reqlog - Linux kernel patch to monitor disk reads/writes

		      Copyright (c) 1999, Malcolm Beattie

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 1, or (at your option)
    any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
    General Public License for more details.

This is reqlog version 0.3.

reqlog is a small patch to the Linux 2.2.12 kernel which writes brief
details of each physical disk read/write to a ring buffer from where a
userland program can make use of the information. Example uses of this
information are for hot migration of live in-use block devices, as by
the bmigrate suite available at
    ftp://ftp.ox.ac.uk/pub/linux/bmigrate-0.3.tar.gz
or detailed disk I/O monitoring utilities (I'm going to write one Real
Soon Now).

INSTALLATION

This patch relies on one other patch to applied: the bufflink patch
which allows fast, lightweight communications between kernel and
userland. You can get the bufflink patch from
    ftp://ftp.ox.ac.uk/pub/linux/bufflink-0.3.tar.gz

Once the bufflink patch is applied (if you've only just applied it,
you can apply this reqlog patch now before compiling your kernel),
apply the reqlog patch with
    cd /usr/src/linux
    patch -p1 < reqlog-2.2.12.patch

Run your favourite kernel configuration tool (e.g. make config,
make menuconfig, make xconfig, make oldconfig) and say Y to the
options CONFIG_EXPERIMENTAL and CONFIG_REQUEST_LOG (and if you've
only just applied the bufflink patch then ensure you also remember to
turn on CONFIG_BUFFLINK_KERNEL and CONFIG_BUFFLINK as described in the
bufflink documentation). Once you've built your new kernel, ensure you
create a device node for reqlog:
    mknod -m 0600 /dev/reqlog c 120 0
Note there the "-m 0600" which is equivalent to
"chmod 0600 /dev/reqlog". The semantics of the permissions on
/dev/reqlog are that anyone with read permission can read which
sectors on disk have been read/written (interfering with any other
process doing the same since the data can only be read once) and
anyone with write access to /dev/reqlog can enable/disable logging.

Compiling and running the utility program "reqlog" in this
distribution shows:
    Usage: reqlog enable|disable|read device
Doing "reqlog enable /dev/reqlog" turns on kernel logging of disk
I/O requests and "reqlog disables /dev/reqlog" it. You can use
    reqlog read /dev/reqlog
as a simplistic way of displaying the information that reqlog
records: for each physical disk I/O you will see a line such as
    device 3/1 sector 107936 write 2
which means that a disk write has just been issued for 2 sectors
(2 * 512 bytes = 1KB) at sector 107936 on the block device with
major 3 and minor 1 (/dev/hda1).

Malcolm Beattie
mbeattie@sable.ox.ac.uk
1 November 1999