Re: best way to sych a mirror SATA drive to RIAD array

Rahul wrote:
David Brown <david.brown@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in news:CoGdnYeeUtIPzbnXnZ2dnUVZ8rOdnZ2d@xxxxxxxx:

day, you'd be better off using a snapshot backup system to a large disk (i.e., a disk array). Put 4 1.5 GB Sata drives in a Linux software raid10 (raid 10 is preferably to raid 5 if you can afford the extra disk) and use rsnapshot for the backups. If the backup machine is off-site, you don't need the tapes at all.


Thanks David. The tape is legacy. I am learning as I go that it might be a dinosaur. I am very open to changing to disk-only. Just googled rsnapshot.

With something like this is it possible to set a granular scedule? Say recovery ability to go back on a each day basis for last 15 days and then on each week for 6 months etc. and so on?


Check out rsnapshot or dirvish - they do this.

That's what we have set up via tape currently. I need to guard against HW failure but also for recovery from user errors.

The other question of course is how much storage will I need. As I understood it tape (although slow) was way cheaper $/GB. Althoguth am not sure by how much!


Tape is cheaper per GB, but you need vastly more GB's, since you can't use hard linked copies. You need to have full copies on every tape if you want to be able to do a restore while retaining your sanity (incremental backups are a nightmare for restores - and if one tape is bad, you lose a lot of data). Tape also costs for the tape drive, and if disaster strikes and your tape machine is stolen/destroyed, you can have endless joys finding a compatible drive to the one you bought XX years ago, then finding compatible software, then hoping that you didn't have an issue such as drive alignment errors that mean your tapes are only readable on the same drive as they were written. So, do you have a second tape drive on a second machine that is kept off-site, and do you test your restores on it on a regular basis?

Our current tape-schedule does involve periodic "full backups" (as opposed to incremental). I always found those wasteful but seems a way of life with the constraints of slow tape.

Maybe disk backups take me out of that trap.

Another question: Is software RAID giving acceptble speeds these days? When we acquired our RAID boxed about 3 years ago it seemed hardware was the realistic way to go.


I don't have a raid setup myself, other than a test box. But yes, software raid should be fine for most uses. For a backup machine, you don't need high speed anyway. But watch out for the costs and risks involved with very large disks in a raid setup. There have been a number of articles recently about the risks of raid 5 with > 1 TB disks - if one disk fails, then rebuilding on a spare disk takes so long that there is a very real risk of unrecoverable read errors on one of the other disks during the rebuild. A lot of people are recommending raid 10 as the way around this, to get high speed and high reliability. Using software raid on Linux you have more options than many hardware raid solutions - in particular, you have raid 10 in "far" mode for highest speed of reads. It also makes your raid independent of the controller and other software - if the system dies, you can put the same disks in a new machine and run your raid again, without having to worry about buying a compatible hardware raid card.

As for performance, a good hardware raid card probably still beats software raid on an average machine. But for the price of the hardware raid card, you could get an above average machine (more ram is particularly useful), and get even better performance.
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