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Very slow SSD internal transfer on NUC7i7DNKE


I have a reasonable CPU, reasonable Ram, and a reasonable SSD in this box.

The cpu is 2-3% busy during the transfer (from top). There is no other load/process. 0% memory usage. I'm using dd with a block size of 1MB, so it should be super efficient, but I'm only getting about 20MB/sec, which is slower than spinning disk performance.

OS is Ubuntu 18.10.

memory = 32GB


iostat thinks that the disk is 100% busy, which is perplexing (and probably a bit meaningless)


Device           r/s    w/s    rkB/s    wkB/s  rrqm/s  wrqm/s %rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util

loop0           0.00   0.00     0.00     0.00    0.00    0.00  0.00  0.00   0.00   0.00  0.00    0.00    0.00  0.00  0.00

loop1           0.00   0.00     0.00     0.00    0.00    0.00  0.00  0.00   0.00   0.00  0.00    0.00    0.00  0.00  0.00

loop2           0.00   0.00     0.00     0.00    0.00    0.00  0.00  0.00   0.00   0.00  0.00    0.00    0.00  0.00  0.00

loop3           0.00   0.00     0.00     0.00    0.00    0.00  0.00  0.00   0.00   0.00  0.00    0.00    0.00  0.00  0.00

loop4           0.00   0.00     0.00     0.00    0.00    0.00  0.00  0.00   0.00   0.00  0.00    0.00    0.00  0.00  0.00

loop5           0.00   0.00     0.00     0.00    0.00    0.00  0.00  0.00   0.00   0.00  0.00    0.00    0.00  0.00  0.00

sda            14.00  19.50 15360.00 18464.00    0.00    7.50  0.00 27.78 170.29 5118.36 143.51 1097.14  946.87 29.91 100.20


So, I adjust the elevator algorithm to noop. This is much better.

I can read (write to /dev/null) on an uncached file at 500MB/sec. I can write (read from /dev/zero) at around 200MB/sec -- why only half the speed? - FS = ext4, fwiw). When I try to do read and write to/from the same ssd on 2 different ext4 filesystems, the throughput drops to about 120MB/sec.




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2 Replies
Super User Retired Employee

First thing that you need to do is update your BIOS to the latest version available.


If you are currently running BIOS 50 (DNKBLi7v.86A.0050.2018.0716.1602) or older, remember that you need to install BIOS 52 (DNKBLi7v.86A.0052.2018.0808.1344) using the jumper-based BIOS Recovery method. This method is documented here: I would note that it has been anecdotally suggested that you remove the M.2 SSD drive from the NUC while performing this step. Once you are running on BIOS 52, you can upgrade to the latest BIOS (BIOS 60, DNKBLi7v.86A.0060.2018.1220.1528) using the normal F7 method.


If you are already running BIOS 52 (DNKBLi7v.86A.0052.2018.0808.1344) or newer, upgrade to BIOS 60 using the jumper-based BIOS Recovery process described above. Do this even if you are running BIOS 60 already! Again, it has been anecdotally suggested that you remove the M.2 SSD drive from the NUC while performing this step.


Hope this helps,


New Contributor II

WDS100T1B0B is WD Blue M.2 1TB SATA SSD. If I remember correctly this SSD uses planar NAND flash instead of newer 3D NAND.


Specs of this SSD say "Sequential read speeds of up to 545MB/s" and "sequential write speeds of up to 525MB/s".


When copy a lot of data from one partition to another on the same disk, the reads from one partition and the writes to another partition will compete for the SATA 6Gbps bandwidth, so neither the reads nor the writes will reach their max speeds. Also, assuming this SSD features SLC write caching, once the SLC cache is filled by some amount of writes, the write throughput will drop significantly as the disk needs to internally flush the SLC cached data to TLC NAND, in additional to having to handle the reads and writes for the copy operation, so the NAND becomes more busy so the write throughput drops further. I don't find this surprising.


All this is to say you may be experiencing the inherent performance (or lack thereof) of your SSD under specific usage conditions, as opposed this being a NUC issue.