January 13, 201511 yr I just installed my first cache drive, a 250gb Samsung 840 EVO SSD. This should be a fast cache drive and I expected write speeds 70+MB/s, but my write speeds have dropped from 37MB/s down to 30MB/s after installing the cache drive, for the same media copy. How could this be?
January 13, 201511 yr Cache use is configured per user share. Have you set any of your user shares to use cache?
January 13, 201511 yr Author I have not, I will set that now. Strange though, I have been writing to my array all day at 37MB/s, and as soon as I install the cache drive it drops to 30MG/s. I checked my main unraid page, and while I was copying the reads and writes to my cache drive were going up. But you are correct, I checked the contents of my cache drive and it is blank. So the cache drive has inserted some sort of overhead to writing to my user shares that are not included in the cache drive, I don't like that!
January 13, 201511 yr I have not, I will set that now. Strange though, I have been writing to my array all day at 37MB/s, and as soon as I install the cache drive it drops to 30MG/s. I checked my main unraid page, and while I was copying the reads and writes to my cache drive were going up. But you are correct, I checked the contents of my cache drive and it is blank. So the cache drive has inserted some sort of overhead to writing to my user shares that are not included in the cache drive, I don't like that! It has not. It just doesn't work like that. But if you want to prove it to youself either way, remove the drive and see if speeds go back up. There are any number of reasons speed could have dropped.
January 13, 201511 yr Author I just set my user shares to use the cache drive and ran another 10gig file copy test. 35.5 MB/s, slower than without a cache drive. What can I check/test?
January 13, 201511 yr Author And I just checked the contents of my cache drive, still blank even though I set the user share I copies to, to use cache drive..... What could be preventing unraid from copying to my drive? I set the min free space to 100gb on a 250gb drive.
January 13, 201511 yr Author Ran another 10gb test, I am back to my non-cache speed of 37MB/s so the previous tests must be an anomoly. Now to figure out how to get the files to go through the cache drive? The drive is added to unraid, formatted, and shares set to use it and the min space set to below the drive capacity.
January 13, 201511 yr Two things: 2mb/s is not that different, it is noise for the most part. That said, please set shares to cache, reboot, and retest. IIRC there have been reports of that being an issue.
January 13, 201511 yr Author Restarted server, now getting 65-70 MB/s. This is better, is this what I should expect, or is there any tweaking or other things I can check to increase speed? I am on wired gigabit, the cache is plugged into motherboard SATA.
January 13, 201511 yr What are you feeding the SSD-cache with? another SSD? a VERY fast HDD? From its outer tracks? there is a script somewhere on the board (sorry I can't search for it right now) that runs a quick (about 30sec/drive) speed test. It can confirm for you that your SSD is at least operating at high speed as configured on your board. then you'll want to test the drive your feeding it from (hdtune or bench32 perhaps). After that, I'd start looking at the network. How many switches / routers between the machines. Perhaps swap around cables / eliminate hops as a test. If you're savvy enough try loading up iPerf on two machines to confirm network speeds.
January 14, 201511 yr Author The system that I am copying from frequently feeds terabytes of data to USB3.0 hard drives at 85MB/s average so that is what I was expecting. I will look into the networking as that could be the issue, thank you!
January 14, 201511 yr Well truth be told, 85m/s isn't that fast. Add to that some network overhead and I can't say I'm that surprised. Still, seems a bit slow, but not that slow. Confirm your unraid SSD speed in place when you get a chance just to make sure.
January 15, 201511 yr Somewhere here on the forums is a drive speed test script. I can't find it at the moment. If that continues, and no one else pipes up with it, I'll try to post it tonight when I get home. It tests all your drives (using dd iirc) and posts the results to an html on your flash you can view.
January 15, 201511 yr Found it http://lime-technology.com/forum/index.php?topic=31073.msg280120#msg280120
January 19, 201511 yr Author I ran the speed test, my cache drive is running at 447 MB/s I am going to upgrade my networking gear, and swap out the spinning disc for a ssd in the windows box that I typically copy from. Will report back my speeds as I make these changes.
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