Every now and then you get an eye opener

This one is while we are conditioning a Forte NVMe unit, and I am running our OS install scripts. Running dstat in a window to watch the overall system …

----total-cpu-usage---- -dsk/total- -net/total- ---paging-- ---system--
usr sys idl wai hiq siq| read  writ| recv  send|  in   out | int   csw 
  2   5  94   0   0   0|   0    22G| 218B  484B|   0     0 | 363k  368k
  1   4  94   0   0   0|   0    22G| 486B  632B|   0     0 | 362k  367k
  1   4  94   0   0   0|   0    22G| 628B  698B|   0     0 | 363k  368k
  2   5  92   1   0   0| 536k  110G| 802B 2024B|   0     0 | 421k  375k
  1   4  93   2   0   0|   0    22G| 360B  876B|   0     0 | 447k  377k

Wait … is that 110GB/s (2nd line from bottom, in the writ column) ? Wow …

Likely a measurement oddity. But it made me do a double take

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2 thoughts on “Every now and then you get an eye opener

  1. Depending on the FS in play here, you’re likely seeing TRIM commands. mkfs.ext4 in particular will trim the entire partition at the beginning, and dstat thinks these trims include data. Since almost all blocks in the partition for newly created file systems are being issued TRIMs, dstat aggregates that and comes up with rather wild numbers.

  2. This is xfs, and it does this unless you use -K to avoid the discard. I often forget this aspect as I make the file systems … and often I see wild numbers. Its amusing, but not “real”.

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