How many of you run a pair of parity drives?

[Kash76]

I have read about the benefits and fully understand. I am pulling two 5TB drives from my QNAP NAS and am debating using them as a pair of parity drives or just use the second one for my array.

 

I am curious if many of you run pairs or not.

 

Thanks!

 

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[ashman70]

For me, it comes down to how many drives you have in your array. Personally I think once you go beyond 10-12 data drives, you should have a second parity drive, so in my monster unRAID server where I have 27 data drives, I have two parity drives, but there is no rule, you can have two parity drives with only six data drives, its really a convenience more than anything. Whats more important is having a backup of your data. My monster unRAID server is a backup server meaning it contains duplicate data. You should always have a backup and not be in a situation where your unRAID server is the only place your data resides.

[SSD]

The risk of dual drive failure is very low, but the truth is most drive "failures" here have nothing to do with a failed drive. They have to do with cable fails and user errors. One of the most common use cases is that a drive appears to fail, user swaps it and nudges a cable causing the rebuild to fail as another drive drops offline. Two drive failures. Can you recover with one parity - probably, with help from the forum pros. But that extra parity would have let that second drive drop and allowed the reconstruct to complete. Would take a lot of stress and anxiety out of the picture. If you're an old pro that knows how to manage your array and what to do if things go wrong, dual parity is much less likely to be needed than if you are a new user and a little unsure of how this whole thing works. If this is you, you need that second parity disk! I don't care if you have 5, 15, or 25 drives.

[Kash76]

All great points, thank you for sharing. I tend to go the safe route so that's why I am asking.

 

I already have a pair of cache drives.

 

My only concern (and this was a risk in my NAS) is that my two 5TB drives were purchased together. The likelihood of them falling at the same time is not likely but also scary if they were both parity and data if they did decide to go together.

 

I will have backups of my most important data to a NAS (via Resilio Sync) at my parents house as well as a portable drive stored in my safe.

 

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[bgrantp]

I've been debating this myself. I have 1 parity and 3 data HDDs, plus 2 SSDs for cache, with room for two more HDDs. It's a bit of a low ratio, but I've been leaning towards adding a second parity drive just for that extra safety. I do have another backup on a 2-bay Synology of my most important data, so I have that as well, but I'd really like to take measures to avoid having to fall back on that, hence the second parity.

[BobPhoenix]

I have two server that I run dual parity and are powered up 24x7.  Drives are spun down most of the time.  I also have 3 servers that are single parity and are powered up on demand with IPMI.  Didn't want to use sleep.  Wanted them to be somewhat hard to power up without a purpose.

[METDeath]

I use dual parity on my primary server, (20+ array drives) and only single parity on my back up server (3 array drives).

[trurl]

Another very important thing you can do to help avoid multiple disk failures is to setup Notifications. Numerous examples of "set it and forget it" users who were unaware that they had multiple drives deteriorating until it was too late. Notifications will tell you before it happens to you.

[TUMS]

I have 11 data drives & 2 parity.

[garycase]

... I tend to go the safe route so that's why I am asking.

 

In which case you should go with dual parity, as this definitely is safer vis-à-vis data loss due to drive failures.

 

 

...  scary if they were both parity and data if they did decide to go together.

 

Actually, if you're going to have two drives fail at the same time (with a single parity system), it'd be BEST if they were parity and data => that way you'd only lose one drive's worth of data.    If you had two data drives fail at the same time you'd lose two drive's wroth of data.

 

[Kash76]

Interesting, I thought that parity could build both data drives if they went. Thanks, that reinforces dual parity.

 

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[landS]

... I tend to go the safe route so that's why I am asking.

 

In which case you should go with dual parity, as this definitely is safer vis-à-vis data loss due to drive failures.

 

 

...  scary if they were both parity and data if they did decide to go together.

 

Actually, if you're going to have two drives fail at the same time (with a single parity system), it'd be BEST if they were parity and data => that way you'd only lose one drive's worth of data.    If you had two data drives fail at the same time you'd lose two drive's wroth of data.

 

So with a single parity drive,  you are protected from a single hard drive data failure...  But if a second data drive failure occurs you loose both data drives data?  I did not realize that.  I'm glad I use dual parity.

[JonathanM]

Interesting, I thought that parity could build both data drives if they went. Thanks, that reinforces dual parity.

 

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Parity does not have any sensible data on it, it's the answer to an equation from all the rest of the drives. If only 1 drive is gone, the parity bits can be used along with ALL the other data drives to calculate what was on the missing drive. The second parity drive is a different much more complicated equation that does the equivalent of the same thing, but with the extra requirement that all remaining data drives be in the correct order.

[Kash76]

So have a screen capture of your disk config available?

 

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[JonathanM]

So have a screen capture of your disk config available?

Always a good idea. A current backup of your flash drive taken with the array stopped is valuable as well.

[SSD]

Interesting, I thought that parity could build both data drives if they went. Thanks, that reinforces dual parity.

