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SMR?RESILVER?

HoneyBadger

Mushroom! Mushroom!
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#21
It's only a hypothesis, but you would have to know the rough size of the CMR/cache partition on the SMR drive. Blasting it with small, random I/O would cause the drive to write it to the cache partition, once that fills up then you'll have to write straight to the SMR portion and that's when you should see the latency spike.

Maybe I'll have to get one of these smaller SMR drives and expose the scenarios where they fall down.
 

tfran1990

FreeNAS Experienced
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#22
It would be a good thing to test out, as time goes on SMR will be more common. Its important to be able to see there negative effect on freenas with SMR.
rough size of the CMR/cache partition on the SMR drive
This^ differs from the 256Mb specified right?
 

tfran1990

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#24
In that case, lets say the CMR zone is 50G, after it fills up with small files wouldn't it the 256Mb portion?
 

Johnnie Black

FreeNAS Guru
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#25
Most models now have a multi tiered cache, to minimize as much as possible the SMR writes issues, first goes to the small RAM cache, then some models can have a small but larger FLASH zone, then they can have a CMR zone and finally SMR, they are filled in this order, if/when all these caches are filled you hit the SMR "wall".

https://www.seagate.com/files/www-c...-tier-caching-technology-white-paper-2017.pdf

The difficultty is finding out detailed info on what type of cache a specific model has.
 
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#27
Its not uncommon during a resilver the disc being written to will run higher % used(maybe 50%) the rest that are reading do like 25%. Its going to take time to see how the SMR discs playout.
It might not be a bad thing if using smr drives the price down a lot. less heat. longer life span. less noise. higher density, could it be be worth it?
In my experience of the last couple weeks, the WD60EZAZ SMR drives run 100% busy during a resilver, according to gstat. It was absolutely the bottleneck in the operation. The other SMR drives being read from ran from 30-50% busy, and the PMR drives being read from were <10% busy.

The hot-spare style resilver that I just killed was writing at about 6 megabytes/sec, at 100% busy. I'm ditching them, at a loss, to switch to PMR drives.

I could see a use case for them, but resilvering times for arrays with multiple TB used per drive are going to be so long that it might be worth moving to raidz3 for additional protection.
 
Last edited:

tfran1990

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#28
I checked my zpool status but the resilver reset, i had a scrub run a day ago.
Is it possible to pull a log for the resilver info for you?
 
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