Memory overcommit (was Magic SysRq Key Sequences over Remote DRAC5 Connection)

Rainer M. Canavan rainer-dellpe at 7val.com
Fri Jun 4 03:26:43 CDT 2010


Bryce Nesbitt wrote:
> We have Dell 1950 and 2950 servers with DRAC version 1.20 firmware, and
> normally use putty or OpenSSH to connect.  As far as well can tell the
> serial 'break' does not equate to the SysRq key as documented.
> 
> These servers run Debian Linux, and we periodically have problems with out
> of control memory situations.  We could save servers from a hard reboot by
> sending Magic SysReq commands such as "m" and "f" (to dump Linux Kernel
> memory stats and invoke the Out of Memory Killer (oom-killer).  Or make a
> reboot nicer by syncing disks first.  These commands work fine directly from
> an attached keyboard (e.g. Alt-SysRq-m).  Can we get Magic SysRq over DRAC5

I unfortunately had an opportunity to test this with a 1950 and 
DRAC Firmware 1.51. While the Macros (Alt+SysRq, Alt+Printscreen) are 
useless, the Java based virtual Console under Windows in full screen 
mode does pass Alt+SysRq properly. I'd suspect that this could work 
from Linux clients as well if Magic SysRq ist disabled locally. 

I have used serial Break to trigger Magic SysRequests on my personal
Linux Server(s) in the past, but it appeared to be rather picky 
about the timing. I assume that this requires the system console to 
be on that specific serial port, just having a getty listening will
not be sufficient.

Rainer



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