Here is what I need to achieve. I have a work PC with 2xSSD one for O.S. and software, and the second smaller for storing data. Currently I backup my data on an external HDD and on the cloud, I also do O.S. image to recover the full system with software. The data backup on external HDD is daily, the cloud backup is weekly and the O.S. image is monthly.

I wanted to improve this pattern by replacing my external HDD with a NAS, so the first question is: would a NAS used for storing backup files be a good idea or an overkill?

The very second question is: what kind of NAS do I need to store what above? I have read about the various RAID levels, and as of now I'm undecided between 1, 6 or 10 what is the possibility that: a) two disks will go bad at the same time or b) the second disk goes bad before I'm able to replace the first fault?

Another option could be just replacing my current external HDD with a bigger one, that can house more sets of data, but this would not really improve the security the way I had in mind.

The Short Answer is: it depends

The Longer Answer is regarding:

Would a NAS used for storing backup files be a good idea or an overkill?

• In Fact, it is a commonly used practice to do so for short-Term backup
• Long-Term Backups should be saved on Tapes or other medias.

what kind of NAS do I need to store what above?

The Short Answer is: it depends

The Long one is:

It depends on how many drives may you want to have failed at once to still be able to recover.

You may want to read about the Different Raid Level and Information on Wikipedia, as it would explode the Answer dramatically with non-related stuff.

A Commonly used Raid Level for Storing Backups are Raid 5 or Raid 10 - and yes, again it depends on your purposes and needs.

Remind: Raid is NOT a Backup, its just saves you from n-X failed drives.

Where n is your current drives and X the drives where can fail depending on the chosen raid level.

To answer your question about the possibility that two drives fail at the same time: It could happen and it might be more likely than you think. I have seen it happen on a RAID5 system which then got all its data wiped.

Usually the RAID system detects that a drive has gone bad and starts rebuilding on a hot spare or manually replaced bad drive. When this rebuilding takes place all drives in the system will have to work hard and usually all the remaining drives are of the same age, make and model as a drive which has just failed.

On the other hand, I have also seen a raid6 system fail when 6 disks of the same make, model and age decided to give up at the same time. Raid6 would allow 2 disks to give up at the same time, but when 6 of 16 disks gave up all data was lost.

Reexamine what point in time you need to recover if the data is lost. Losing a week of work is bad in any organization, but may be tolerable to yours. And test restores, which is the entire point of backups.

Your current backups are good in that you have three copies of the data, and one at a different location. Assuming the weekly backups meet your business continuity goals if both the computer and the external were lost.

A storage array improves reliability by surviving a disk failure. Yes, it could make sense to replace the external with a NAS dedicated to backups. Note that if the primary storage is still one disk, it will fail at the same rate as before and some day require a restore. Which may be suitable for your needs.

Beware that an always-connected file share is not a cold offline backup. Ransomware will encrypt such files if it finds them. Assume your backup user is compromised, what damage can it do to backups? Typical file permission schemes cannot easily implement a user that can write to files, but cannot change them or delete them. (Try doing so with SMB or NFS.) Unplugging the backup NAS from the network when not in use may seem tedious and paranoid, but perhaps limiting access to the daily backups is useful. Confirm that the cloud backups cannot be easily altered or deleted, such as using credentials that do not have delete permissions.

Array type is a choice to make, each RAID flavor you mentioned could be decent. RAID 1 of a pair of drives is easy to do, but only 50% usable space. RAID 6 of a handful of drives will survive 2 failures, very slightly slower due to parity calculations. RAID 10 is striped RAID 1, can survive at least 1 failure, and is quite fast, but 50% usable capacity may be too expensive compared to RAID 6. All of the above are better than the single point of failure that is a single physical drive.

Backup to tape could be a different option. Superior in that cold offline backups are easy, stored tapes are not being read or written to. Further, using a different media type means a backup copy has different wear behavior, and is not vulnerable to specific flaws such as in the primary storage's controller firmware. Challenging in that tape library hardware and software can get expensive.

I had the same problematic as you a couple years ago, and ultimately decided that a NAS and Raid setup wouldn't be what I need.

I decided to disregard the risk of virus and ransomwares, my house burning and most of let's say "less common" problems which may occur, and decided to only focus to protect some medias against disk failure. And at the end, I just decided to add, 2 big HDD, on top of my 2 SSD and 1 HDD in my gaming PC. I copy weekly the content that i want to save on my "1st new" HDD, and i keep the 2nd new HDD unavailable and make a copy on the 2nd HDD when i feel that enough new medias where added to the "1st new HDD", and then on top of that i still had the most important stuff backed up on an external drive, and another pc. (Not that much into cloud solution, and my stuff is located at different places, so I believe it should be fine this way)

And for all of this jungling I use a software made for doing this kind of backups / copies (bvckup2), instead of going all out with a NAS, and all ..

TL:DR : If you "just" want to secure you Datas and are not interested in the others benefits of NAS, making a system only for storing backups can be easier done and used than a whole NAS, with Raid and all, also i hesitated to use Raid 1, but then just decided not to use it, so that i could have one of the 2 drive permanently unavailble, unless i manually re-activate it, so that at least this drive can be partially protected from virus, and those kind of problems.

If using a small 2-bays NAS your only option is RAID1 (obviously).

If using a 4+ bays NAS for sequential read/write workload (ie: storing files with small or no random access at all), RAID6 should be preferred due to smaller parity overhead (at 5+ bays) and better fault tolerance (any 2 disk can die without data loss). In this case, keep in mind that replacing a disk is going to cause much reduced performance and significant stress on the others.

If random reads/writes are expected to have a significant role (ie: you will be running a database or some VMs off the NAS) or you can't stand the massive performance impact of a parity RAID rebuild, I strongly advise to use RAID10 (parity RAID without BBU caches are very slow in random write patterns).

Bonus point: use a NAS which support snapshots, so you can have point-in-time backup and recovery. And remember that RAID is not a backup, so be sure to keep other copies of your valuable data.

RAID 1

To answer the question posed in your title quickly and succinctly, Raid 1 will always be the most reliable, secure raid level.

If you have ten drives in a raid1 array, NINE OF THEM can fail and you are still online. no other single raid level can survive more than 2 failures