Re: Linux mdadm superblock question.

[Rudy Zijlstra]

Kyle Moffett wrote:
> On Tue, Feb 16, 2010 at 21:01, Neil Brown <neilb@xxxxxxx> wrote:
>   
> > On Tue, 16 Feb 2010 16:03:43 -0900 (AKST) "Mr. James W. Laferriere" <babydr@xxxxxxxxxxxxxxxx> wrote:
> >     
> > >       I am unaware of any record from Neil or other maintainer(s) of the
> > > /md/ device tree saying that they will not remove the 0.90 table and the
> > > autoassembly functions there .  I'd very much like to hear a statement saying
> > > there will not be a removal of the autoassembly functions for 0.90 raid table
> > > from the kernel tree .
> > >       
> > I will not be removing 0.90 or auto-assemble from the kernel in the
> > foreseeable future.
> > None the less, I recommend weaning yourself from your dependence on it.
> > initramfs is the future, embrace it.
> >     
>
> What are people's reasons for pushback against initramfs?  I've heard
> lots of claims that "it's not trustworthy" and "it breaks", but in 7
> years of running bootable software RAID boxes on weird architectures
> (even running Debian unstable) I have only once or twice had initramfs
> problems.
>
> As a software capability, initramfs makes it possible to use
> *anything* as a root filesystem, no matter what is necessary to set it
> up.  For example, I have seen somebody use DRBD (essentially network
> RAID-1) as a root filesystem with a few custom hook scripts added to
> the initramfs-tools configs.  Other examples include using Sun ZFS as
> a root fs via an initramfs FUSE daemon, a feat which even Solaris
> could not accomplish at the time.  Encrypted root filesystems also
> require an initramfs to prompt for encryption keys and decrypt the
> block device.  Multipath block devices are another example.
>
> You should also take a look at your distro installers.  There is not a
> single one made in the last several years which does not use an
> initramfs to start networking or access the installation media.  In
> fact, of all the distro installers I have had the most consistent
> behavior regardless of system hardware from the ones which operate
> entirely out of their initramfs.
>
> From a reliability perspective, an initramfs is no more essential
> than, say, /sbin/init or /boot/vmlinuz-2.6.33.  Furthermore, all of
> the modern initramfs generation tools automatically keep backup copies
> exactly the same way that "make install" keeps backup copies of your
> kernel images.  The two times I've managed to hose my initramfs I was
> able to simply edit my grub config to use a file called something like
> "/boot/initramfs-2.6.33.bak" instead.
>
> In fact, I have had several times where an initramfs made my boot
> process *more* reliable.  On one of my LVM JBOD systems, I was able to
> pull a group of 3 SATA drives whose backplane had failed and drop them
> all in USB enclosures to get the system back up and running in a half
> an hour.  With just straight partitions on the volumes I would have
> been hunting around for 2 hours to figure out where all my partitions
> had gone only to have the USB drives spin up in a different order
> during the next reboot.
>
> If you're really concerned about boot-process reliability, go ahead
> and tell your initramfs tool to include a fully-featured busybox,
> coreutils, bash, strace, gdb, and a half-dozen other developer tools.
> You may wait an extra 20 seconds for your bootloader to load the damn
> thing during boot, but you'll be able to track down that annoying
> 10-second hang in your /sbin/init program during config-file parsing.
>
> I've built specialized embedded computers with stripped-down chipset
> initialization code, a tiny Linux kernel and a special-purpose
> initramfs burned into the flash.  By using the fastboot patches and
> disabling all the excess drivers, my kernel was fully operational
> within the first half-second.  It used the tools on the initramfs to
> poke around on the hard disk as a bootloader, then kexec() to load the
> operational kernel.
>
> Counting up all the problems I've had with system boot... I've had an
> order of magnitude more problems from somebody getting careless with
> "rm", "dpkg --purge", etc than with initramfs deficiencies.
>
>
>   
We are looking at 2 different use-cases i think.

for the power-user system manager, who manages all his servers and has 
knowledgeable backup, initrd may indeed work as above.

I have to keep in mind, that when there is a problem while i am 
travelling (and that happens), there is no sys-admin present. Also, i am 
supporting systems remote where no-one has the knowledge to debug using 
a initrd. In such cases, initrd is an additional step. And each 
additional step is an additional source of mistakes.

1/ distro tools assume that the kernel being build will run on that 
machine. For servers this is often not true. There are very valid 
security reasons to exclude compilation capability from many servers.
2/ For most small shops, there is need for RAID (disks are fallible, 
shop cannot do without server), the RAID should work without being 
visible. If there is a problem with the RAID that causes auto-assemble 
to break, it means i need to travel (>100KM) to trouble shoot. The 
simpler the setup, the more i like it. This is also why i almost always 
use HW raid for the system partitions. The ones i use have userland 
tools in Linux which warn on disk failure, ensure auto rebuild, etc...
Still, for large storage needs it is SW RAID over SATA.
3/ for my home systems, if i need to remote-support to get things 
working again (i am often travelling for my work), the added layer of 
initrd is an added layer of possible mistakes.



Cheers,


Rudy
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