Disk mirroring is growing increasingly popular in enterprise IT settings. In fact, many larger companies have gone from solely using it to promote high availability to using it exclusively to bolster data protection efforts. While mirroring offers an effective way keep your databases and websites online, viewing it as the end-all solution to keeping your data safe is one of the biggest mistakes you can make.
Mirroring vs. Backing Up
Mirroring is often described as a backup method. This is not the case. Although the terms are regularly used interchangeably, they are two different tactics designed to meet two different objectives. Whereas mirroring strives to keep your systems up and running in the event of hard disk failure, backup provides full data protection and recovery capabilities in the event that the whole drive becomes inaccessible. The following scenarios will further touch on the differences and explain why mirroring is not a reliable backup replacement.
Let’s say you’re using RAID 1, the most common disk mirroring configuration, between hard disks on two different servers to improve the availability of your database system. The RAID controller or software will provide fault tolerance by automatically and seamlessly copying data from one disk to the other, which essentially acts as a mirror of sorts. So if one disk mysteriously fails out of the blue, you can keep your database operations flowing by shifting to the mirrored disk.
Mirroring becomes problematic when issues of data corruption and security come into play. Let’s say your database is corrupted due to an improper shutdown. Because all data is copied between the two disks, both your primary and mirrored databases are now corrupted. Likewise, if the primary database is infected by a virus, the infection will be duplicated on the mirrored database as well. In this particular scenario, mirroring doubles your trouble and enables disaster to spread quickly.
Now let’s apply a backup strategy to that scenario. Instead of RAID 1, you’re using an image-based software program like ShadowProtect to protect your data. If your primary disk falls victim to corruption or infection, you can load a clean copy of your data onto another disk, physical or virtual, and restore the database to a point before the problem occurred. A good backup strategy will provide reliable data protection in a wide range of scenarios, and pick up the slack where mirroring falls short.
Additional Mirroring Concerns
The inability to deliver complete data protection is just one glaring issue associated with mirroring. Other drawbacks include:
Performance drops. While mirroring is highly recommend for applications that require high availability and high performance, the fact that data is written twice tends to lead to slight, but noticeable dips in performance.
More expenses. In many cases, incorporating mirroring means you need to buy RAID controllers and dedicated hard drives, both of which can drive up your operating costs quite a bit.
Maintenance challenges. By implementing disk mirroring, you are also tacking on additional maintenance challenges in the form of firmware updates, consistency checks, and day-to-day management tasks.
Capacity loss. In order to ensure that the mirrored disk can fully support the primary disk, your total disk capacity must be cut in half. Therefore, a 500 GB drive configured with RAID 1 will actually have around 250 GB of total space.
False sense of security. It’s fairly easy to view backups as an afterthought when you know the data on your server is fully duplicated somewhere else. Unfortunately, this false confidence can put your company in jeopardy. Mirroring can’t protect you from malware, file corruption, accidental file deletion, and the host of other factors that pose a threat to your data.
To Mirror or Not to Mirror
Considering that it provides convenient and immediate access to full copies of mission-critical data, it’s fair to say that mirroring certainly has a place in data storage and business continuity strategies. It’s the role in data protection and disaster recovery that needs to be revisited. If you demand around the clock access to your email server, database system, or high-traffic website, this is a viable option worth considering. Just remember that while mirroring is a nice optional luxury, a backup strategy is a must-have. Something you can’t afford to go without.
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