On 2023-02-17, Eric Johnson <726960+openbsd0...@pm.me> wrote: > Ask yourself what happens when someone writes a file to a mirror? > Answer: It means that both drives in the mirror will then contain the > file. If you make a mistake in the file, it means that you have the > issue on both drives and no copy of the original file.
Also it means that e.g. any filesystem errors are just replicated to two drives instead of one. There's also an issue if the drives don't agree about the stored data, either due to media errors, or partial writes following a crash. You can't tell which is correct. (there are mirror-like filesystems on some OS which use checksums/hashes to improve chances to identify this, but a standard mirror doesn't). > Create a mirror if you must (why not go to higher Raid levels instead?) They're often slower (especially in failure conditions) and more complex. Reconstructing RAID5/6 after a drive failure is pretty intensive on the other disks. > Do not bet the business on a mirror instead of a backup. Yep. The various forms of RAID allow keeping running in some conditions where hardware has failed but it's something to do *in addition* to backups. Ideally offsite, or at least stored offline - physically disconnected not just unmounted.
