diff options
Diffstat (limited to 'doc/stonith/README.rcd_serial')
| -rw-r--r-- | doc/stonith/README.rcd_serial | 186 |
1 files changed, 186 insertions, 0 deletions
diff --git a/doc/stonith/README.rcd_serial b/doc/stonith/README.rcd_serial new file mode 100644 index 0000000..8b4abb4 --- /dev/null +++ b/doc/stonith/README.rcd_serial @@ -0,0 +1,186 @@ +rcd_serial - RC Delayed Serial +------------------------------ + +This stonith plugin uses one (or both) of the control lines of a serial +device (on the stonith host) to reboot another host (the stonith'ed host) +by closing its reset switch. A simple idea with one major problem - any +glitch which occurs on the serial line of the stonith host can potentially +cause a reset of the stonith'ed host. Such "glitches" can occur when the +stonith host is powered up or reset, during BIOS detection of the serial +ports, when the kernel loads up the serial port driver, etc. + +To fix this, you need to introduce a delay between the assertion of the +control signal on the serial port and the closing of the reset switch. +Then any glitches will be dissipated. When you really want to do the +business, you hold the control signal high for a "long time" rather than +just tickling it "glitch-fashion" by, e.g., using the rcd_serial plugin. + +As the name of the plugin suggests, one way to achieve the required delay is +to use a simple RC circuit and an npn transistor: + + + . . + RTS . . ----------- +5V + or ---------- . | + DTR . | . Rl reset + . | T1 . | |\logic + . Rt | ------RWL--------| -------> + . | b| /c . |/ + . |---Rb---|/ . + . | |\ . (m/b wiring typical + . C | \e . only - YMMV!) + . | | . + . | | . + SG ---------------------------RWG----------- 0V + . . + . . stonith'ed host + stonith host --->.<----- RC circuit ----->.<---- RWL = reset wire live + (serial port) . . RWG = reset wire ground + + +The characteristic delay (in seconds) is given by the product of Rt (in ohms) +and C (in Farads). Suitable values for the 4 components of the RC circuit +above are: + +Rt = 20k +C = 47uF +Rb = 360k +T1 = BC108 + +which gives a delay of 20 x 10e3 x 47 x 10e-6 = 0.94s. In practice the +actual delay achieved will depend on the pull-up load resistor Rl if Rl is +small: for Rl greater than 3k there is no significant dependence but lower +than this and the delay will increase - to about 1.4s at 1k and 1.9s at 0.5k. + +This circuit will work but it is a bit dangerous for the following reasons: + +1) If by mistake you open the serial port with minicom (or virtually any +other piece of software) you will cause a stonith reset ;-(. This is +because opening the port will by default cause the assertion of both DTR +and RTS, and a program like minicom will hold them high thenceforth (unless +and until a receive buffer overflow pulls RTS down). + +2) Some motherboards have the property that when held in the reset state, +all serial outputs are driven high. Thus, if you have the circuit above +attached to a serial port on such a motherboard, if you were to press the +(manual) reset switch and hold it in for more than a second or so, you will +cause a stonith reset of the attached system ;-(. + +This problem can be solved by adding a second npn transistor to act as a +shorting switch across the capacitor, driven by the other serial output: + + + . . + . . ----------- +5V + RTS ----------------- . | + . | . Rl reset + . | T1 . | |\logic + . Rt | ------RWL--------| -------> + . | b| /c . |/ + . T2 --|---Rb---|/ . + . | / | |\ . (m/b wiring typical + . b| /c | | \e . only - YMMV!) + DTR ------Rb--|/ C | . + . |\ | | . + . | \e | | . + . | | | . + SG ----------------------------------RWG------------- 0V + . . + . . stonith'ed host +stonith->.<--------- RC circuit ------->.