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I need to validate my understanding on RAID.
I'm planning to buy a Synology (or QNAP) NAS and plan to mount 4 HDD (4To each) in a RAID5 group.

I’m considering several crash scenarios :

  1. one hard drive crash
    I can replace failed disk with a new similar one and embedded software will rebuild the entire system (it will be long, but it will be OK)
  2. the system crash (power supply, motherboard, software…)
    How can I recover my data : can I plug each disk an a unix system to restore data by copying manually it ?
    Will it possible by USB mounting (external harddrive)

I heard about hot spare disk configuration : one dedicated disk for checksums & data recovery. But in my case, if I plug also the 3 others useful disks, will they contains clearly my data ?

I've already ask the question to different people answers are not really convincing. They've pretty said : it depends, in case 2, you will have to buy exactly the same NAS to recover data and others sounds like that...

I know that RAID != backup, but I'm suprized that's even newest versions (RAID 5/6) are not more reliable... Isn't there any solution which allows me to not buy 2 systems to backup each others... ?

In a global consideration, could you tell be an estimation on data ownership. My estimation are on a minimum of 100$/year/To. Am I right ?

Regards

Damien C
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2 Answers2

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First: DON'T ever use a RAID5, especially not with disks that size. The reason is that it's very likely you end up with an error during a rebuild after a failed disk, which will make the array a total failure.

To your questions:

one hard drive crash I can replace failed disk with a new similar one and embedded software will rebuild the entire system (it will be long, but it will be OK)

Yes, but you should use a RAID6, which can sustain a failure of two drives and increases the likelihood of a full recovery after a single failure.

the system crash (power supply, motherboard, software…) How can I recover my data : can I plug each disk an a unix system to restore data by copying manually it? Will it possible by USB mounting (external harddrive)

This depends on the manufacturer and model. Some NAS vendors basically use Linux software RAIDs, others use proprietary solutions that require you to put them into similar devices from the same vendor to read the data. Synology in particular can be recovered with a standard Linux rescue system, and yes, putting them into USB adapters would work. (https://www.synology.com/en-us/knowledgebase/faq/579). Beware though, this is dangerous if you don't know exactly what you are doing.

A hot spare disk has a completely different purpose: It's an empty disks in your RAID system that can be used by the system immediately to begin a recovery process in case of a disk failure. This is desirable in large environments and where you can't easily replace the disk yourself immediately, due to weekends or the server in a remote data center. The idea is to keep the time without or with reduced redundancy as short as possible.

What you mean is called a dedicated parity disk and this is a very specific setup that is only used in very special circumstances.

I know that RAID != backup, but I'm suprized that's even newest versions (RAID 5/6) are not more reliable... Isn't there any solution which allows me to not buy 2 systems to backup each others... ?

The main reason you can't consider a RAID a backup isn't necessarily the possibility of data loss due to an hardware error, but the fact that a well designed backup will protect you from your own errors ("I really didn't want to delete this file! Help"), software errors, malware and stuff like that. Also, a good backup is offsite and protects you from theft, fire, water damage etc.. This can't be done with any possible software solution.

Oh, and a RAID5 or RAID6 doesn't mean it's the fifth version of this principle, it's just a type designation. http://en.wikipedia.org/wiki/Standard_RAID_levels

Sven
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  • Yep, just checked my watch, it's definitely 2015 - not the era of R5 I'm afraid, in fact I think we should start banning any questions about R5 - it's like people don't *want* to keep their data! – Chopper3 May 03 '15 at 15:57
  • @Chopper3: Well, the web is still full of sites talking about how great RAID5 is and how secure it will make your setup :( – Sven May 03 '15 at 16:00
  • I wish you weren't right, wonder how many are from the last decade though, I was an R5 user for a long time - they were called the nineties! – Chopper3 May 03 '15 at 16:18
  • Ok Guys. I did not know this facts about RAID5. Sure Raid5 had glory days because of economic approach. To sum up: you advice me to * Build RAID 10 (and also dedicate half of my capacity on that) * Build a second system for my backup – Damien C May 04 '15 at 07:02
  • About Raid6, it seems the leakness are quite the same (even if it is more fault tolerant...) – Damien C May 04 '15 at 07:05
  • @DamienC: I am not advising a RAID10 but that you take your performance requirements into consideration and choose accordingly. A RAID10 has better performance characteristics, but it is less fault tolerant than a RAID6. A RAID6 can withstand any two drive failures, while the failure of a mirror pair in a RAID10 will mean data loss (of course, a RAID6 with two dead drives has the same recovery failure risk as a RAID5 with one dead disk, but you still have the *chance* for a recovery). – Sven May 04 '15 at 07:33
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Raid5 is a pain in the event of system failure. Each manufacturer does it slightly differently, so generally for quick recovery, you need an identical system.

As a result, it will not be possible to plug the drives into another system to pull the data off. That said, there is software available that allows you to rebuild data from a RAID5 array, but it is slow and expensive.

In my experience, I've had more data recovery scenarios from a failure of the RAID5 system than I have from drive failure!

As a result, whenever I configure a RAID5 NAS, I set it up with a hot spare to minimise loss due to multiple hard drive failure (be aware, that in the event of a drive failure, the RAID5 is then rebuilt to the hot spare meaning that you have a period of time with no redundancy). I also plug in a cheap USB drive in to the NAS and configure a daily backup to that. In the event of a problem, this CAN be connected to another system for data recovery.

Since an additional hard drive for the hot spare and an external USB device is fairly cheap, it offers a cost effective solution covering you against multiple points of failure.

ChadH360
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