Re: Install and RAID
On Sat, 20 Jan 2001, Russell Coker wrote:
> RAID is vitally important for serious machines. This means all server-type
> machines and workstations for people who do important work.
RAID is far less important, in my mind, than a journaling fs. The purpose of
RAID is to ensure that hardware failure doesn't result in downtime. But in my
experience, your average "workstation for a person who does important work"
suffers much less downtime and data loss from hard drive detonation than from
unexpected power outages. Many of the problems RAID is intended to solve
can also be dealt with by throwing more hardware at the problem (SCSI disks
with lower failure rates and higher access speeds, or hardware RAID
controllers). OTOH, there's no hardware solution that solves the problems
posed by the lack of a journaling filesystem.
If you're going to be modifying the installer in such a dramatic way, one
which will certainly break lots of things that work now (as anything this big
is bound to lead to problems in the short-term), I would strongly encourage
you to focus your efforts on getting good journalling support into the default
install.
> To solve this I believe that it is best to create a RAID-1 array in "degraded
> mode" (basically telling the system that one drive has failed). This makes
> the system operate and perform as if it was a single disk not a RAID array.
> Then after the main install has completed the system could be instructed to
> go into full RAID mode and sync the disks.
> Now if we do this then someone who only has a single hard drive may as well
> go through the RAID install process and just skip the second stage of turning
> on the RAID. Then if at some future time (months or years later) they
> purchased a second hard drive then RAID mirroring could easily be turned on!
Above all, it's important that the user be in control of this process. Even
if there are multiple drives in the machine at time of install, they should
NOT be set up as a RAID array unless the user explicitly chooses this option.
I don't see a problem with doing the necessary setup on the partitions so that
they can later be RAIDed together, so long as this doesn't cause instability
or performance problems; but any decisions about whether given partitions
should be set up as mirrors should be made by the user, not by the
installation software.
> So the question is, is it worth still allowing installation to non-RAID
> partitions?
Yes. Backwards-compatibility, code maturity, and user comfort level are among
the reasons I can think of for supporting installation to non-RAID partitions.
> A RAID partition is the same as a regular partition apart from having a small
> amount (less than 100K) of disk space reserved at the end.
This seems reasonable to me; however,
> If you choose to not use the RAID device you can just edit /etc/fstab to
> refer to the raw devices instead and change the partition type to something
> other than 0xFD, and reboot.
this seems to imply that the user must know quite a bit about Linux already in
order to opt out of the RAID configuration. Effectively, it seems to me we
would be saying "we support RAID; if you don't want RAID, you can change it,
but you're on your own at that point." I think that we would be setting
ourselves up for a fall in doing that. I'm sure there are plenty of Debian
users, as well as developers, who aren't comfortable enough with RAID yet that
they'd want to use it on all of their systems. And as rigorously tested as
the Linux RAID support is, there are bound to be situations where using the
RAID driver isn't a viable option (c.f. the discussions about bootloaders).
> In this scheme if the user sets partitions as type 0xFD then it will install
> as RAID and start RAID at every boot.
> If the user sets partitions as other types then it would install as RAID but
> put the raw devices into /etc/fstab. So the RAID driver would not be used at
> boot, but this could easily be changed without requiring a rebuild.
So long as setting up RAID partitions is given as an /option/ to the user
during install, I think this would be great.
Steve Langasek
postmodern programmer
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