Well RAID 1 (Mirroring) isn't just about downtime (even though that is usually its main purpose for something that needs to be up all the time).
You need to understand how data loss happens and what steps can help to prevent it.
Here are a few of the major ones:
1) User deletes data.
2) Program deletes data. (this includes viruses)
3) Disk fails.
4) Disk controller or other hardware other then drive fails destroys data (writing to both drives), include power loss in here too.
5) Physical damage, like NAS dropped.
6) Physical damage, like house burns down.
Which one(s) of these will mirrored drives help? On #3 (and maybe #5 since being two drives one might get lucky and not be damaged, but it certainly is meant to protect against this)
What is protected by a backup, and what are the limitations?
Backups can protect against all of the above, within certain limitations.
If you do backups once a day. Then clearly you can lose up to one day's worth of data.
So frequency of backup matters.
For either 5 or 6 you need your data to be backed in a physically different location. And in the case of 6 it can't be in your house (and extended this what happens in a disaster?). Having two NASes should help with #5 provided they are both and the same table that gets tipped over. But clearly for 6 you need offsite backup. And again how often or how much is going to factor into this. As in up link speed of a network is much slower then down speed, and much slower then local network, and of course hard drive speed. So if there is a huge amount of data to transfer you might not have enough time in the day to do it. Also you have to factor in the cost of offline storage so given this you might only backup what is really important to you.
One of the things I think factors into this is the likelihood of something happening.
I can tell you by far that the number one cause of data loss is the user(s).
This is mostly done with the running the wrong command or such, but I will also throw in the user dropping the NAS/machine or what not.
Then hard drive failures (On my personal machines I believe I have lost about 2 drives in 35+ years, at working with maintaining dozens of build servers at a time, each with multiple hard drives, maybe a half a dozen). Then again there was a product we put out and the manufacture clearly had a problem, because a one year one in four machines had a hard drive failure (out of 4 drives) (We dropped that manufacture of hard drives and never had the problem again).
I have only seen a couple of times hard drive controllers have failed and caused data loss (usually they fail without writing bad stuff to the drive).
That is from 35+ years of experience with computers.
For my own data.
All personal data is on the DNS-323 (note that if your data is on a local drive and you backup to the NAS, then you have two copies already, but in my case the data on the DNS-323 is the first copy)
Once a night I backup the data to the second drive in the DNS-323.
Once a night I backup the data to offline storage.
Occasionally I will backup the second drive to another drive through the USB port.
Where can I loses data? Basically any data that was created since the last backup might be lost.
That is fine with me. And actually my data like software development is stored in SVN, so I have another copy of it.
One other source of real loss of data I should point out. Either because of user error, or because of problems with hardware, people have had the situation that a hard drive goes down, they think they are safe because they mirror, but in the process of "recovering" they destroy the data on the second drive.
One of the things that strikes me all the time is that that even when people realize that they need to do backups (or mirroring) they forget to see if they even can recover the data. Like the cases where I saw people back up to tapes, and then when they lost data and went to those tapes they found they couldn't recover it because the backup tapes had either been worn out or the backup done wrong.
When you have something like a hard drive fail it is a stressful time, and if you have never rebuilt a drive you can make fatal mistakes. Learning how to restore your data after you have had data loss is a really bad time to learn.
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Topic: Is mirroring the safest way of not losing data? (Read 8394 times)