Very slow writes to cache pool


toptable

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Hello. Hoping someone can offer some help. I'm seeing extremely slow writes to the cache of my new unraid server. I'll  try to set out what I've been doing below, along with things I've tried. I'll gladly provide any more info if you need it. I've attached my diagnostics as seems to be the norm, and also a screengrab of a large transfer (c. 80GB) to show the issue. I took the diagnostics during the transfer in the screenshot.

 

I also noticed that the CPU is pretty much pinned while the transfer is going on (screenshot also attached), but when I looked in the processes tool, I couldn't see anything with high usage. Is this normal? CPU is an i5-4590.

 

My setup is: 

 

2x 240GB SATA SSDs for cache. These are set to the typical Raid 1. 

I have the trim plugin installed and set to daily. I also ran it from the terminal. 

I have the cache pool set to maintain 20GB of free space, and there is plenty left on the SSDs. 

Cache is set to "Yes" in the share I'm writing to. I've also confirmed it is indeed being written to the cache pool. 

Speeds were ok for the first couple of transfers I made, about 85MB/s, then suddenly dropped mid transfer to 20MB/s and have pretty much stayed there since. 

 

Many thanks for any help you can offer!

 

 

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old-hen-diagnostics-20201218-1602.zip

Edited by toptable
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9 minutes ago, JorgeB said:

This look like a device problem, is it faster if you transfer directly to the array with turbo write enable?

I turned on reconstruct write and copied a 13GB file to a share with cache set to "no" to test this. The copy averaged about 95MB/s. Stable speeds throughout. 

 

I've also attached a screengrab of me running trim 3 times in a row on the SSDs. The results are always the same. Is this right? 

Trim.PNG

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Fair enough, they are economical SSDs, however I'd expect them to manage more than 20MB/s. I'd also expect them not to drop to 350KB/s pretty much ever!  Wouldn't you?

 

I've used my fair share of cheap SSDs in my time and never seen performance like that! 

 

Thanks for taking the time to help me! 

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Hello! I'm checking back in for completeness. Hopefully this will help someone with the same issue in the future. The advice I was given was bang on. My crappy ADATA SSDs were to blame. Use decent SSD drives with 3D TLC NAND. I'm using WD Blues. 

 

My cache drives are now running at line speed, all the time. See attached screengrab for the results. 

 

Thanks so much for the help. 

upgrade.PNG

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