FAQ: inconsistent random 4K write speed results

AmorphousDiskMark

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katsura
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Joined: Mon Sep 18, 2017 4:28 pm

FAQ: inconsistent random 4K write speed results

Post by katsura »

FAQ: inconsistent random 4K write speed results

The five-second measurement duration time limit was added to ADM2.5 in order to avoid the impractically lengthy 4K read/write measurement time (many minutes and hours) on inherently slow hard drives, to avoid unnecessary writes on SSDs, and to be consistent with CrystalDiskMark, you may get different or inconsistent random 4K write speed results.

The random 4K write result could be faster, slower, or about the same with the five-second limit. I suspect that this is due to how the write cache on the SSD device itself behaves.
  • Faster: It is possible that the write cache was used during the five-second measurement. Not actually written to the flash memory.
  • Slower: It is possible that the write cache didn't kick in during the five-second measurement.
  • Same: It is possible that the write cache was used at the same rate during the first five-second and after the first five-second, or there is no significant write cache on the device.
Crucial MX500 1TB SATA SSD (it appears that the write cache boost occurred at the beginning of the five-second measurement.):

Code: Select all

$ iostat -w1 disk1
               disk1       cpu    load average
    KB/t   tps  MB/s  us sy id   1m   5m   15m
    0.00     0  0.00  26  4 70  1.82 1.58 1.47
    4.02 12425 48.73  25 36 38  1.82 1.58 1.47
    4.00  8507 33.23  28 41 31  1.82 1.58 1.47
    4.00  6436 25.14  24 36 39  1.82 1.58 1.47
    4.00  4812 18.80  35 38 28  1.82 1.58 1.47
    4.00  4331 16.92  38 37 25  1.99 1.62 1.49
    4.14  1973  7.97  31 20 49  1.99 1.62 1.49
    0.00     0  0.00  20  4 76  1.99 1.62 1.49
Crucial P1 1TB NVMe SSD (it appears that the write cache boost occurred at the beginning of the five-second measurement):

Code: Select all

$ iostat -w1 disk0
               disk0       cpu    load average
    KB/t   tps  MB/s  us sy id   1m   5m   15m
    0.00     0  0.00   0  0 99  1.24 1.21 1.45
    4.00 10397 40.62   3 23 74  1.24 1.21 1.45
    4.00 12858 50.25   1 35 64  1.30 1.22 1.46
    4.00  9139 35.70   1 33 67  1.30 1.22 1.46
    4.00  7446 29.09   1 32 67  1.30 1.22 1.46
    4.00  6754 26.38   0 31 69  1.30 1.22 1.46
    6.96  6050 41.12   0 27 73  1.30 1.22 1.46
   10.28  6418 64.41   2  9 89  1.28 1.21 1.45
    0.00     0  0.00   0 13 87  1.28 1.21 1.45

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