Disk mirroring is an old-school technique traditionally used to ensure data is always available, in a newsroom or bank, for example.
But as a back-up strategy it is disastrous. Despite this, it is becoming increasingly popular with big companies for back-up. It is even sometimes described as a back-up method.
Mirroring may seem like a simple, budget-friendly data storage option but it is fraught with dangers.
At its simplest, disk mirroring involves the use of two disks, one that is a mirror of the other; essentially the same data is saved to both disks. It is useful for keeping a system running in the event of disk failure, but it can’t provide full data protection and recovery capability should the primary disk become inaccessible.
By contrast, a proper back-up will provide full protection, allowing you to recover your data should your hard disk fail.
The increasing pressure – on banks in particular, for example – to provide customers with 24/7 access means there is a tension between ensuring permanent online availability and ensuring data is backed up regularly. Data replication and mirroring is part of the 24/7 strategy employed by at least one local bank, the BNZ.
How back-up by data mirroring can fail
So how does mirroring fail? Let’s take the simplest example, the use of RAID 1 (which stands for Redundant Array of Independent Disks – Level 1). This sees two disks being written to at the same time – there is also RAID 2, 3, 4 and 5, which sees more disks being written to, but the principle is the same.
Because data is mirrored onto a second disk, if one disk fails the second disk can take over.
This sounds great, but there are problems. For example, suppose your database becomes corrupted because of an improper shut down, because the data is copied to both disks, both databases are now corrupted. Similarly, if your primary data is infected with a virus, your mirrored data will be too.
Should you, instead, back up your data conventionally, preferably off-site, then you could load a clean copy of your data up to the point where the problem occurred and you are back in business.
There are other problems too. For example, local users regularly experience an inherent problem: one disk fails but the first time you realise this is when the second one fails. This is because the RAID mirroring system keeps on working until it can’t.
Computer Forensics’ MD Brian Eardley-Wilmot thinks mirroring is “bad choice for a back-up strategy” for an even simpler reason.
“You have two disks on the same PC and one is mirroring the other. The only advantage is if one disk fails you switch to the other, but suppose there is a power spike? Both disks can be affected – fatally.”
Natural disasters like earthquakes can cause such power surges, but so can hot weather. In November, wild spring weather saw hail and lightning strikes in North Canterbury, resulting in power surges.
Other mirroring problems
There are other problems too. Suppose you accidentally delete a critical file? If you using mirroring, both disks will be equally affected, so the file really is lost. It would not be if you had been backed up instead.
There is also a drop in performance when data is copied to two disks, and capacity is also halved.
” Mirroring can give you a false sense of security,” says Eardley-Wilmot.
“Obviously it can’t protect you from viruses or from files being corrupted. It’s not that it doesn’t have its place – it does, especially for 24/7 continuity. Which is what the banks use it for. But it really isn’t a safe alternative to backing up, which is why it’s a concern when big companies rely on it for this.”