Plane or train? Tape or disk? Reg readers speak

Disk speed versus tape economy and removabiity

Evan Unrue - Product Specialist at Magirus UK

Chris Evans

What is the role of tape in IT departments by and large today? 

There has been a big drive from a number of storage vendors pushing disk-based backup media as the cure to all ills backup-related, especially with the advent of technologies such as deduplication. However, tape-based technologies still remain a staple diet of most businesses from the SMB (small and medium business) through to the enterprise.

I don't often see disk-based backup necessarily as the nemesis of traditional tape-based backup; the two (at least for today) appear more complimentary. In my experience most organisations today have a healthy balance of both tape and disk technology to accommodate their backup needs. In most cases organisations will run initial backup jobs to disk for incremental/differential backups and then take secondary copies of these backup jobs off to tape for longer term retention (monthly/annual backup runs).

There are a number of reasons for this; firstly the perception of disk being a faster media and lending itself to keeping backup jobs within defined backup windows. Secondly, most restore requests tend to be for data which has been backed up within the previous week/month in most cases (Last known non-corrupt point in time, recent accidental deletion of data, restoring data to recover from a recent server failure).

Storing daily/weekly backup jobs to disk facilitates a less painful exercise when restoring data. 

Tape does have its challenges compared to disk; tapes aren't always reliable, restores can be nail-biting moments and performance is typically slower in many cases than that of disk.

However tape definitely has its place. Portability is the key elements to tape's continuing success, especially for those companies which don’t have a second site to replicate disk-based data to. Having the ability to eject a tape and store it at a secondary site, a tape management outsourcing company, or even at home has a big appeal.

Tape also makes longer term retention less painful to the wallet. Accommodating longer term retention of backups on disk can be costly from a CAPEX perspective, but also, disks keep spinning, so doing this comes with a larger physical footprint in the datacenter and a larger power bill. Tape scales by adding cartridges which don’t spin when not being use and don’t take up space in the IT room as they scale (albeit the tapes need to be stored somewhere).

Disk-based deduplication technologies alleviate some of the cost implications of longer term retention on disk however, but these also come with a cost. The question is, how much is it costing you to track, store and restore your tapes financially and in man hours vs the cost of a reduplicated, disk-based backup solution? Also, it is worth bearing in mind that some companies (or associated regulatory bodies) mandate that a tape must be vaulted offsite for compliance purposes; in which case, there is no avoiding tape.

Bearing in mind that backup environments tend to be quite sticky and troublesome to rip and replace, a lot of organisations are less willing to refresh their backup infrastructure unless they have a compelling event. Some disk-based backup technologies dictate that their own front-end backup software must be used, which is less than desirable unless you are already wanting to move away from your existing backup software.

VTL functionality on many disk-based appliances however gives you all the benefits of disk, whole masquerading as a tape library to the backup software, causing minimal disruption. This is all well and good, but you still have to manage virtual cartridges (which can’t be written to and read in parallel). So, by emulating tape, you can find yourself constrained by some of the limitations of how tape fundamentally works.

In conclusion, I don’t think it’s a case of disk versus tape, but in most cases finding a balance between the two. Certain new technologies such as disk-based deduplication may prompt a revisit of the above scenario, but its always worth going through the process of determining the management/CAPEX/OPEX costs of your current set up versus the newer technologies. Newer disk-based technology may have a higher CAPEX impact, but will it save you time and money in the long run ?

Evan Unrue is a Product Specialist at Magirus UK, the UK arm of the Stuttgart-based IT systems and services supplier. He writes a blog at www.interestingevan.wordpress.com.

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