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A drive dropped out of my raid 5 array yesterday. It looks like the reason was due to a bad controller so I switched it out and attempted to re-add the drive but mdadm claimed it couldn't do it. So I zerod the superblock and just added the drive normally and left it to resync.

When I came to check on the array this morning I was unable to mount it at all and it's now showing as CLEAN FAULTY with two drives missing. The two missing drives are listed as spare and faulty spare.

Is there anything I can do in this situation or is the array gone?


The disks appear to be fine - except maybe for enough bad data on one of the disks for mdadm to get annoyed and kick that disk from the array too.

I was able to recreate the array by marking the disk as working and forcing the assembly so I'm currently just making sure that all my backups are up to date.

So I can probably change this question to: RAID5 seems to be a problem with large disks (3x3TB). I'm considering changing to mirrored RAID-Z arrays but is there anything else I should consider instead?

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You didn't say if the two drives are actually working, or not. If they are effectively dead, then there is no hope. Otherwise the answer would depend on what have actually happened - please attach the relevant syslogs. – Adam Ryczkowski Oct 3 '12 at 6:41
Don't have access to the logs at the minute. I'll post them tonight. – Dean Reilly Oct 3 '12 at 10:45
If you value your data, also stop using consumer-grade SATA drives. – Michael Hampton Oct 3 '12 at 13:07
up vote 3 down vote accepted

It is not recommended to use raid-5 on 3TB consumer disks because it take ages to resync (might be well more than 24h), and during that time it is likely that yet another one might fail, and in such case all your data (or at least the part, that haven't managed to get resynced) will be lost.

Raid-Z has small the advantage that it resyncs (resilvers) only that part of hard drive, which actually is used by files as compared to standard raid implementation which is filesystem-agnostic.

Another advantage of zfs is that it is said (I've never tried it myself. See article on that you can specify the order of files that are resilvered; files important can get high priority, which translates to being first to resilver.

I suggest to go with mirroring, which is faster and better error-proof for such large drives.

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In general, RAID-6 is much preferred over RAID-5, but both of them have the issue of long rebuild times. Using "--bitmap=internal" on the arrays may help, but switching to a RAID 1+0 or RAID-10 array is also recommended (shorter rebuilds). – tgharold Mar 27 '13 at 11:40

RAID-5 with just three disks is very risky because:

  • You only need one drive to go pop and one more drive failure will take out the array completely

  • The remaining two disks will be under increased load when one drive is out of the array, increasing the chance they could fail at that particular time

  • Consumer grade SATA drives are even more likely to fail (they aren't designed for 24/7 RAID I/O), plus if you bought them all at the same time from the same batch it is even more likely

I used to deploy three disk RAID-5 setups years ago but quickly moved on. RAID-5 has a higher write penalty as well which is something to consider.

It's hard to make a specific recommendation to improve your setup without knowing exactly what application you use your RAID array for, but you might want to look at getting an additional disk and using RAID-10 across all 4 or I tend to steer more towards ZFS these days, RAID-Z is interesting as well.

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So it's risky because of risks that he knew about when he created the array? Do you have any methodology to quantitatively assess the risk in business terms?? – Chris S Apr 29 '14 at 1:50
@ChrisS I think it is a fairly safe assumption that the vast majority of businesses would class their RAID array crashing and potentially losing all their data as some kind of risk.. – unixdude Apr 29 '14 at 1:57
The loss of more than one drive in any RAID 5 array, regardless of the number of drives in the array is fatal. This isn't specific to a three drive RAID 5 array. – joeqwerty Apr 29 '14 at 2:20
@joeqwerty yes apologies I got a bit focused on the three disks in the original question and should of pointed out this is a bad thing about RAID-5 in general. – unixdude Apr 29 '14 at 3:03
@chriss as I said in my comment it's hard to say specifically without knowing exactly what he uses the array for and how valuable the data is to the business, if they are using some form of RAID in the first place though I would assume it's important. There is no magic number that applies to every single business. – unixdude Apr 29 '14 at 3:03

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