Raid 1 og 5 for best recovery possibilities at HW-crash

Posted on 2012-08-13
Medium Priority
Last Modified: 2016-12-08
I'm not sure if it is best to use Raid 1 or 5 in the following Situation:

When running t.ex. a NAS-server, and the Hardware crashes (not the disks),  when running Raid 1 it is possible to mount one of the mirrored disks in any HW that supports the inteface of the disks.
if running Raid 5 it seems to me that it is necessary to mount the disks in similar Hardware for mounting the Raid 5 configured disks, or is there an easy way to get all the data from the Raid 5 disks ?
(Did have the Issue on a older windows-server configured as Raid 5, where I had to buy a similar used server to get the data back).
Question by:olefisk
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Expert Comment

ID: 38289875
No.  Raid controllers all have vendor/product specific layouts.  Even some controllers made by the same manufacturer don't support what you want to do.

Furthermore, most NAS devices don't even use hardware RAID.  They are just embedded LINUX boxes running software RAID.  (and no, you just can't mount those either on any arbitrary system).
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Accepted Solution

gmbaxter earned 668 total points
ID: 38289880
You should choose the raid level based on reliability and performance, not recoverability. Raid is not a backup, it merely is used to add resilience and/or performance to a disk system. Never substitute backups for raid.

Raid 1 is a mirror and with a hardware failure, sometimes you can pull a disk, put it in a caddy and get the data off, but this isn't always the case - some raid controllers will write metadata at the start of the drive meaning that many OS's will not recognise the disk/data.

Raid 5 can be reassembled with software, but this gets complicated due to parity etc.

As I said, do not let this decide what raid level to go with - a good backup ensures you can restore onto any system.
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Assisted Solution

noxcho earned 668 total points
ID: 38290252
RAID1 for reliability for sure. RAID5 is mostly for accessibility because you can replace one or two HDDs in case they fail (5 hdds config fr example).
And with RAID1 if ne drive fails then you can use the second one from the mirror for booting or using your daa.
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Assisted Solution

David earned 664 total points
ID: 38290290
RAID5 has an order of magnitude more things that can go wrong with it than RAID1.  I'm not talking simple drive failures, either.   There are numerous paths to data loss because of the complexity and multiple failure scenarios.  

To quantify, about 75% of the source code for a RAID stack deals with a long list of failure scenarios.  RAID1 is immune to most of them.  So in total RAID1 is maybe 10X more stable than RAID5 everything else being equal.

Case in point, just about any cheap desktop drive or even a pair of USB sticks can easily be qualified on a RAID controller or stack.   But those same devices will fail when running data integrity tests on a RAID5 config.  Go to the WD site and see how their Black, blue and green drives FAIL RAID5 qualifications and even WD says don't use those disks in a RAID5 config.

(I will throw it out there, but since you are using UNIX, you could easily do things like have a 3-way redundancy using 3 disks, or even 3-way redundancy (think triple mirror) with 2 drives.

Just get 1 x 2TB and 1 x 1 TB disk, partition the 2TB into 2 x 1 TB, and now you can mirror all 3.  You still have same protection of a drive failure, but you now have 3 copies of every byte of data so data loss due to unreadable blocks is much more unlikely.

Author Closing Comment

ID: 38291084
Thanks for the good explainations.

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