<---- RWL = reset wire live + host . . RWG = reset wire ground + + +Now when RTS goes high it can only charge up C and cause a reset if DTR is +simultaneously kept low - if DTR goes high, T2 will switch on and discharge +the capacitor. Only a very unusual piece of software e.g. the rcd_serial +plugin, is going to achieve this (rather bizarre) combination of signals +(the "meaning" of which is something along the lines of "you are clear to +send but I'm not ready"!). T2 can be another BC108 and with Rb the same. + +RS232 signal levels are typically +-8V to +-12V so a 16V rating or greater +for the capacitor is sufficient BUT NOTE that a _polarised_ electrolytic should +not be used because the voltage switches around as the capacitor charges. +Nitai make a range of non-polar aluminium electrolytic capacitors. A 16V 47uF +radial capacitor measures 6mm diameter by 11mm long and along with the 3 +resistors (1/8W are fine) and the transistors, the whole circuit can be built +in the back of a DB9 serial "plug" so that all that emerges from the plug are +the 2 reset wires to go to the stonith'ed host's m/b reset pins. + +NOTE that with these circuits the reset wires are now POLARISED and hence +they are labelled RWG and RWL above. You cannot connect to the reset pins +either way round as you can when connecting a manual reset switch! You'll +soon enough know if you've got it the wrong way round because your machine +will be in permanent reset state ;-( + + +How to find out if your motherboard can be reset by these circuits +------------------------------------------------------------------ + +You can either build it first and then suck it and see, or, you need a +multimeter. The 0V rail of your system is available in either +of the 2 black wires in the middle of a spare power connector (one of +those horrible 4-way plugs which you push with difficulty into the back +of hard disks, etc. Curse IBM for ever specifying such a monstrosity!). +Likewise, the +5V rail is the red wire. (The yellow one is +12V, ignore +this.) + +First, with the system powered down and the meter set to read ohms: + + check that one of the reset pins is connected to 0V - this then + is the RWG pin; + + check that the other pin (RWL) has a high resistance wrt 0V + (probably > 2M) and has a small resistance wrt to +5V - between + 0.5k and 10k (or higher, doesn't really matter) will be fine. + +Second, with the system powered up and the meter set to read Volts: + + check that RWG is indeed that i.e. there should be 0V between it + and the 0V rail; + + check that RWL is around +5V wrt the 0V rail. + +If all this checks out, you are _probably_ OK. However, I've got one +system which checks out fine but actually won't work. The reason is that +when you short the reset pins, the actual current drain is much higher than +one would expect. Why, I don't know, but there is a final test you can do +to detect this kind of system. + +With the system powered up and the meter set to read milliamps: + + short the reset pins with the meter i.e. reset the system, and + note how much current is actually drained when the system is in + the reset state. + +Mostly you will find that the reset current is 1mA or less and this is +fine. On the system I mention above, it is 80mA! If the current is +greater than 20mA or so, you have probably had it with the simple circuits +above, although reducing the base bias resistor will get you a bit further. +Otherwise, you have to use an analog switch (like the 4066 - I had to use 4 +of these in parallel to reset my 80mA system) which is tedious because then +you need a +5V supply rail to the circuit so you can no longer just build it +in the back of a serial plug. Mail me if you want the details. + +With the circuit built and the rcd_serial plugin compiled, you can use: + +stonith -t rcd_serial -p "testhost /dev/ttyS0 rts XXX" testhost + +to test it. XXX is the duration in millisecs so just keep increasing this +until you get a reset - but wait a few secs between each attempt because +the capacitor takes time to discharge. Once you've found the minimum value +required to cause a reset, add say 200ms for safety and use this value +henceforth. + +Finally, of course, all the usual disclaimers apply. If you follow my +advice and destroy your system, sorry. But it's highly unlikely: serial +port outputs are internally protected against short circuits, and reset pins +are designed to be short circuited! The only circumstance in which I can +see a possibility of damaging something by incorrect wiring would be if the +2 systems concerned were not at the same earth potential. Provided both +systems are plugged into the same mains system (i.e. are not miles apart +and connected only by a very long reset wire ;-) this shouldn't arise. + +John Sutton +john@scl.co.uk +October 2002 |