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Slow read speed - Hardware limitation or Drivepool?


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Currently have a Dell T30 Server (i7-6700 with 16GB RAM) with a pool of 2x8Tb and 1x10TB HD. The drives are Red or White drives, so pretty sure they are lower RPM drives and all spin at 5400 RPM, more or less.

The issue is file transfer speeds over my gigabit network. Copying a 2GB file from my desktop to the pool saturates the network and I get speeds of 110 MB/s. However, read speeds from the pool are much less. Copying the same file from the pool to my desktop transfers at 60-70 MB/s, sometimes less. I've tinkered with DP settings, used different network cards, and tweaked network settings. Nothing really seems to make a difference.

Testing the pool speed with CDM (see screenshot) shows speeds in line with a 5400 RPM hard drive. 

Is the slow speed just a limitation of the slower RPM drives or is it a Drivepool issue? It seems strange as the write speed is great but the read speed is significantly lower.
 

2020-12-17_210430.png

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1 hour ago, Screwtape said:

Is the slow speed just a limitation of the slower RPM drives or is it a Drivepool issue? It seems strange as the write speed is great but the read speed is significantly lower.

I suspect your write speed is significantly higher for a 2GB file transfer because you have 16GB RAM on your system, and Windows will initially cache a file transfer in RAM before it starts to write to the slower HDDs. To verify this, you could queue up a much larger transfer, say 40GB, and watch your transfer write speed in real time. If your computer is anything like mine, you will soon see the high transfer rate of the write to slow down and remain steady when Windows is done caching the transfer in RAM and has to now write to the speed of your HDD.

I don't know exactly how much RAM cache Windows 10 "automagically" uses in file transfers, but you can watch the RAM fill up in a file transfer by observing the stats in Task Manager. With 16GB RAM on your system, it may initially fill the entire 2GB file in RAM before writing it to the HDDs, which gives you a very fast write speed.

As far as a slower read speed, I find the limit not due to DrivePool, but rather the limit is the speed of the HDD containing the data. In other words, the data transfer cannot be faster than the pool drive itself can send out the data. However, DrivePool has a feature called "Read Striping" which can speed up reads from DrivePool. If you have duplication set on your transfer folder(s), it will read from both copies at the same time from 2 different drives, which potentially really increases the speed.

I recently added a SSD to the front end of my DrivePool. I just ran a 15GB transfer write speed test to DrivePool (with SSD cache) and compared that to writing directly to the SSD non-DrivePool. The write transfer rates (~115 MB/s) were the same in my small test. I then copied files back (read test) from DrivePool (still in SSD cache) and my stand alone SSD. The read transfer rates (~115 MB/s) were also the same. In that case, I see no difference between transfer on DrivePool or not.

I did run a test on a file in my DrivePool on the archive HDDs, and the transfer read speed was ~90 M/bps. But I have a mix of hard drives 5400 rpm - 7200 rpm in my pool and that file was transferred from a 7200 rpm drive. I suspect your 60-70 MB/s would be the speed on one of my 5400 rpm drives. Again, DrivePool or not, I see no difference in transfer rates.

I have my DrivePool running on a ~7 year old HP computer with 8GB RAM. My DrivePool HDDs are all attached external USB 3.0 drives some connected directly but most of them connected via hubs. In other words, it's not a very fast setup. But it works for me as a media storage center for Plex/Kodi. Your computer system is much better than mine, but your 5400 rpm drive(s) are probably slowing down your performance.

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one other thing

the speed of the read will be dependant on where on the disk the file is located

as its a disk the speed of the read will be faster when the file is on the outside of the disk vs the inside (near spindle) 

so an empty disk will preform near the quoted speed of the disk as files are added to the outside first

and a full disk with a new file added will go to the inside....

thats simplified as other factors can influence where a new file gets written e.g. you have just deleted a load of files before the new write etc etc

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