02-27-2018 11:46 AM
Poor performance of disks on the RAID controller Perc H700.
# megacli -LDInfo -Lall -Aall
Adapter 0 -- Virtual Drive Information:
Virtual Drive: 0 (Target Id: 0)
Name :Virtual Disk 0
RAID Level : Primary-1, Secondary-0, RAID Level Qualifier-0
Size : 931.0 GB
Sector Size : 512
Mirror Data : 931.0 GB
State : Optimal
Strip Size : 64 KB
Number Of Drives : 2
Span Depth : 1
Default Cache Policy: WriteThrough, ReadAheadNone, Direct, Write Cache OK if Bad BBU
Current Cache Policy: WriteThrough, ReadAheadNone, Direct, Write Cache OK if Bad BBU
Default Access Policy: Read/Write
Current Access Policy: Read/Write
Disk Cache Policy : Enabled
Encryption Type : None
Bad Blocks Exist: No
Is VD Cached: Yes
Cache Cade Type : Read Only
02-27-2018 12:44 PM
It is a limitation of the TLC NAND technology. TLC drives are meant for average consumer use (meaning small amounts of writing, but lots of reading). I actually had an MX300 (uses TLC NAND) and I tested it over USB3. It will go at near 500MB/s for 30-40 seconds until the cache is filled, then it drops to about 250MB/s. As I continued testing the MX300 eventually dropped until a spinning hard drive is faster. In fact the MX300 actually got locked into the lower speed of 50MB/s and the only way I could recover it was by a Secure Erase. I tried several MX300s to be sure it was not a bad drive. This was before websites were giving proper true reviews of the TLC SSD performance. I believe I read that the TLC SSDs may also be throttled when the temperatures rise, but I'm not sure if that is correct.
Do the test again, but use dstat to monitor the SSD while you do the "dd" test.
Use: dstat -tdD sda
I understand the MX500 should be better than the MX300s due to some design changes, plus they are using a different controller. Personally I'll stick to the MLC SSDs for as long as possible. Surprisingly the BX300 series uses MLC NAND.
Here is an very good article on Tom's Hardware giving the current status of SSD technology.
02-28-2018 05:23 AM
Does it have a cache? We're testing my memory here (never a good thing!) but at least in the olden days SSD performance was terrible on 'proper' RAID controllers that lack a cache...