Using USB attached drives for Storage and Parity?


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I have a server with 6 internal drive bays and I'd like to attach a few more drives via USB for both storage and parity functionality.

 

Has anyone seen any problems with doing this as long as they're all attached to a UPS keep outages in sync with the server being on?

 

I plan on using a couple of SSDs or a F80 for cache so I'm thinking that this might be okay...

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Posted (edited)

Gee, I just tried it and it's letting me do it for a parity device.................................

 

Are you differentiating Parity from Array and Pool even thought Party devices are listed under Array?

Edited by unraidfan
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Posted (edited)

As a backup parity as in this case is it reasonable? Could you explain what my expectations should be from the following?: 

 

screenshot.jpg

 

Obviously in this case I have ONLY used the USB drive for Parity and not Storage, but I was going to add storage to USB 3 as well at some point... But yeah, I'd like to know what I should expect with this and then perhaps we can expand discussion to storage after we're more clear on the big picture and any bottlenecks or problems that might arise that relate to performance or otherwise...

Edited by unraidfan
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43 minutes ago, unraidfan said:

As a backup parity as in this case is it reasonable? Could you explain what my expectations should be from the following?: 

 

screenshot.jpg

 

Obviously in this case I have ONLY used the USB drive for Parity and not Storage, but I was going to add storage to USB 3 as well at some point... But yeah, I'd like to know what I should expect with this and then perhaps we can expand discussion to storage after we're more clear on the big picture and any bottlenecks or problems that might arise that relate to performance or otherwise...

 

The problem is that USB drives are prone to a temporary disconnect and then re-connecting with a different device identifier.  Unraid is not "hot plug" aware and cannot handle the device id changing for any array or parity drives so it then treats the drive as failed and disables it.  You then need to do a rebuild of the drive contents to get it back into operation which is a lengthy process with a good chance that the problem could occur again shortly.

 

You are welcome to try a USB connected drive but you need to be aware of this issue as it can be very annoying.  It is possible that your hardware may not show these symptoms but they are very common.

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Thank you @itimpi for your remarks! I find this very interesting regarding the device identifier changing. I thought that USB drives such as this had a GUID that always was able to identify it. Bootloaders such as GRUB, OpenFirmware, OpenCore, and so on rely on this information and reliably boot to USB drives even if they change ports... so I find your remark very curious and I would like to hear a developer chime in on this topic!

 

Since this is a Parity drive we are first specifically talking about, I have to believe that it won't impact read/write performance or create other headaches. Furthermore I would think the SSD cache might further reduce impact if there is any kind of impact. The USB device should, even if it disconnects, be able to recover rapidly and since the system continually checks consistency I don't think corruption would occur or at least occur in any significant amount (?)...

 

Again, thank you and I find this very interesting. It does sound like a STORAGE component to the Array would be more problematic and I probably should avoid that.

 

I guess one more point is that I am confused about the point of how many Parity drives can a system have? I thought that the more Storage drives it has the more Parity drives it would or should need. Is 2 Parity drives the only number it will ever have or will this count go up as more actual drives are plugged in? I keep having the thought that USB attached Parity drives seem almost ideal since they're easy to replace if and when they go bad (but I suppose always having an internal one would be sane maybe).

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21 minutes ago, unraidfan said:

I guess one more point is that I am confused about the point of how many Parity drives can a system have? I thought that the more Storage drives it has the more Parity drives it would or should need. Is 2 Parity drives the only number it will ever have or will this count go up as more actual drives are plugged in? I keep having the thought that USB attached Parity drives seem almost ideal since they're easy to replace if and when they go bad (but I suppose always having an internal one would be sane maybe).

Unraid is not RAID. The Array uses parity system that is explained there : https://wiki.unraid.net/Manual/Overview#Parity-Protected_Array

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Thank you, so it appears as though the two Parity drives are essentially identical and simply offer a redundancy in case one goes bad.

 

I was looking for an FAQ but cannot seem to find one in order to understand: how many drives can fail at one time given any size before losing data?

 

Only one or more?

 

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1 minute ago, unraidfan said:

so it appears as though the two Parity drives are essentially identical and simply offer a redundancy in case one goes bad.

No, with dual parity you can lose two drives, with single just one.

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9 hours ago, unraidfan said:

Since this is a Parity drive we are first specifically talking about, I have to believe that it won't impact read/write performance or create other headaches.

Unraid parity is realtime, so any writes update the parity drive(s).  Writing is limited to the slowest participant, even more so when simultaneous writes to different data drives are done.

 

It's already been said, even though Unraid will allow you to configure USB drives in the parity array, it's far from ideal. Feel free to experiment, the worst that can happen is lost data, so as long as you keep good backups all you will be out is time. The good backups thing is just general advice, regular RAID or Unraid is high availability when a disk fails, there are MANY more ways to lose data that don't involve drive failure. Accidental or malicious deletion or corruption must be restored from backups.

 

Also, the second parity slot is very unique, the math used to calculate it is more complex than the simple sums used on parity slot 1, so it takes more CPU to compute. It's perfectly ok to only have one parity disk in slot 2, but that disk is not valid in slot 1, if you remove it from slot 2 and assign it to slot 1 parity will have to be recomputed from scratch.

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