Tape backup vs disk backup

With disk-based backup coming down in cost and offering advantages such as rapid user-led restores, where does tape backup fit into a contemporary backup and archive strategy?

Tape backup is giving way to disk as a backup target, but tape still has numerous benefits that mean it is likely to be with us for a long time yet. While disk is less fussy about the throughput of your backup systems and is superb at rapid restores it is still more expensive as a medium than tape. Meanwhile, tape is cheap and boasts incredible write speeds.

So, how best to combine disk and tape in your backup setup?

In this interview, SearchStorage.co.UK Bureau Chief Antony Adshead speaks with Ian Lock, service director for storage and backup with GlassHouse Technologies (UK), about the benefits and challenges of disk and tape in backup and how best to use tape in a contemporary backup strategy.

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SearchStorage.co.UK: What are the pros and cons of tape as a backup target in comparison with disk?

Ian Lock: Disk-based backup and the advanced functionality it brings has transformed the world of backup in the last five years or so. Just about every backup solution I have designed and installed in this period has been primarily based on a disk target of some description. Tape, though, still has a place, which is why we have recently seen the roadmap for LTO extended out to an eighth generation.

Pro No. 1 for tape is still its cost per gigabyte, although as large capacity disk drive prices continue to fall, this is becoming a much closer contest. A single LTO-5 tape cartridge will cost between £60 and £100 and a single drive around £2,500. For 10 TB, that equates to around 28 pence per gigabyte of backup storage capacity, assuming 2:1 compression. For 10 TB of branded direct-attached SATA storage, one might expect to pay around £5,000, equating to 50 pence per gigabyte, or about double the cost of tape. I should say here that a more advanced appliance with data deduplication features will cost a lot more than this. So, tape still has an advantage, particularly for larger backup volumes, but that advantage is definitely shrinking.

Pro No. 2 is mobility. It is very easy to pick up a box of tape cartridges and ship them from your production location to an offsite facility for long-term storage. If you do this regularly, you have the basis of a simple DR protection policy. It is somewhat more difficult to ship disk drives in an array between sites. More realistically, a disk-based solution will involve copying data between two storage systems over a WAN from one site to another. Once this is set up, of course it is much simpler than handling tapes every day, but it does come at a cost -- you have to pay for the WAN links and usually for an expensive replication software option. My company is frequently asked to write business cases to illustrate the benefits of one method over the other.

The third pro is raw, straight-line speed. The maximum throughput of an LTO drive is 280 MB per second, assuming 2:1 compression. That makes reading and writing to a single LTO tape drive faster than a single disk drive and as fast as many simple SATA-based drive arrays. The problem -- and so our first "con" -- is getting the tape drive up to that speed and keeping it there. To feed the tape drive at this speed, you either need a very powerful backup server and a well-configured source drive array, or you need a whole string of network backup clients multiplexing their backups at the same time.

The cons now start to come thick and fast.

The next con of tape versus disk in backup is normally reliability. In a tape drive there are lots of complex moving parts, both in the drive and the media, which can and do break. Tape drives are also susceptible to bits of dust and grit in the data centre air and on the cases of tape media. Tape drives also start and stop a lot as the data stream from the backup server comes and goes. All this causes wear and tear in a typical tape-based backup environment and explains why reliability levels can seem to drop steeply in busy environments after three years.

Other cons include access speed, as tape drives can take several minutes to load, mount and position before a single byte can be read from them. Also, much of the move away from tape to disk-based backup has been driven by concerns over the security risks of manual tape handling and the high-profile legal cases linked to losses of backup tapes containing personal data in the US.

SearchStorage.co.UK: Where should tape fit into a contemporary backup/archive strategy?

Ian Lock: Despite all those cons, tape does still fit into a contemporary backup and archive strategy in a couple of key areas.

Firstly, it still makes sense for long-term storage of large volumes of data. By long-term, we really mean those month-end or quarterly backups that have to be kept for maybe five or seven years. It makes very little sense to keep these large static backup images on disk, consuming power, cooling and maintenance budget when the data is accessed so infrequently.

A sensible strategy will keep daily and weekly backups (those with shorter retentions) on disk for the first month or two, with monthly and quarterly backups, then kept on tape and held offsite. To provide protection from disaster, the daily and weekly backups on disk should ideally be replicated to a second site. This can be achieved in several different ways, either through an option of the backup application, through array Replication or through advanced functionality of an intelligent virtual tape library (VTL) or data deduplication appliance. I often advocate that tape is used in this way, as the final backup location for long-term retention after daily and weekly backups have been protected on deduplicated disk to cut down their size, with these deduplicated images replicated to a second site.

Tape also still has a place for off-site disaster recovery protection where disk-based replication is not an available option; for example, where a company only has a single site. In this case, creating a second copy of backups onto tape, which is then shipped offsite provides a basic form of disaster recovery (DR) protection. The primary backup copy might still best reside on disk, providing flexible backup and high-speed restore.

Finally, tape is, of course, still a good option if cost is an issue. A single LTO-4 tape drive can be purchased for just over £1,000, and an LTO-4 tape cartridge for between £15 and £20. This gives you a backup target for a small environment of a few servers and as data volumes grow you only need to buy more tapes. This is still cheaper than buying a disk array or appliance, which would need to be constantly expanded over time to accommodate increasing numbers of backups.

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