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I have a synology NAS and I don't understand the point of RAID 1 system. Why bother having a mirror? If I delete a file by accident it's deleted on both drives.

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    I would always use RAID 1 in a two disk NAS. What if a disk dies? Without RAID you have lost your backups, gg. – Džuris Jul 8 '19 at 19:36
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    If you can live with the downtime when a disk fails - I.e. you don’t mind to restore your backup to a newly purchased drive not using RAID1 is an budget option. You will get double the useable space (writes might also get a bit faster, usually reads will get a bit slower). Keep in mind that you will lose all modifications since the last backup in that case. – eckes Jul 8 '19 at 23:16
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    Do you backup your NAS to your NAS ? – Criggie Jul 9 '19 at 4:04
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    @Džuris Without RAID you have lost your backups Absolutely, utterly wrong. RAID of any level is no backup. RAID does nothing to protect you against rm -f -r ~/All/My/Data. RAID provides nothing but availability - it improves your access to your data, but it does not protect your data at all. – Andrew Henle Jul 9 '19 at 11:29
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    @AndrewHenle What is wrong in what I said? Backup to NAS protects you from losing data by that rm. RAID on NAS protects you from losing the backups when one disk on the NAS dies. – Džuris Jul 9 '19 at 14:26
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RAID is not a backup mechanism; it's a redundancy mechanism, and it does a completely different job – one protects against disk failures, the other protects individual files. So you wouldn't use one instead of the other; you'd normally use RAID along with backups or snapshots.

The main advantage of a redundant system is that it will not go down completely when a complete disk failure happens – the mirror allows you to continue using the NAS without interruption while the array is rebuilding.

(In other words: If you had a backup system but no RAID, you'd have to spend time restoring the complete system from backups every time a disk failed. If you have a backup system and a RAID 1 mirror, you only have to use backups when both disks fail at once, which happens much less often.)

Likewise, the redundant array should also allow you to replace disks that are only about to fail (e.g. if you see bad sectors increasing), and even to swap them with larger ones (if you're running out of space), without any downtime.


1 Doesn't apply to RAID 0.

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    If losing an hour of data is OK but losing all your data isn't, then backups are more important than RAID. – gmatht Jul 8 '19 at 8:54
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    @harrymc, is there a RAID rebuild that requires the system go offline? Sure you might have reduced performance, but rebuild is done while the array is operational. – JPhi1618 Jul 8 '19 at 15:59
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    For emphasis (for future readers): RAID is not a backup mechanism – FreeMan Jul 8 '19 at 18:36
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    Backups are also suppose to be off-site. If your server was stolen, flooded, set on fire, offline, crypto-ransomed, etc, you still have a backup. They do different things. – Nelson Jul 9 '19 at 8:35
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    Am I blind, or is the footnote not referenced anywhere in the text of the answer? – Falco Jul 9 '19 at 11:26
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Raid 1 isn't meant to protect you from deleted files. It only provides protection (redundancy) in the case of disk failure, wherein if one drive fails, the other has a complete copy of all your data.

RAID 1 consists of an exact copy (or mirror) of a set of data on two or more disks; a classic RAID 1 mirrored pair contains two disks.

This layout is useful when read performance or reliability is more important than write performance or the resulting data storage capacity.

The array will continue to operate so long as at least one member drive is operational.

From wikipedia

Incremental backups, on the other hand, will help you in case you end up deleting a file by mistake, but if your main drive fails, you will lose all the data that hasn't been backed up.

It is always recommended that you use one in conjunction with another, such as by having your NAS work in a RAID 1 configuration, and then taking regular backups on a third drive (or possibly another RAID config!).

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    @Fractale It is of use in cases where each and every bit of data is important, and where you can't afford any downtime - such as servers – rahuldottech Jul 8 '19 at 8:04
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    @Fractale If you are confident that you'll be okay with losing some data in the case of disk failure, sure. Be sure to test your backups and ensure that they're functioning as expected. – rahuldottech Jul 8 '19 at 8:07
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    @Fractale Let us continue this discussion in chat. – rahuldottech Jul 8 '19 at 8:11
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    @Fractale, it's not just an hour of lost work you'd avoid. It's also the extended period of downtime needed to restore from backup. For example, I estimate that if I needed to restore my home fileserver from backup, it would take on the order of a week (10 hours restoring files from an external hard drive, and the rest of the time spent re-ripping CDs and DVDs). – Mark Jul 8 '19 at 22:31
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    Also noting that it may be expensive for 1 hour of work in your situation, for a business or even home office situation, the cost of a drive vs a team's wages is pretty much rounding error. – Hugoagogo Jul 9 '19 at 0:14
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Some points the other answers have glossed over.

  • A backup is a point-in-time copy of your data. A RAID1 or higher array is a right-now redundant storage of your data. So if you did a daily backup (and it completed quick enough) then you could be restoring data that is up to 24 hours out of date. Can your use-case cope with losing a day's changes?

  • Cost - you mentioned in a comment that RAID1 feels wasteful. It is. But if the cost of losing your data is high enough then the cost of doubling the drives is miniscule. The cost of downtime also has to be considered.
    Would I RAID and backup my /photos directory? Absolutely!
    Would I RAID and backup my /TV+Movies directory? No, not at all.

