Expanding array size and parity drives

[inghamio]

Hi All,

 

I have been using Unraid since Dec22 and loving it so far! My current setup has 2x 8TB Seagate IronWolf drives, with one in the array and one as a parity drive (I am also using a nvme drive for appdata/domains/system). I have been reading about how the parity disk works (https://wiki.unraid.net/Parity) and I am curious about expanding the array size...

 

If I leave my setup like above, and the single drive in the array fails. Am I right in thinking that I would be able to recover data on the array using the single parity drive?

 

If I were to add an additional 8TB drive to the array (which would then be 2x 8TB drives in array, 1x 8TB parity drive), would the single parity drive allow me to rebuild if one of the two array drives fails? But if both drives in the array were to fail, the single parity drive would not be enough?

 

Ultimatley, I am looking to expand and wondering at what point you would want to add an additional parity drive!

 

Many thanks

[itimpi]

1 hour ago, inghamio said:

If I were to add an additional 8TB drive to the array (which would then be 2x 8TB drives in array, 1x 8TB parity drive), would the single parity drive allow me to rebuild if one of the two array drives fails? But if both drives in the array were to fail, the single parity drive would not be enough?

Correct.    The simple way to look at it is if you have more drives fail than you have parity drives then the data on the failed drives cannot be rebuilt.    Data on any non-failed drives would be OK.

 

it is worth pointing out that a ‘disabled’ drive at the Unraid level is not necessarily a failed drive that is completely unusable.   it can be just that a write to it failed for some reason so it is no longer in sync with parity.   It may well still be possible to get most of the data off such a drive when in data recovery mode.

[Hoopster]

1 hour ago, inghamio said:

wondering at what point you would want to add an additional parity drive!

There is no hard and fast rule on this but many consider 8+ data disks a good threshold at which to start considering a 2nd parity.  Personally, I have two Unraid servers with 5 data disks and they are both single parity systems.

 

Only once in the 11 years I have been using Unraid have I had a disk red-balled. That happened because of a bad SATA connection that eventually resulted in write errors which disabled the disk.  I did not replace the disk.  I just fixed the cabling issue and rebuilt the disk back onto itself and all has been well for a couple of years.

[inghamio]

1 hour ago, itimpi said:

Correct.    The simple way to look at it is if you have more drives fail than you have parity drives then the data on the failed drives cannot be rebuilt.    Data on any non-failed drives would be OK.

 

it is worth pointing out that a ‘disabled’ drive at the Unraid level is not necessarily a failed drive that is completely unusable.   it can be just that a write to it failed for some reason so it is no longer in sync with parity.   It may well still be possible to get most of the data off such a drive when in data recovery mode.

Ah ok, so I guess if you start noticing lots of errors on the drives all of a sudden, or if unraid fires a warning notification (I'm guessing this is a thing?), then it's time to go into data recovery mode...

 

In which case, I'm guessing the ideal scenario would be to connect up new additional drive(s) and move the data across? Then you are free to shutdown and remove the bad drive(s). I'm yet to look into how the data recovery process works, I should read up on that!

 

1 hour ago, Hoopster said:

There is no hard and fast rule on this but many consider 8+ data disks a good threshold at which to start considering a 2nd parity.  Personally, I have two Unraid servers with 5 data disks and they are both single parity systems.

 

Only once in the 11 years I have been using Unraid have I had a disk red-balled. That happened because of a bad SATA connection that eventually resulted in write errors which disabled the disk.  I did not replace the disk.  I just fixed the cabling issue and rebuilt the disk back onto itself and all has been well for a couple of years.

Ah ok, interesting. So running a 2 disk array with a 2 disk parity would be completely overkill? I'm guessing it would work fine if you decided to set it up this way, but a tad unnecessary. Having more parity drives than array drives on the other hand, would not really achieve anything, right?

 

Thanks for your input 😁👍

[Hoopster]

2 minutes ago, inghamio said:

running a 2 disk array with a 2 disk parity would be completely overkill? I'm guessing it would work fine if you decided to set it up this way, but a tad unnecessary

I would consider it overkill, yes.  But others may have a different opinion.  Parity drives protect against disk failures so one or two parity drives is really a question of your risk tolerance for multiple simultaneous disk failure.

