Re: [Qemu-devel] Quick nbdkit question

[Richard W.M. Jones]

On Wed, Aug 07, 2019 at 02:30:18PM -0700, ivo welch wrote:
> hi richard---please forgive us.  another quickie.  we created an nbd
> device.  just a block device...no partitions, etc.  in the guest vm, we
> wrote a short C program that writes one sector (open file, fseek, fwrite,
> close file) and then does it again.  all is working just fine.  the host
> console (filter) prints out the write requests, as it should.  (we added
> some more print code.)
> 
> but we are confused about that after the first [but not the second] write,
> we see about 30 (4096b) sector reads, starting from 4096, 12288, ... .
> (the file below has both the C code and the nbdkit messages.)

I guess it's happening because of readahead in the qemu NBD driver, or
generally in the QEMU block layer.  Eric or Kevin will probably have a
better idea.

> there is no file system layer here, just an nbdkit block device. what is
> reading sectors here?  is it the stdio.h library in the guest, something
> magic about access into an unformatted block device in the guest VM linux,
> or nbdkit itself?!     does this sound familiar?
> 
> regards,
> 
> /iaw
> --
> Ivo Welch (address@hidden)
> http://www.ivo-welch.info/

> #include<stdio.h>
> 
> int main() {
>       FILE *fp;
>       char str[4096] = "THIS IS A TEST";
>       
>       fp = fopen("/dev/sdb", "r+");
>       fseek(fp, -4096, SEEK_END);
>       fwrite(str, 1, sizeof(str), fp);
>       fclose(fp);
> 
>       fp = fopen("/dev/sdb", "r+");
>       fseek(fp, -8192, SEEK_END);
>       fwrite(str, 1, sizeof(str), fp);
>       fclose(fp);
> }
> 
> 
> 
> 
> nbdkit: file.2: debug: filter: pwrite count=4096 offset=8589930496 flags=0x0
> nbdkit: file.2: debug: 
> 
> 
> WRITE [4096:8589930496:0:0]: 'THIS IS A 
> TEST.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................'
> 
> 
> 
> nbdkit: file.2: debug: pwrite count=4096 offset=8589930496 fua=0
> nbdkit: file.13: debug: filter: pread count=4096 offset=0 flags=0x0
> nbdkit: file.13: debug: pread count=4096 offset=0
> nbdkit: file.13: debug: filter: pread count=4096 offset=4096 flags=0x0
> nbdkit: file.13: debug: pread count=4096 offset=4096
> nbdkit: file.13: debug: filter: pread count=4096 offset=12288 flags=0x0
> nbdkit: file.13: debug: pread count=4096 offset=12288
> nbdkit: file.13: debug: filter: pread count=4096 offset=8589869056 flags=0x0
> nbdkit: file.13: debug: pread count=4096 offset=8589869056
> nbdkit: file.1: debug: filter: pread count=4096 offset=8589930496 flags=0x0
> nbdkit: file.1: debug: pread count=4096 offset=8589930496
> nbdkit: file.1: debug: filter: pread count=4096 offset=8589799424 flags=0x0
> nbdkit: file.1: debug: pread count=4096 offset=8589799424
> nbdkit: file.1: debug: filter: pread count=4096 offset=8589901824 flags=0x0
> nbdkit: file.1: debug: pread count=4096 offset=8589901824
> nbdkit: file.6: debug: filter: pread count=4096 offset=8589803520 flags=0x0
> nbdkit: file.6: debug: pread count=4096 offset=8589803520
> nbdkit: file.6: debug: filter: pread count=4096 offset=8589729792 flags=0x0
> nbdkit: file.6: debug: pread count=4096 offset=8589729792
> nbdkit: file.10: debug: filter: pread count=4096 offset=8589631488 flags=0x0
> nbdkit: file.10: debug: pread count=4096 offset=8589631488
> nbdkit: file.10: debug: filter: pread count=4096 offset=8589586432 flags=0x0
> nbdkit: file.10: debug: pread count=4096 offset=8589586432
> nbdkit: file.8: debug: filter: pread count=4096 offset=8589557760 flags=0x0
> nbdkit: file.8: debug: pread count=4096 offset=8589557760
> nbdkit: file.0: debug: filter: pread count=4096 offset=8589467648 flags=0x0
> nbdkit: file.0: debug: pread count=4096 offset=8589467648
> nbdkit: file.3: debug: filter: pread count=4096 offset=8589434880 flags=0x0
> nbdkit: file.3: debug: pread count=4096 offset=8589434880
> nbdkit: file.9: debug: filter: pread count=4096 offset=8589426688 flags=0x0
> nbdkit: file.9: debug: pread count=4096 offset=8589426688
> nbdkit: file.9: debug: filter: pread count=4096 offset=8589447168 flags=0x0
> nbdkit: file.9: debug: pread count=4096 offset=8589447168
> nbdkit: file.9: debug: filter: pread count=4096 offset=8588353536 flags=0x0
> nbdkit: file.9: debug: pread count=4096 offset=8588353536
> nbdkit: file.9: debug: filter: pread count=4096 offset=16384 flags=0x0
