[lkml]   [2019]   [Sep]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[PATCH] Documentation: sysrq: don't recommend 'S' 'U' before 'B'
This advice is obsolete and slightly harmful for filesystems from this
millenium: any modern filesystem can handle unexpected crashes without
requiring fsck -- and on the other hand, trying to write to the disk when
the kernel is in a bad state risks introducing corruption.

For ext2, any unsafe shutdown meant widespread breakage, but it's no longer
a reasonable filesystem for any non-special use.

Signed-off-by: Adam Borowski <>
Documentation/admin-guide/sysrq.rst | 20 +++++++++-----------
1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/Documentation/admin-guide/sysrq.rst b/Documentation/admin-guide/sysrq.rst
index 7b9035c01a2e..72b2cfb066f4 100644
--- a/Documentation/admin-guide/sysrq.rst
+++ b/Documentation/admin-guide/sysrq.rst
@@ -171,22 +171,20 @@ It seems others find it useful as (System Attention Key) which is
useful when you want to exit a program that will not let you switch consoles.
(For example, X or a svgalib program.)

-``reboot(b)`` is good when you're unable to shut down. But you should also
-``sync(s)`` and ``umount(u)`` first.
+``reboot(b)`` is good when you're unable to shut down, it is an equivalent
+of pressing the "reset" button.

``crash(c)`` can be used to manually trigger a crashdump when the system is hung.
Note that this just triggers a crash if there is no dump mechanism available.

-``sync(s)`` is great when your system is locked up, it allows you to sync your
-disks and will certainly lessen the chance of data loss and fscking. Note
-that the sync hasn't taken place until you see the "OK" and "Done" appear
-on the screen. (If the kernel is really in strife, you may not ever get the
-OK or Done message...)
+``sync(s)`` is handy before yanking removable medium or after using a rescue
+shell that provides no graceful shutdown -- it will ensure your data is
+safely written to the disk. Note that the sync hasn't taken place until you see
+the "OK" and "Done" appear on the screen.

-``umount(u)`` is basically useful in the same ways as ``sync(s)``. I generally
-``sync(s)``, ``umount(u)``, then ``reboot(b)`` when my system locks. It's saved
-me many a fsck. Again, the unmount (remount read-only) hasn't taken place until
-you see the "OK" and "Done" message appear on the screen.
+``umount(u)`` can be used to mark filesystems as properly unmounted. From the
+running system's point of view, they will be remounted read-only. The remount
+isn't complete until you see the "OK" and "Done" message appear on the screen.

The loglevels ``0``-``9`` are useful when your console is being flooded with
kernel messages you do not want to see. Selecting ``0`` will prevent all but
 \ /
  Last update: 2019-09-03 18:09    [W:0.044 / U:66.320 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site