If your budget is limited, RAID may not be feasable. However good backups are priceless. You can't replace some data like family photos, scans and documents.

TL:DR Backups are mandatory, RAID is optional.

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    In fairness, photos are probably among the easier to keep multiple copies of on disparate media anyway. Any time I've been photographing, I'll copy the files from the memory card to the computer (where they're stored redundantly, but that's not important here), thus creating a second copy. Only after the first backup after that has run without problems (thus creating a third copy) do I delete the files from the card (thus reducing to two copies). If I'm not in a hurry, I might even wait until the cloud backup has run as well as having switched backup drives, for five to four copies. – a CVn Jul 10 '19 at 15:56
  • @aCVn fair points - I was trying to show the relative "value" or irreplaceability of different types of files. You can't go back and re-take the same photo, but "MASH-final-episode.avi" is in another category. – Criggie Jul 10 '19 at 20:54
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If your aim is to protect against deletes, then RAID 1 is not for you.

RAID 1 will reduce the available disk-space by half, by making two disks serve as one disk, with the additional inconvenience that if one disk fails and is replaced, then the RAID might be inaccessible or very slow to access while it is rebuilding itself.

As your aim is backups, rather than sub-second data accuracy, you would be better served by using the two disks as two stand-alone disks and keeping two copies of the data, one perhaps somewhat behind the other.

With that simple setup, you would avoid the problems that can cause a RAID to fail, as some RAID failures may result in the total lose of data of both disks (some such cases are found on our site).

From your post and comments I get the impression that resiliency and resistance for wrong deletes are the most important to you. In that case two classic backups are better and safer than one RAID backup.

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Aside from the redundancy and uptime benefits of RAID highlighted in other answers, there is another factor: data corruption.

A decent implementation of RAID will protect you from bit-rot (unrecoverable read errors) automatically. Backups only provide manual* protection against bit-rot. This is reduced if you discard old backups (you could lose the last intact copy of a file) or the backup medium itself suffers bit-rot.

You can further improve the catching of bit-rot by performing regular scans of your RAID, if the implementation allows it. Even better is to use a filesystem-based RAID like BTRFS or ZFS, where checksumming of data is done in software, reducing the reliance on disks to report ECC errors correctly.

If bitrot is something that concerns you, you should use RAID or a checksumming filesystem (plus backups). Ideally, use both.

* For example you could perform regular drive scans, and then cross-reference any read error sectors to files using filesystem debug tools, and then replace the referenced files with backed-up copies.

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    I'm not sure about Btrfs, but ZFS can do data checksumming and validation just fine without any redundancy. It can even do (up to double = three copies) redundancy on a single disk, if you want to, without you needing to do crazy stuff like combining multiple partitions within a vdev. And when there's a data error, as long as something still works, it can tell you which files are affected, often by name. For all their good sides, I don't think many hardware solutions can do that. (As an aside, don't run ZFS on top of a RAID array. Give ZFS the raw disks. Your later self will thank you.) – a CVn Jul 10 '19 at 16:01
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In addition to everything else mentioned here, RAID also improves performance. A large file can be read in parallel from both physical disks, one half from each disk, reducing read time by nearly half.

  • Assuming that the RAID implementation is intelligent enough to do so. Not all are, so this might be a side benefit, but it's not something I'd count on happening without verifying with the particular RAID implementation. – a CVn Jul 10 '19 at 16:03
  • This is tricky for one sequential read stream because it introduces seeking. If the RAID alternates reads between drives with chunks that are too small, it won't be anywhere near 2x and could even be worse than 1 drive. But with aggressive readahead in large chunks from alternating drives then yes it can certainly win for one large sequential read with rotational media. It's a bigger win if there's parallelism in the accesses, like two programs both reading data from the filesystem, or parallel requests for multiple small files. – Peter Cordes Jul 10 '19 at 17:20
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I have built about 20 RAID 1 systems as home computers for individuals who have no technical knowledge. Over the years, about half a dozen of these have had single drive failures. From the point of view of the non-technical user, nothing much happens when half of a RAID 1 fails - the computer continues to work, but gives an error message complaining about a problem with the RAID drive. The computer owner phones me, and I check out the problem. The solution is relatively simple - identify which drive has dropped out of the array, remove it, and install a new disk. The array is rebuilt over a period of perhaps 5 or 10 hours. The clients are impressed when I tell them that had I not insisted on building them a RAID system, they would have lost some or all of their files. But what about using a single drive with regular backups. The problem is that non-computer literate individuals struggle to successfully do a single backup (I have gone through coaching a senior citizen how to do it, and it isn't easy from their point of view). Forget about doing monthly backups, let alone backups every day.

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    You could configure a scheduled back up. Most backup software support that. The only thing to worry about is making sure the backup media stays connected to the computer. – Tsahi Asher Jul 11 '19 at 13:04
  • And disconnected otherwise. Else randomware will have your main data and your backups. And yes, getting people to do that can be a challenge. – Hennes Aug 10 '19 at 13:46

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