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32
Mirror seeing half the write IOPS on one disk than the other, is this normal? (discourse.practicalzfs.com)
submitted 2 months ago* (last edited 2 months ago) by [email protected] to c/[email protected]
15 comments fedilink hide all child comments
 

I'm syncoiding from my normal RAIDz2 to a backup mirror made of 2 disks. I looked at zpool iostat and I noticed that one of the disks consistently shows less than half the write IOPS of the other:

                                        capacity     operations     bandwidth 
pool                                  alloc   free   read  write   read  write
------------------------------------  -----  -----  -----  -----  -----  -----
storage-volume-backup                 5.03T  11.3T      0    867      0   330M
  mirror-0                            5.03T  11.3T      0    867      0   330M
    wwn-0x5000c500e8736faf                -      -      0    212      0   164M
    wwn-0x5000c500e8737337                -      -      0    654      0   165M

This is also evident in iostat:

     f/s f_await  aqu-sz  %util Device
    0.00    0.00    3.48  46.2% sda
    0.00    0.00    8.10  99.7% sdb

The difference is also evident in the temperatures of the disks. The busier disk is 4 degrees warmer than the other. The disks are identical on paper and bought at the same time.

Is this behaviour expected?

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[–] [email protected] 11 points 2 months ago (1 children)

It might be that the data to both disks saturates a common link before the second disk reaches full iops capability, and thus the driver then writes at full speed on one disk and at half speed on the other, for twice as long.

[–] [email protected] 4 points 2 months ago (1 children)

I put the low IOPS disk in a good USB 3 enclosure, hooked to an on-CPU USB controller. Now things are flipped:

                                        capacity     operations     bandwidth 
pool                                  alloc   free   read  write   read  write
------------------------------------  -----  -----  -----  -----  -----  -----
storage-volume-backup                 12.6T  3.74T      0    563      0   293M
  mirror-0                            12.6T  3.74T      0    563      0   293M
    wwn-0x5000c500e8736faf                -      -      0    406      0   146M
    wwn-0x5000c500e8737337                -      -      0    156      0   146M

You might be right about the link problem.

Looking at the B350 diagram, the whole chipset is hooked via PCIe 3.0 x4 link to the CPU. The other pool (the source) is hooked via USB controller on the chipset. The SATA controller is also on the chipset so it also shares the chipset-CPU link. I'm pretty sure I'm also using all the PCIe links the chipset provides for SSDs. So that's 4GB/s total for the whole chipset. Now I'm probably not saturating the whole link, in this particular workload, but perhaps there's might be another related bottleneck.

[–] [email protected] 3 points 2 months ago (1 children)

I'm not fully familiar with the overheads associated with all things going on on a chipset, but it's not unreasonable to think that this workload, plus whatever the chipset has to do (hardware management tasks mostly), as well as the CPU's other tasks on similar interfaces that might saturate the IO die/controller, would influence this.

B350 isn't a very fast chipset to begin with, and I'm willing to bet the CPU in such a motherboard isn't exactly current-gen either. Are you sure you're even running at PCIe 3.0 speeds too? There are 2.0 only CPUs available for AM4.

[–] [email protected] 1 points 2 months ago* (last edited 2 months ago) (1 children)

B350 isn’t a very fast chipset to begin with

For sure.

I’m willing to bet the CPU in such a motherboard isn’t exactly current-gen either.

Reasonable bet, but it's a Ryzen 9 5950X with 64GB of RAM. I'm pretty proud of how far I've managed to stretch this board. 😆 At this point I'm waiting for blown caps, but the case temp is pretty low so it may end up trucking along for surprisingly long time.

Are you sure you’re even running at PCIe 3.0 speeds too?

So given the CPU, it should be PCIe 3.0, but that doesn't remove any of the queues/scheduling suspicions for the chipset.

I'm now replicating data out of this pool and the read load looks perfectly balanced. Bandwidth's fine too. I think I have no choice but to benchmark the disks individually outside of ZFS once I'm done with this operation in order to figure out whether any show problems. If not, they'll go in the spares bin.

[–] [email protected] 2 points 2 months ago (1 children)

Oh wow congrats, I'm currently in the struggle of stretching an ab350m to accept a 4600G and failing.

You're right, you should hit PCIe 3 speeds and it's weird, but the fact that the drives swap speeds depending on how they're plugged in points to either drivers or the chipset.

[–] [email protected] 1 points 2 months ago* (last edited 2 months ago) (1 children)

On paper it should support it. I'm assuming it's the ASRock AB350M. With a certain BIOS version of course. What's wrong with it?

[–] [email protected] 2 points 2 months ago (2 children)

It's a gigabyte ab350m gaming-3 rev 1.0. it boots grub fine but then crashes right after displaying "loading Linux 6.x", CPU led flashes then dram led stays on, I have to turn it off with the PSU switch.

Either it's a rev 1.0 bug which is a thing on those motherboards, or the CPU (or igpu) is defective.

https://superuser.com/questions/1854228/proxmox-doesnt-boot-after-cpu-change

I'm currently waiting on support from both the seller and gigabyte but I don't expect anything out of it, though I'm still yet to test it in a different motherboard.

[–] [email protected] 1 points 2 months ago (1 children)

I think the board has reached the end of the road. 😅

[–] [email protected] 1 points 1 month ago

Sorry I 'ever saw this, that sucks.

Turns out mine was broken too. I put the CPU in my gaming rig and it worked fine, so I bought a new motherboard and the problem is gone.

[–] [email protected] 1 points 2 months ago* (last edited 2 months ago)

Iiinteresting. I'm on the larger AB350-Gaming 3 and it's got REV: 1.0 printed on it. No problems with the 5950X so far. 🤐 Either sheer luck or there could have been updated units before they officially changed the rev marking.