> nbdkit: file.9: debug: pread count=4096 offset=16384
> nbdkit: file.9: debug: filter: pread count=4096 offset=32768 flags=0x0
> nbdkit: file.9: debug: pread count=4096 offset=32768
> nbdkit: file.9: debug: filter: pread count=4096 offset=65536 flags=0x0
> nbdkit: file.9: debug: pread count=4096 offset=65536
> nbdkit: file.14: debug: filter: pread count=4096 offset=131072 flags=0x0
> nbdkit: file.14: debug: pread count=4096 offset=131072
> nbdkit: file.14: debug: filter: pread count=4096 offset=262144 flags=0x0
> nbdkit: file.14: debug: pread count=4096 offset=262144
> nbdkit: file.1: debug: filter: pread count=4096 offset=524288 flags=0x0
> nbdkit: file.1: debug: pread count=4096 offset=524288
> nbdkit: file.1: debug: filter: pread count=4096 offset=1048576 flags=0x0
> nbdkit: file.1: debug: pread count=4096 offset=1048576
> nbdkit: file.1: debug: filter: pread count=4096 offset=2097152 flags=0x0
> nbdkit: file.1: debug: pread count=4096 offset=2097152
> nbdkit: file.1: debug: filter: pread count=4096 offset=4194304 flags=0x0
> nbdkit: file.1: debug: pread count=4096 offset=4194304
> nbdkit: file.1: debug: filter: pread count=4096 offset=28672 flags=0x0
> nbdkit: file.1: debug: pread count=4096 offset=28672
> nbdkit: file.1: debug: filter: pread count=4096 offset=61440 flags=0x0
> nbdkit: file.1: debug: pread count=4096 offset=61440
> nbdkit: file.1: debug: filter: pread count=4096 offset=8192 flags=0x0
> nbdkit: file.1: debug: pread count=4096 offset=8192
> nbdkit: file.1: debug: filter: pread count=8192 offset=20480 flags=0x0
> nbdkit: file.1: debug: pread count=8192 offset=20480
> nbdkit: file.1: debug: filter: pread count=24576 offset=36864 flags=0x0
> nbdkit: file.1: debug: pread count=24576 offset=36864
> nbdkit: file.1: debug: filter: pread count=61440 offset=69632 flags=0x0
> nbdkit: file.1: debug: pread count=61440 offset=69632
> nbdkit: file.1: debug: filter: pread count=126976 offset=135168 flags=0x0
> nbdkit: file.1: debug: pread count=126976 offset=135168
> nbdkit: file.12: debug: filter: pread count=258048 offset=266240 flags=0x0
> nbdkit: file.12: debug: pread count=258048 offset=266240
> nbdkit: file.13: debug: filter: pread count=16384 offset=8589410304 flags=0x0
> nbdkit: file.13: debug: pread count=16384 offset=8589410304
> nbdkit: file.13: debug: filter: pread count=4096 offset=8589430784 flags=0x0
> nbdkit: file.13: debug: pread count=4096 offset=8589430784
> nbdkit: file.13: debug: filter: pread count=8192 offset=8589438976 flags=0x0
> nbdkit: file.13: debug: pread count=8192 offset=8589438976
> nbdkit: file.13: debug: filter: pread count=16384 offset=8589451264 flags=0x0
> nbdkit: file.13: debug: pread count=16384 offset=8589451264
> nbdkit: file.13: debug: filter: pread count=86016 offset=8589471744 flags=0x0
> nbdkit: file.13: debug: pread count=86016 offset=8589471744
> nbdkit: file.6: debug: filter: pread count=24576 offset=8589561856 flags=0x0
> nbdkit: file.6: debug: pread count=24576 offset=8589561856
> nbdkit: file.15: debug: filter: pread count=40960 offset=8589590528 flags=0x0
> nbdkit: file.15: debug: pread count=40960 offset=8589590528
> nbdkit: file.10: debug: filter: pread count=36864 offset=8589635584 flags=0x0
> nbdkit: file.10: debug: pread count=36864 offset=8589635584
> nbdkit: file.8: debug: filter: pread count=57344 offset=8589672448 flags=0x0
> nbdkit: file.8: debug: pread count=57344 offset=8589672448
> nbdkit: file.0: debug: filter: pread count=65536 offset=8589733888 flags=0x0
> nbdkit: file.0: debug: pread count=65536 offset=8589733888
> nbdkit: file.3: debug: filter: pread count=61440 offset=8589807616 flags=0x0
> nbdkit: file.3: debug: pread count=61440 offset=8589807616
> nbdkit: file.2: debug: filter: pread count=28672 offset=8589873152 flags=0x0
> nbdkit: file.2: debug: pread count=28672 offset=8589873152
> nbdkit: file.7: debug: filter: pread count=20480 offset=8589905920 flags=0x0
> nbdkit: file.7: debug: pread count=20480 offset=8589905920
> nbdkit: file.11: debug: filter: pwrite count=4096 offset=8589926400 flags=0x0
> nbdkit: file.11: debug: 
> 
> 
> WRITE [4096:8589926400:0:0]: 'THIS IS A 
> TEST.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................'
> 
> 
> 
> nbdkit: file.11: debug: pwrite count=4096 offset=8589926400 fua=0
> 

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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