 

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There has been a rather pervasive misunderstanding of the very basis of what unRaid is. That somehow a single parity disk, by itself, has the ability to rebuild any disk in the array. This would make the parity disk, in essence, able to store the data of every disk in the array. What a trick that works be, a single party 4tb drive being able to rebuild any other of 10 other 4tb drives in an array. To do that the poor parity would be storing 40tb of data. If this were so, Tom would win the Nobel Prize for mathematics!

 

If we think for a second we realize that is impossible. As Scotty would say, we'd be breaking the laws of physics! Parity is not magical. In fact it is pretty pedestrian. And by itself it is nothing of value (except in a 2 drive array). It, in combination with all but one data disk, can rebuild the one missing disk. With a second parity disk and a bit of higher powered math, a second party would allow rebuilding a two failed disks at the same time. Theoretically, more parity disks could be added each one allowing the rebuild of one more disk. However, the math gets more complex and performance would suffer if Tom tried to go beyond 2.

 

If you spend a couple minutes though, and look at the real world problems users have, very rarely are they talking about drives failing in combination with their neighbors. The only time that I remember clearly was a case where someone's server was in a flood. Some of his drives underwater! He had multiple drive failures, although somehow managed to restore most if not all his data with little if any help from parity. No, if you read the real world problems, a huge percentage relate to drives APPEARING to fail because of bad or loose cabling. An investment of a couple bucks per locking cable would dramatically reduce these failures and arguably would be a better and cheaper way to protect your data! As would turning on notifications of drive issues, which often give advance warnings before a drive fails. But we mods and others providing forum support have been preaching these steps for years, but continue to see these problems daily.  And a second parity WILL thankfully help users with cable issues too!

[Kash76]

Thank you, all great info

 

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[JorgeB]

Although they are rare I had once as dual disk failure (not complete failure, but bad sectors on one disk while rebuilding another failed disk), at the time unRAID only supported single parity, I've also seen a few more dual failures on the forum.

 

Now all my servers with 10 data disks or more have dual parity (except my backup server), seems a small price to pay for added redundancy and piece of mind, eg, a couple of weeks ago had a disk on my oldest server (22 x 2TB) develop some bad sectors, the replacement was much less stressful due to me knowing I was still protected against another disk failure.

[SSD]

Although they are rare I had once as dual disk failure (not complete failure, but bad sectors on one disk while rebuilding another failed disk), at the time unRAID only supported single parity, I've also seen a few more dual failures on the forum.

 

Now all my servers with 10 data disks or more have dual parity (except my backup server), seems a small price to pay for added redundancy and piece of mind, eg, a couple of weeks ago had a disk on my oldest server (22 x 2TB) develop some bad sectors, the replacement was much less stressful due to me knowing I was still protected against another disk failure.

 

Johnnie, what happens when you get read errors on a disk as you are rebuilding another? Does unRaid persevere and rebuild the rest of the disk? Or roll up the carpet and give up?

 

Never had it happen to me so curious what happens.

[JorgeB]

Johnnie, what happens when you get read errors on a disk as you are rebuilding another? Does unRaid persevere and rebuild the rest of the disk? Or roll up the carpet and give up?

 

Never had it happen to me so curious what happens.

 

Rebuild continued, rebuilt disk had some corrupt files due to the read errors.

[SSD]

Johnnie, what happens when you get read errors on a disk as you are rebuilding another? Does unRaid persevere and rebuild the rest of the disk? Or roll up the carpet and give up?

 

Never had it happen to me so curious what happens.

 

Rebuild continued, rebuilt disk had some corrupt files due to the read errors.

 

Thanks Jimmie. That's what I thought.

 

Anyone reading, this is a big philosophical difference between unRaid and RAID5/6. UnRaid will generally "do its best" to complete operations even if it creates a less than perfect data outcome. And give you means, like trust parity option, that give the user flexibility to recover from a wide range of issues. RAID is much more strict, and if there is corruption it will invalidate the array and expect you have a full.backup and roll forward logs if needed.

 

Not sure about you, but for media storage, I like unRaid's priority better. But for most business solutions, you'd really want true RAID.

[JorgeB]

See here for a situation where not even dual parity is enough, user is running 24 older disks so more prone to issues, but he would need quad parity 

[jbrodriguez]

But wouldn't the alternative be something like ZFS with realtime checksum integrity (plus periodic scrubs), at the expense of hard-to-expand vdevs ?

 

I guess I'm saying there are pros & cons ... caveat emptor applies

[garycase]

The "alternative" -- no matter how many parity sets you have -- is to have a good set of backups.

 

NO RAID system is a substitute for that ... fault tolerance is nice, and can help significantly reduce the number of times you need to access your backups -- but it is nevertheless NOT a backup.

 

As for dual parity -- I absolutely agree it's relatively inexpensive extra insurance.  Definitely worth having.  All but one of my servers have dual parity -- including my backup server.  So I'd have to have 6 disks fail before I'd lose any data (3 on one server and also 3 on the backup server) -- and even then whether I'd lose any data would depend on which specific disks had failed.    [And actually I also have a complete set of off-line backups stored in a fireproof, waterproof, data-rated safe that I update monthly  ]

 

[Can0n]

I run 10 disks for data totally 65TB and run two 8TB drives for parity, as I have 2 Data disks of the 8TB variety