Re: anyone aware of a high availability setup that relies on fully redundant install?

[Miles Fidelman]

On 4/18/16 12:48 AM, valdis.kletni...@vt.edu wrote:
> On Sun, 17 Apr 2016 10:47:55 -0400, "Robert P. J. Day" said:
>>i figure this is as good a place as any to ask ... is anyone here
>> aware of anyone using a linux config and install that, for the
>> purposes of reliability or high availability or whatever you want to
>> call it, relies on a second, completely independent installation of
>> linux on the same hard drive?
> 
>
> Most implementations  of "high availability" would see the phrase "on the same
> hard drive" and start pointing and laughing at the single point of failure.
>

Yup.  That was my reaction.  HA starts with either 2 servers and either 
a) mirrored drives (e.g., using DRBD) or b) a HA RAID cluster.

Miles Fidelman

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Re: anyone aware of a high availability setup that relies on fully redundant install?

[Valdis . Kletnieks]

On Mon, 18 Apr 2016 06:29:21 -0400, "Robert P. J. Day" said:

>   that makes sense -- a *minimal* bootable system for recovery and
> troubleshooting. but not a fully independent previous install.

No, it's a *complete* system - your kernel boot image, /, /usr, /var, and
whatever other file systems you specified to go into 'rootvg'.  The use case is
that before doing maintenance or whatever, you run a shell script that clones
the entire rootvg over to alt_rootvg.  Or you can apply system updates to
the alternate boot, so instead of taking an outage for 2 hours to apply
all your fixes, you apply fixes onto the alternate, and your outage window
is only what it takes to reboot to the updated image - and you still have
the old image to fall back to.

Incredibly useful for that set of systems that you need to minimize downtime,
but the application in question isn't one that you can run multiple instances
behind a load balancer.  We've used it for everything from a Listserv server
to a TSM backup server.

http://sureshaix.blogspot.com/2008/07/alternate-disk-installation.html
http://www.drdobbs.com/aix-alternate-disk-installation/199101222

(Yes, that's an article from 1991.. :)


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Re: anyone aware of a high availability setup that relies on fully redundant install?

[Robert P. J. Day]

On Mon, 18 Apr 2016, valdis.kletni...@vt.edu wrote:

> On Sun, 17 Apr 2016 10:47:55 -0400, "Robert P. J. Day" said:
> >   i figure this is as good a place as any to ask ... is anyone here
> > aware of anyone using a linux config and install that, for the
> > purposes of reliability or high availability or whatever you want to
> > call it, relies on a second, completely independent installation of
> > linux on the same hard drive?
>
> IBM's AIX has for a long time had the concept of an 'alternate boot
> volume', which can be another logical volume on the same physical
> hard drive.  But it's not intended for high-availability, it's for
> "if this software upgrade goes pear-shaped I have an easy backout
> procedure".  And it avoids most of the "you have to keep two
> version" issues by providing a tool to copy your *current* system
> onto the alternate boot.  I'm sure some Linux distros have stolen
> the concept.

  that makes sense -- a *minimal* bootable system for recovery and
troubleshooting. but not a fully independent previous install.

> Most implementations of "high availability" would see the phrase "on
> the same hard drive" and start pointing and laughing at the single
> point of failure.

  trust me, i'm aware of that. :-) perhaps i shouldn't have used the
phrase "high availability", this proposal was more for just the
ability to back out of a botched or flawed upgrade.

rday

-- 


Robert P. J. Day Ottawa, Ontario, CANADA
http://crashcourse.ca

Twitter:   http://twitter.com/rpjday
LinkedIn:   http://ca.linkedin.com/in/rpjday



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of device trees and powerpcs and low memory

[Robert P. J. Day]

  a question about device trees for those (probably numerous) members
of this list that know more about them than i do.

  recently, was given the following patch to be applied to a device
tree source file for a fairly elderly (MPC83xx-based) powerpc platform
that has 1G of RAM:

memory {
device_type = "memory";
linux,phandle = <300>;
-   reg = <0x 0x4000>;
+   reg = <0x 0x2000>;
};


i was puzzled by the change that now seemed to define 512M of RAM
instead of the full 1G. the explanation i was given is that that is
how one defines where kernel low memory ends, thereby leaving the
remaining 512M for high memory in kernel space.

