course have their options and some very nice options indeed, but for some organizations that need storage availability, many of the proprietary solutions do not scale to the available budgets.

This is where DRBD comes to the table. DRBD is graciously maintained, supported, and provided as open source by the people over at Linbit. As the website states: "DRBD can be understood as network based raid-1". Technically speaking, DRBD is a block device driver you layer into the device chain just like LVM or MD. It can be put anywhere in the stack: on top of the disk device, on top of LVM or MD, at the top just under the file-system, wherever it makes sense. Its specialty is taking block level changes, keeping track of them, and sending them over a network to another system where they are duplicated. DRBD has a few specific traits to highlight. First off, it is smart (and dumb?). It works at the block level and knows which bits may be out of sync and will only send those bits across the wire. It knows nothing of file-systems, files, etc, so the application using it can be anything: samba, iSCSI, NFS, EXT3, XFS, or others. Secondly it can be non-destructively added to existing data volumes. It is not necessary to go through backup/install/restore process - only install, configure and replicate.

Linbit also offers a closed source product, DRBD Proxy, that is designed for long haul, high latency(200ms) connections as well as greater than 2-node replication situations. If you want to replicate outside of a LAN using DRBD, then you'll need it. Recently, Linbit became the steward of the Heartbeat cluster messaging codebase. Heartbeat is the application that handles the monitoring and fail-over activities between the systems. DRBD and Heartbeat are a tried and true tag team for storage high availability / fail-over situations.

So what does a simple, highly available, solution look like? Two nodes with appropriate storage use DRBD in their block device stacks to replicated the data over a gigabit or better connection; one is primary (the source), the other secondary (the destination). Heartbeat runs on both systems monitoring for system and service availability. If Heartbeat detects a failure it coordinates the startup of services on the backup node including all DRBD and service (could be nfs, samba, iscsi, etc) related commands. It is a beautiful thing to see in action.

I'm not one to rehash out the commands needed to install, configure and run these solutions. The DRBD website has an excellent user manual that would be absolutely futile to reproduce here, and Heartbeat is relatively simplistic to get established and working. Both products have a multitude of options not necessary for basic operations but are very handy for more fine grained control and tuning.

What are the keys parts to a good solution? DRBD relies on TCP/IP networking to move data around. Make sure gigabit or better is being used along with quality cables and switching. If at all possible keep DRBD traffic on its own network - or even direct cross-over connections. Don't skimp on your backup node. A backup node that can not keep up with replication will cause IO blocking on the primary. Design connectivity to handle loss of a switch. If in doubt - ask Linbit or the community for assistance. DRBD, DRBD+Heartbeat have been in use for a while and they are sound technologies.


About the author: Jeff Hengesbach is a System Administrator with a history of working in smaller organizations. He is an avid proponent of virtulization and linux (and Microsoft, where appropriate) technologies. Between work and family he occasionally has the opportunity to post articles on his blog at:

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Anonymous said...

I think DRBD is excellent for small-scale solution. I wasn't sure if it is possible to scale in a large environment though, in cases like you have 100 machines making up a single logical storage.

Anonymous said...

Anon: it scales only to a few nodes, but most often people will run HA-iSCSI between two nodes, and let their 100 nodes access the iSCSI target. Other designs include 100 pairs of servers, each redundant and replicated. The possibilities are endless.

rosy said...
This comment has been removed by a blog administrator.
Anonymous said...

drbd could be good for lower writing loads. Forget dumb ideas like put-your-db-on-it. Don't believe who said it's cool with a smart smile. They don't know what they are talking about.