Replacing 2 drives at once on 6.8.3?


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Hi All,

I've just received 2 new drives for my Unraid setup, to replace 2 older drives in the array.

I'm currently using 6x 8TB drives (double parity and 6 data) and 2x 6TB drives (2 data).

The 2x 6TB drives are the candidates for replacement/upgrade with 2x 8TB disks.

 

I've been searching around and googling and haven't been able to find any info apart from a thread using unraid 5.0.3, which was also using single parity, and he didn't seem to follow the correct process that I'm familiar with.

 

Anyhow...

 

So my question is, does the current stable 6.8.3 (When using double parity) support replacing 2 disks at once?

Has anyone attempted this, or is it a no-go idea?

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If everything is healthy and parity is in sync it should work fine.

 

The major thing that could go wrong is if one of the drives you are NOT replacing happened to fail during the rebuild. When was your last parity check with zero errors?

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I wondered about this also, as a valid and healthy parity seems to be a key requirement.

 

I've had a good parity history, and run at the beginning of every month:

image.png.58df386faa8f13a5c74b7de310eb55c9.png

 

So I had a good parity check about 10 days ago.

 

As far as I can tell, none of my drives have any SMART errors and are recent drives purchased within the past year.

 

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As always, the typical disclaimer that RAID or Unraid is NOT backup, it's only redundancy, applies.

 

As long as your backup strategy is working ok for you, then you should be fine to go ahead with a double rebuild.

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Thanks, that's great to know that things should work.

 

As an aside question, I've been wondering why my parity checks run at about 110MB/second, but my drives are capable of above 200MB/s.

Am I missing something in the calculation or is it a case of slowest drive, or something else?

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Just now, jonathanm said:

All of them over their full capacity?

Sorry I'm not sure what you mean.

 

The drives have had a couple TB free on each for some time, but the speed has been consistently at around 110MB/s.

Recently they have been filling up a bit more, but the parity speed has been the same.

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52 minutes ago, KptnKMan said:

As an aside question, I've been wondering why my parity checks run at about 110MB/second, but my drives are capable of above 200MB/s.

Am I missing something in the calculation or is it a case of slowest drive, or something else?

Generally the speed is good at first and is going down with time as the heads read from the outer of the platters to the center.

If I look at my example, I start at 190ish MB/s and I end up on the 90ish for an average of 145MB/s.

Edited by ChatNoir
fixed my mistake
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Disks are typically somewhat slower on the inner cylinders simply due to data density. The shorter tracks have less data because they are shorter, but it is running at the same RPM as the outer tracks which are longer. And the outer tracks are used first, so slower the fuller it gets.

 

And of course, parity operations are more complicated than single disk read/write.

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Ok so I downloaded and ran Diskspeed yesterday, and have been running it a few times with mixed results:

image.png.c46b09aec0810a16a86e201681984526.png

 

Unfortunately, 2 of my disks are slower, but these are the 2x 6TB disks that I'm replacing... so no surprise there.

What is surprising, however, seems to be that one of the 6TB disks (Disk1/sdl) is bandwidth capped, and the other (Disk2/sdm) is causing issues finishing the tests, and seems to be retrying with Speed Gap errors every time I ran it:

image.thumb.png.df7cd1797d190f26cb21ea7f030da5d6.png

 

Interestingly, I never knew about this Diskspeed tool but it's interesting and has definitely confirmed my suspicions about these 6TB drives, being that they are (1) slowing down operations and (2) are proving a bit inconsistent alongside the newer 8TB drives.

If anyone hasn't done so, I'd recommend checking out the Hard Drive Database associated with the Diskspeed tool to see how results of drives with the same model number perform. I assume that the Diskspeed tool uploads test results to this database.

 

So at this point, looks like a good idea to get these guys out of there.

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Rebuild running on Disk1 and Disk2 concurrently:

image.png.bce6615198527af16956daddcd58750c.png

 

This with all VMs and Dockers running, although that should require few reads/writes on the array.

I'm already pretty happy with the performance.

 

When it's done, I'll see if I can run another Diskspeed test.

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15 hours ago, KptnKMan said:

Unfortunately, 2 of my disks are slower, but these are the 2x 6TB disks that I'm replacing... so no surprise there.

What is surprising, however, seems to be that one of the 6TB disks (Disk1/sdl) is bandwidth capped, and the other (Disk2/sdm) is causing issues finishing the tests, and seems to be retrying with Speed Gap errors every time I ran it:

image.thumb.png.df7cd1797d190f26cb21ea7f030da5d6.png

 

Interestingly, I never knew about this Diskspeed tool but it's interesting and has definitely confirmed my suspicions about these 6TB drives, being that they are (1) slowing down operations and (2) are proving a bit inconsistent alongside the newer 8TB drives.

If anyone hasn't done so, I'd recommend checking out the Hard Drive Database associated with the Diskspeed tool to see how results of drives with the same model number perform. I assume that the Diskspeed tool uploads test results to this database.

 

So at this point, looks like a good idea to get these guys out of there.

 

The bandwidth capped means that the drive is likely outputting data faster than the system can utilize it and is represented by a flat'ish line for a portion of the graph.

 

Question - when you were getting the Speed Gap errors, was the max allowed size increasing? It's supposed to increase every time it retries to eventually pass but here it doesn't look like it was doing that.

Edited by jbartlett
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20 minutes ago, jbartlett said:

 

The bandwidth capped means that the drive is likely outputting data faster than the system can utilize it and is represented by a flat'ish line for a portion of the graph.

I can't say the line looked very flat'ish but I'm not sure if this is an issue with these older drives bursting data of something.

I thought it would have had the opposite issue, with not keeping up with the other 8TB drives.

Guess I was mistaken.

 

20 minutes ago, jbartlett said:

 

Question - when you were getting the Speed Gap errors, was the max allowed size increasing? It's supposed to increase every time it retries to eventually pass but here it doesn't look like it was doing that.

Is that the data in the other graph? That was somewhat linear like this graph, but seemed to have got stuck retrying at 90%. I'm not sure if you're referring to that. 

 

 

I feel like I've summoned a genie by accident. 

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Hi @jbartlett and thanks for making this tool.

I've gained a lot of insight into how my drives are performing.

However, I'm still not sure of your question regarding the "Max allowed size", I think you mean the threshold of measured throughput, but I don't know where I can see or verify this. It didn't seem to be increasing above 45MB. I also tried running the same test with "Disable Speed Gap detection" enabled, and it still would not finish the test.

 

So once the parity rebuild finished, looks like it was a little faster overall, despite the system being under normal load:

image.png.930deb8e4fae7e5b13876f1dd081f73d.png

 

Ran the Diskspeed test again (The test finished this time):

image.png.7ff712a940fcac188ed5969733d9d08f.png

 

The results look a lot more pleasing, with all the drives performing quite well together.

Interestingly, there was a still a bandwidth cap on Parity1 and (the now new) Disk1:

image.png.3fa49203e5d7c18bb44ccdcf13c9a114.png

 

I'm not sure what to do with this result as yet, but it looks like the system is at least behaving normally.

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