Seeking for support! Exchanging parity and data disk causing trouble

[schark]

Hello,

 

I have at the moment an awkward issue with my unraid machine which is otherwise pretty stable. So normally I do not need to do anything - it is running flawlessly. But now

it is driving me mad. So, the situation is the following.

 

I am running 5.0 version of unraid (I think 5.0.3.) - plus version.

(BTW: Changes are always about parity and Disk 4)

 

I had installed the following array of disk

 

Parity: WD 3TB

Disk 1: WD 2TB

Disk 2: WD 2TB

Disk 3: WD 2TB

Disk 4: Samsung 1,5 TB

Disk 5: Seagate 2 TB

 

No cache drive installed.

 

Then I bought a WD 4 TB drive. I exchanged the parity drive as this one always needs to be the biggest one. I let unraid calculate

the parity and afterwards I made a parity check resulting in 0 errors.

 

What I noticed during the parity calculation and parity check is, that the first 2,5 TB are around 27MB/s and then it runs like

hell with around 95MB/s.

I do not have any clue why this is the case.

 

Then I had an array like this:

 

Parity: WD 4TB

Disk 1: WD 2TB

Disk 2: WD 2TB

Disk 3: WD 2TB

Disk 4: Samsung 1,5 TB

Disk 5: Seagate 2 TB

 

I then wanted to exchange the smallest one (Samsung 1,5TB) with the WD 3 TB. So I shut down the server and exchanged the disk.

 

So it was then like this:

 

Parity: WD 4TB

Disk 1: WD 2TB

Disk 2: WD 2TB

Disk 3: WD 2TB

Disk 4: WD 3TB

Disk 5: Seagate 2 TB

 

I then started again unraid and what I did not notice at first (because I had already made a parity check) was the blue ball

beside the parity drive.  So parity drive was recognised as new one and I had a new drive in the array.

 

Result was that drive 4 could not be calculated. I still have the Samsung 1,5TB drive.

 

So I was thinking, shuting down the server, replacing the new WD 3TB drive with the old Samsung 1,5TB and a new calculation

of parity would solve the issue.

 

Unfortunately I always get a message that the new drive (Samsung 1,5TB) is smaller then the old one (WD 3TB) and that this is not

allowed.

 

So I ended up with an array where I do not have a valid parity and one drive is missing. Not good. What I could do is save all data

on the 1,5 TB Samsung drive somewhere else and put it back on the 3TB WD drive.

 

But I wanted to avoid this. Can I force unraid to accept the smaller 1,5TB somehow? Or is there another easy solution for that?

 

 

 

[DaleWilliams]

schark,

Someone with excellent advice will be along shortly.

Meantime,

1.  Do you have another Slot available? Why did you remove the 1.5 drive? Why not just leave it in and add the old parity drive to the array?

2. Did you preclear or otherwise change the old parity (disk4). Is it still a valid parity (assuming you could go back?)

 

[i should add, none of the above are criticisms...there's another thread running in which the discussion is 'the thought process' users follow when they make physical changes to the array.  Turns out the best process may depend on a lot of different constraints as well as users' skill levels.]

[dgaschk]

Set a New config and assign the working data disks including the 1.5T disk4. Rebuild parity and then try the swap again.

[schark]

DaleWilliams,

 

I do not have another slot for data drive available as I own the plus key only.

 

I did  not preclear the old parity drive. I just added. unraid asked me to format it. I pressed "yes", it started for a second and then it was done, but I could not mount it as parity was not valid. Even it was still valid there have been changes (e.g. the 4TB parity drive is now the leading one).

 

[schark]

Set a New config and assign the working data disks including the 1.5T disk4. Rebuild parity and then try the swap again.

Dgaschk,

could you please post a little more detailed information on this as I consider myself (although I have been using my unraid rig already for four years now) as a rookie. 

[dgaschk]

Set a New config and assign the working data disks including the 1.5T disk4. Rebuild parity and then try the swap again.

Dgaschk,

could you please post a little more detailed information on this as I consider myself (although I have been using my unraid rig already for four years now) as a rookie.

 

1. Select Utils-New Config.

2. Assign original data disks and new parity drive.

3. Start array and rebuild parity.

4. Run a parity check.

5. Assign new 3T disk4 and rebuild disk4.

6. Run a parity check.

 

[schark]

Okay, so far so good. I am now at step 3. Parity sync is running.

 

Again with about 26MB/s - and I am pretty sure the last TB will be calculated with 99MB/s.

 

Any idea why this is the case? Is the bottleneck here the slowest drive in the array? What might be the root cause here?

[dgaschk]

Attach a syslog.

[DaleWilliams]

schark,

I believe the speed difference as the Parity progresses is normal in your situation.

Terabytes 2-3 will be very fast, and

The last Terabyte of 3-4 will be very very fast...

 

Your data drives #1-5 have no data above 3TB...calculating parity (when there's no data) is trivial.

During that portion, your new 4TB parity drive is simply being filled with zeros.

The other drives won't be read. (they aren't defined at that size range.)

 

[itimpi]

Again with about 26MB/s - and I am pretty sure the last TB will be calculated with 99MB/s.

 

Any idea why this is the case? Is the bottleneck here the slowest drive in the array? What might be the root cause here?

At any point in the parity calculation speed is governed by the slowest drive that is being used.  As a result is not untypical for the parity check o speed up as it gets beyond the size of disks of a given size as they are then no longer involved in the parity process. 

 

This can be complicated slightly that within a given disk size speeds tend to be fastest at the beginning (which is the outer edge) and slow and the heads move inwards) so the speed is always fluctuating.

[schark]

Hi,

 

I followed these steps and it worked perfectly.

 

0. Make screnshot for the right order of discs

1. Select Utils-New Config.

2. Assign original data disks and new parity drive.

3. Start array and rebuild parity.

4. Run a parity check.

5. Assign new 3T disk4 and rebuild disk4.

6. Run a parity check.

 

Thank you for your help!