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SubjectLinux RAS
On Fri, 15 Sep 2000 10:42:54 +0100, wrote:
>I think the case for the kernel debugger is better stated as the case for
>RAS (Reliability, Serviceability and Availability) in the kernel [snip]
>Customers knew we never sent
>developers on site to debug OS/390 (or MVS as it was called in those days).
>They also knew that we never rejected a problem because it was not
>re-creatable and we never even asked for a re-creation scenario. The reason
>for this was that we had appropriate RAS capability in MVS which allowed
>data to be captured automatically at fault time combined with a certain
>amount of self-healing capability and automated recovery.

It would be nice if Linux had the same level of RAS as MVS. But
consider the different environments that MVS and Linux run under.

MVS. Standard and external I/O controllers. Fix the data pages,
build the CCW, issue SIO, forget about it until it interrupts.
I/O requires little or no OS support, it can even be done from
bare hardware.

Linux. 50+ different I/O controllers. Most require continual hand
holding by the operating system. Doing I/O from bare hardware
is difficult if not impossible.

MVS. Multiple systems keys (0-7) so different system components can
be segregated. If VTAM (key 7) dies the rest of the OS (keys
0-6) can run. Multiple system keys stop most runaway
components from stamping on other component's data. Key
switching is relatively cheap.

Linux. On ix86, all of the kernel runs in key 0. No protection of one
part of the kernel from another. Key switching is relatively

MVS. High quality hardware. Processors that can detect their own
problems, all memory is ECC or better, disks that report
failures before they occur.

Linux. Most users are on the cheapest possible hardware. No parity on
memory, processors and disks that just die without warning.
Lots of random problems, "one bit different, probably bad RAM".

MVS. Trace of interesting events in the master trace table.

Linux. No kernel trace table (oops backtrace does *not* count).

MVS. Only one hardware model - S/390 with minimum hardware
requirements for each version of OS/390. IBM can get away with
telling customers "to run OS/390 2.9 you need a G5 processor or

Linux. 12+ different hardware models, with no restriction on how old
the hardware is. We cannot tell customers "to run Linux 2.4
you must have a Pentium III or better".

MVS. If all else fails, MVS can do a stand alone IPL to capture the
crash data. It requires that a site plan ahead by building
stand alone IPL text and dedicating an area of disk to receive
the crash dump but apart from that it always works.

Linux. Assuming that your I/O subsystem will run without a working OS
(and lkcd shows how difficult that is), you still need a
mechanism to get the stand alone dump going. On our most common
platform (ix86) we have to pray that the BIOS will not wipe
memory before saving the crash dump. If the BIOS is not
reliable (and most are not) then the stand alone code must be
built into the kernel and protected somehow. Then you have the
problem of activating the stand alone dump.

MVS. Decent hardware support for OS tracing, with Program Event
Registers (PER) that are standardized and easy to use.

Linux. Each platform has different hardware mechanisms for hardware
debugging. Some have no hardware debugging at all.

MVS. The OS developers know everything about the hardware. They
even know the undocumented processor and I/O subsystem

Linux. Nobody knows everything about every bit of hardware that is

MVS. Decent hardware support for cheap cross memory services. OS
data can be separated into multiple data spaces, separating
unrelated data.

Linux. Where is data for subsystem XYZ? Somewhere in the kernel,
along with every other subsystem's data.

MVS. Much of the I/O is controlled by user space applications. MVS
is just a low level provider of I/O services and relies on
complicated code in user applications to do data sharing. This
is changing with Unix System Services and HFS data spaces but I
suspect that the bulk of data movement is still being done by
batch, IMS, CICS and VSAM data spaces, all controlled from user

Linux. All I/O is done by the kernel so the kernel contains all the
complicated code to maintain data integrity.

MVS. IBM controls the levels of software in a system. SMP (System
Management/Mangling Product) may have its faults but as a tool
for building and tracking complicated binaries it makes RPM,
DEB etc. look pitiful. IBM tests new versions of OS/390
before shipping and it knows that its customers will not/cannot
change the running OS.

Linux. Roll your own kernel. Select your own options, and add those
15 patches you found from 6 different ftp sites. Do it all
without any tools for version or reliability checks. Now try
to report a bug based on a binary dump, when you have lost the and config options for this kernel.

Don't get me wrong, I would love to have the same level of RAS and
debugging tools in Linux that OS/390 has. But getting Linux up to the
level of OS/390 RAS is at least an order of magnitude harder because of
the hardware and software constraints that Linux has to run under.

Right now Linux is about at the stage of MVS/SP 1.3, before cross
memory services were introduced. Back in 198x, most MVS data was in
CSA and crashes caused by storage corruption were a lot more frequent.
They were also a lot harder to diagnose, many is the time that IBM
asked for a problem to be reproduced, "try this slip trap, try that
slip trap, try this patch, sorry we cannot reproduce it". I won't
mention how bad IPCS version 1 was at reading crash dumps, nor how
difficult it is to add your own control block definitions even in the
latest IPCS.

Some of the problems above can be fixed in Linux.

* Add a master trace table.
* Move some data into different keys, if the hardware lets us.
* Get a crash dump facility, even if it is only supported on some
* Create decent tools for debugging crash dumps, anything would be
better than reading a crash dump in IPCS.
* Standardize on tracking the and .config with the kernel.
* Use the hardware trace and debug facilities as much as possible.
Dprobes is a nice start.

The rest of the problems are inherent in the hardware that the two OS's
run on or are caused by the fundamentally different software models.

Keith Owens.
Mainframe Sysprog since the days of VS/1, first machine was IBM 1620.
Linux kdb, modutils, ksymoops maintainer, kbuild hacker.

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