gstat more writes per seconds than there are seconds per write

[Bernd Walter]

On Thu, Mar 08, 2018 at 11:06:23AM +0100, Ronald Klop wrote:
> Hello,
> 
> I'm confused by this output.
> w/s = 5, but ms/w is 1108; how can I put 5 writes in a second if the  
> average write takes more than a second?

It is the time a request has to wait until it is done.
But your L(q) says that 17 are queued.

> Is my SD card super parallel? :-)

Maybe, maybe not, flash device vendors don't tell how they work
internally, which is a shame, but you can only request a single
transaction with an SD card.
On USB sticks you might be able to do tagged command queuing,
not that many support that...
Nevertheless you can always queue more than one request at the OS layer.

> Sometimes the numbers are even higher.
> 
> dT: 1.010s  w: 1.000s
>  L(q)  ops/s r/s kBps ms/r    w/s   kBps   ms/w   %busy Name
>    17      5   0    0  0.0      5    420   1108   81.7  mmcsd0
>     0      0   0    0  0.0      0      0    0.0    0.0  da0
>     0      0   0    0  0.0      0      0    0.0    0.0  mmcsd0s1
>    17      5   0    0  0.0      5    420   1108   81.7  mmcsd0s2
>     0      0   0    0  0.0      0      0    0.0    0.0  msdosfs/MSDOSBOOT
>    17      5   0    0  0.0      5    420   1108   81.7  mmcsd0s2a
>    17      5   0    0  0.0      5    420   1108   81.7  ufs/rootfs
> 
> [root at rpi2 ~]# uname -a
> dmesg | grep mmc
> FreeBSD rpi2 11.1-RELEASE-p6 FreeBSD 11.1-RELEASE-p6 #0 r328567: Tue Jan  
> 30 20:10:37 CET 2018      
> root at rpi2:/mnt/usbdisk/usr/obj/mnt/usbdisk/usr/src/sys/RPI2  arm
> 
> [root at rpi2 ~]# dmesg | grep mmc
> mmc0: <MMC/SD bus> on sdhci_bcm0
> mmc0: CMD8 failed, RESULT: 1
> mmcsd0: 2GB <SD SMI   1.0 SN 1736A86C MFG 05/2007 by 27 SM> at mmc0  
> 41.6MHz/4bit/65535-block
> 
> Regards,
> Ronald.
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-- 
B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.