I/O Bottlneck with 1TB cache SSD and 1 parity drive with 7 HDDs... any ideas?


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Setup:

  • 8 x 14 TB Seagate Exos
  • 1 of which is a parity drive
  • 1 Samsung 970 Pro 1 TB Cache SSD
  • 1 PCIe SATA controller
  • Asus 5 3600 CPU
  • MSI ATX 490 motherboard 
  • 64 GB DDR4 RAM 

 

Hi there, I occasionally move large files to the NAS, then the cache drive fills up as the mover can't move the files quickly enough to the HDDs.

I noticed that the mover only writes to one disk at a time when the other disks are idle. Would it not be fair to assume that if the faster SSD were to write to multiple disks at the same time, the mover would be far more efficient? I feel I'm missing the benefit of having 8 drives spinning and the IO bottleneck is quite annoying, as it always interrupts my file operations due to the cache drive filling up too quickly.

 

I'm on a 1Gbit/s LAN so moving files IN to the  NAS at reasonable 100 MB/s and I feel the SSD only writes at about half that speed to a single hard drive... Has anybody else had that issue and found a solution? I have use the onboard SATA for 4 drives and have a SATA controller for the other 4. Usually everything works fine and the speed of the mover is pretty much the same for the motherboard Sata controller and the PCIe one.. 

 

Is this a performance issue by design? I use 1 parity drive to keep things in sync, is that maybe the reason why only one hard drive can be written to at any time? Either way it's really frustrating.

 

Thanks folks, appreciate the help as always!

Edited by theunraidhomeuser
correction
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4 hours ago, theunraidhomeuser said:

Would it not be fair to assume that if the faster SSD were to write to multiple disks at the same time, the mover would be far more efficient?

No, it would be slower since parity would need to be concurrently update for all.

 

4 hours ago, theunraidhomeuser said:

I'm on a 1Gbit/s LAN so moving files IN to the  NAS at reasonable 100 MB/s and I feel the SSD only writes at about half that speed to a single hard drive... Has anybody else had that issue and found a solution?

That's about the max speed you can get with gigabit.

 

For large writes your array disks with turbo write enable should be able to keep up with gigabit, so better just transferring directly to the array.

 

 

 

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I only use cache for "download/process" stuff. Anything that "should" be saved to the array goes directly to the array.

 

I haven't worked on hard drives in a long time, but the "Turbo Write" algorithm... well... it makes as much sense as UnRAID itself.

 

6.

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Thanks folks I have turbo write enabled since the very first day but still not a big win..

 

disabling the cache will solve the io bottleneck but still be epotentially slower than the gigabit lan connection, as the write speeds are somewhat slow and again just to one disk at a time…

 

I really didn’t think about this impact of the parity drive at the beginning as I thought this would be able to cope with data being written to multiple disks at a time..so I guess I’ll just test without cache for now. 
 

thanks everyone 

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2 hours ago, theunraidhomeuser said:

as the write speeds are somewhat slow and again just to one disk at a time…

Your disks should be fast enough for gigabit to be the bottleneck when transferring with turbo write, I can transfer to one of my arrays at 200MB/s+ sustained with disks slower than those.

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  • 1 month later...
On 5/26/2021 at 12:27 PM, JorgeB said:

Note that with gigabit you can't get more than around 115MB/s, diags saved during a transfer might show something, also good idea to run a single stream iperf test to check network bandwidth.

 

 

yes, I'm aware, I'm talking about speeds like 30 MB/s.. so far below that amount. I'd be happy if I managed sustained 110MB/s

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