  that's the first i ever heard of that. is it true? and if it is,
where in the kernel source code can i see that information being
processed and low memory being established based on reading that node
from the DTB?

  in any event, i'd never heard this before -- if i was going to
define low memory on a powerpc, i would think the proper way is to use
this snippet from arch/powerpc/Kconfig:

  menu "Advanced setup"
depends on PPC32

  config ADVANCED_OPTIONS
bool "Prompt for advanced kernel configuration options"
help
  This option will enable prompting for a variety of advanced kernel
  configuration options.  These options can cause the kernel to not
  work if they are set incorrectly, but can be used to optimize certain
  aspects of kernel memory management.

  Unless you know what you are doing, say N here.

  comment "Default settings for advanced configuration options are used"
depends on !ADVANCED_OPTIONS

  config LOWMEM_SIZE_BOOL
bool "Set maximum low memory"
depends on ADVANCED_OPTIONS
help
  This option allows you to set the maximum amount of memory which
  will be used as "low memory", that is, memory which the kernel can
  access directly, without having to set up a kernel virtual mapping.
  This can be useful in optimizing the layout of kernel virtual
  memory.

  Say N here unless you know what you are doing.

  config LOWMEM_SIZE
hex "Maximum low memory size (in bytes)" if LOWMEM_SIZE_BOOL
default "0x3000"


thoughts? i thought i understood device trees reasonably well, but i
had never heard of this alleged configuration.

rday

-- 


Robert P. J. Day Ottawa, Ontario, CANADA
http://crashcourse.ca

Twitter:   http://twitter.com/rpjday
LinkedIn:   http://ca.linkedin.com/in/rpjday



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Re: rmap-test

[Greg KH]

On Mon, Apr 18, 2016 at 10:45:41AM +0200, Jan-Simon Möller wrote:
> does anyone know under what license 
> https://www.kernel.org/pub/linux/kernel/people/mbligh/tools/rmap-test.c
> is released? 
> As it's on kernel.org one may assume its GPLv2, but it is written nowhere ... 

Don't make any such assuption, please contact the author instead.

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Re: fork

[Cihangir Akturk]

On Mon, Apr 18, 2016 at 12:18:00PM +0530, Nitin Varyani wrote:
> Linux kernel development by Robert Love describes the fork process as
> 
> fork() -> clone() -> do_fork() -> copy_process()
> 
> I am unable to find the clone() system call in linux 3.13.
> Can someone explain the proper flow of fork() system call initiated by the
> user?
> 
> Where can I find the libc implementation for fork()? I want the code of all
> the functions involved in fork.

There are a couple of libc implemetations out there, say musl,
dietlibc, uClibc and so on. But If you mean GNU libc aka. glibc
then this is what you are looking for:

https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/nptl/fork.c;h=1a68cbd6476b3268b5b7ac09ae60c35ce7069219;hb=HEAD#l47

__libc_fork() calls clone() syscall internally using a bit of
assembler magic.

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Re: fork

[Alex Wilson]

On Mon, Apr 18, 2016 at 12:48 AM, Nitin Varyani
 wrote:
>
>
> Linux kernel development by Robert Love describes the fork process as
>
> fork() -> clone() -> do_fork() -> copy_process()
>
> I am unable to find the clone() system call in linux 3.13.
> Can someone explain the proper flow of fork() system call initiated by the 
> user?
>
> Where can I find the libc implementation for fork()? I want the code of all 
> the functions involved in fork.
>
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I believe the fork and clone syscalls are found at /kernel/fork.c
under 3.13. (see link below)
They're wrapped in the nice SYSCALL_DEFINE# macros and all seem to
call do_fork().

http://lxr.free-electrons.com/source/kernel/fork.c?v=3.13#L1641

Can't really help you on the libc part, and you would need to specify
which libc implementation you're interested in.

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fork

[Nitin Varyani]

Linux kernel development by Robert Love describes the fork process as

fork() -> clone() -> do_fork() -> copy_process()

I am unable to find the clone() system call in linux 3.13.
Can someone explain the proper flow of fork() system call initiated by the
user?

Where can I find the libc implementation for fork()? I want the code of all
the functions involved in fork.
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