This involves reading every byte of the volume and writing it to the new disk. At a later date, if a disk is added to the volume, a rebuild is started. Therefore, every disk contains exactly the same data if the volume is bootable, every disk in the volume is bootable. Once the volume is set up, every write goes to all the disks in the volume. Let's start by reviewing some of the features of a mirror volume. Backup Strategies which use Mirror Volumes These are the reasons many people think that RAID cannot be part of a good backup solution. Likewise if your building burns down or your Mac is stolen, you have no backup copy of your data to continue your business with. If your mirror volume's file system becomes corrupted, the corruption is written to both disks. If you accidentally delete a file, the file is deleted from both disks. The problem with the "set it and forget it" approach to mirror volumes is that it only protects you from one of the data disasters listed above: hard disk failure. At that point, the other disk provides data protection because your files are redundantly stored on it. This is the "set it and forget it" attitude to RAID you set up the mirror volume with two disks and don't touch it until one of the disks fails. Most people think of mirror volumes as providing protection only from disk failures. In either case, the media should be stored in a safe deposit box at the bank or offsite in a fireproof vault. You should use software which allows you to archive to tape, for long term storage or simply copy files to CD-R or DVD-R media. If you need to maintain an audit trail for financial or legal reasons or if you want to create an offline library of all the work you have ever created, you need some method of archiving your files. This includes: a hard disk failing, a user accidentally deleting a file, theft of a laptop, a file system getting corrupted, a virus infection, or your place of business burning to the ground (can you tell I'm a volunteer fire fighter).įor me, a backup does not include saving multiple copies of a given file or volume for several years or saving copies of all the work you've ever done I call that archiving. What is Backup?īefore I get in trouble with the semanticists reading this article, I need to define what I mean by "backup." For this article, I define backup as the process of protecting files or volumes from any disaster which can destroy data. This article describes how you can use mirror volumes (RAID 1), as part of a backup strategy which can get your users to think of you as a miracle worker. These two items will determine whether the users think you are a miracle worker or just someone who tinkers with computers. The two criteria which matter to your users are: 1) How fast can you get the server back online and 2) How old are the files you are restoring. This is the time when you'll see whether the backup of that server is any good. The users are going to be working through the weekend on a project which is due on Monday and are getting frantic because many of the files they need are on the server. It's 4:30 PM on a Friday and you get a call that a server you administer is down.
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