From: "Richard W.M. Jones" <rjones@redhat.com>
To: ivo welch <ivo.welch@ucla.edu>
Cc: kwolf@redhat.com, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] Quick nbdkit question
Date: Thu, 8 Aug 2019 08:41:05 +0100 [thread overview]
Message-ID: <20190808074105.GD3888@redhat.com> (raw)
In-Reply-To: <CAJrNScTufw-809ag5-DoqhDVJs4aRyYm9EQxSO4aqPVgT_+8gA@mail.gmail.com>
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 (ivo.welch@ucla.edu)
> 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
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine. Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/
next parent reply other threads:[~2019-08-08 7:41 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CAJrNScTufw-809ag5-DoqhDVJs4aRyYm9EQxSO4aqPVgT_+8gA@mail.gmail.com>
2019-08-08 7:41 ` Richard W.M. Jones [this message]
2019-08-08 11:27 ` [Qemu-devel] Quick nbdkit question Eric Blake
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190808074105.GD3888@redhat.com \
--to=rjones@redhat.com \
--cc=ivo.welch@ucla.edu \
--cc=kwolf@redhat.com \
--cc=qemu-devel@nongnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).