 

3 minutes ago, inghamio said:

Having more parity drives than array drives on the other hand, would not really achieve anything, right?

Correct.  Two parity drives is the current maximum and two parity drives to one array drive would really be unnecessary.

[Hoopster]

@inghamio Here is a good Spaceinvaderone video on the subject of Unraid parity.

 

[inghamio]

6 minutes ago, Hoopster said:

I would consider it overkill, yes.  But others may have a different opinion.  Parity drives protect against disk failures so one or two parity drives is really a question of your risk tolerance for multiple simultaneous disk failure.

 

Correct.  Two parity drives is the current maximum and two parity drives to one array drive would really be unnecessary.

OK gotcha. I didn't realise there was a maximum on two parity drives. I guess if you have an insane amount of drives on your unraid setup (for example if you went with 10+ drives) with 2 extra drives for parity, then your data is at most only protected for 2x simulatneous drive failures at once? I'm guessing most in this situation would probably run a second unraid server instead or maybe invest in some power protection or a UPS system instead? (I don't intend on having that many drives btw 😁, just curious really)

 

13 minutes ago, Hoopster said:

@inghamio Here is a good Spaceinvaderone video on the subject of Unraid parity.

 

Thanks, watching now.

[itimpi]

10 minutes ago, inghamio said:

invest in some power protection or a UPS system

I would strongly recommend this as a good investment if you value your data as unexpected power loss is the most likely thing to cause damage.

[Hoopster]

12 minutes ago, inghamio said:

if you went with 10+ drives with 2 extra drives for parity, then your data is at most only protected for 2x simulatneous drive failures at once?

Yes, that is correct.  With dual parity, you are protected against two drives failing at the same time regardless of how many data drives you have in the array.

 

13 minutes ago, inghamio said:

invest in some power protection or a UPS system

A UPS is highly recommended.  It has saved me a couple of time from potential array damage in power outages.

[inghamio]

1 hour ago, Hoopster said:

@inghamio Here is a good Spaceinvaderone video on the subject of Unraid parity.

 

Excellent video btw. Great way to understand how the parity drive works. It doesn't really go into the details of exaclty how the second parity drive works, but it sounds like it works independently of the first parity drive but using a different algorithm. That way you are able to rebuild for all scenarios where one or two drives are bad, even if one of those two bad drives is one of the parity drives.

 

I also didn't realise Unraid will emulate data on the bad drive when a drive is missing or bad (as shown here: https://youtu.be/dX2PvD1qtKw?t=601). Useful to know!

 

51 minutes ago, itimpi said:

I would strongly recommend this as a good investment if you value your data as unexpected power loss is the most likely thing to cause damage.

47 minutes ago, Hoopster said:

A UPS is highly recommended.  It has saved me a couple of time from potential array damage in power outages.

Yep, that's going to be my next purchase 😅

 

47 minutes ago, Hoopster said:

Yes, that is correct.  With dual parity, you are protected against two drives failing at the same time regardless of how many data drives you have in the array.

I think what I didn't appreciate before was how the parity drive actually works. I was foolishly thinking more like a traditional RAID1 setup, probably because that is the only thing I have ever setup in the past!

 

I think what I am going to do is get some additional drives to expand the array and then invest in a UPS. I'm not going to worry about a second parity drive until I the array has more drives in it. As for redundancy, I am using rsync to automate backing up to another NAS periodically.

 

Thanks everyone 😁

Edited March 30, 2023 by inghamio

[Hoopster]

1 hour ago, inghamio said:

I am using rsync to automate backing up to another NAS

This is exactly what I do in addition to backing up shares to external USB drives connected periodically via the  unassigned devices plugin.  I also have a cloud backup of certain shares.  Can't have too many ways to restore your data in case it is lost in one of the many ways that is possible these days.

[icedfusion]

UPS is highly recommended! I used to have an ubuntu raid6 server, but a power outage destroyed 4 drives so I lost all my data (no backup).

Since then I moved to unraid and have a 10+ drive system with a UPS plus another unraid server for backup that has 4 drives (backup stores essential stuff, not movies/tv). I also like the fact that if multiple drives fail you can at least get the stuff of the drives that haven't failed if the failed drives cannot be rebuilt.

The UPS has saved me a few times when we had a period of regular and random outages - best few hundred ££ I have spent (except the unraid license of course! )