[Bug 44609] Re: RAID not implemented (use alternate CD instead)

Fri Jul 22 21:56:55 UTC 2011

I'm currently designing a graphical interface for setting up RAID in the
installer. Thanks to James Troup for helping me understand the
following, but any mistakes are my fault; please provide any corrections
in plain English.

1. The types of RAID it makes most sense to offer in a graphical
installer (where there are likely few disks) are RAID 1, RAID 10, and
RAID 5, in that order. RAID 4 is a bad version of RAID 5. RAID 6 is like
RAID 5, but allows two concurrent disk failures rather than one, in
exchange for being slower. And RAID 0 isn't really RAID at all, but a
close alternative to LVM (bug 43453), though it is possible to run one
on top of the other. Other mdadm configurations such as LINEAR and
MULTIPATH are different enough in kind that they should be designed and
implemented separately.

2. All of the above RAID configurations consist of a minimum of two
partitions *and/or* entire disks.

3. If a RAID device uses a partition rather than an entire disk, the
partition must be on a different disk from every other RAID partition.

4. A RAID device has a filesystem type, a mount point, and a size, like
a normal partition does.

5. The effective size of a RAID 0 device is the total of the
partitions/disks that form it. The effective size of other RAID levels
is, roughly, the minimum size of all the partitions/disks used in the
device.

6. Giving useful advice about which RAID level to choose involves
communicating about (a) read speed, (b) write speed (both as a rough
multiple of normal), (c) space efficiency (exact math), (d) probability
of failure, and (e) time to rebuild from failure.

7. Once set up, a RAID device can itself be partitioned.

Meanwhile, these are the basic design approaches I've thought of so far:

A. You create or format at least two partitions with filesystem type
"RAID partition". Then you choose "Create RAID Device…" somewhere, and
choose which of those partitions should be part of the device. Pro:
Unobtrusive (one extra filesystem type in the menu, and one extra
button), and familiar to Fedora/RHEL users. Con: If you don't know
exactly what you're doing, probably it will be an error message
explaining it to you ("Sorry, you need to set up at least two RAID
partitions before you can set up a RAID device"), and there's little
hint of what the cumulative effect of your choice of partitions/disks
will be or whether you've even set up enough yet.

B. "Set Up RAID..." somewhere opens a secondary assistant for choosing
the RAID type, followed by setting up individual partitions for the
device. Pro: Room to explain the various options, and to communicate the
size and effectiveness of the number of partitions/disks you've set up
so far. Con: Nested assistants (eww), and the interface for setting up
partitions/disks is separate from the usual one.

C: "Set Up RAID..." somewhere uses a variation of the normal "New
Partition" form, and the device then sits somewhere in the window
telling you how many partitions you've specified should be part of it so
far. Pro: Cumulative effect is obvious. Con: Sitting somewhere in the
window is probably weird.

For all of these, after setup, the RAID device starts appearing as a
separate partitionable volume. Other options, wireframes, etc welcome.

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https://bugs.launchpad.net/bugs/44609

Title:
  RAID not implemented (use alternate CD instead)

Status in “mdadm” package in Ubuntu:
  Incomplete
Status in “ubiquity” package in Ubuntu:
  Confirmed
Status in “ubiquity” package in Baltix:
  New

Bug description:
  When trying to install from the Flight 7 Live CD (both Ubuntu and
  Kubuntu) on a system already running Debian Linux with mirrored
  partitions, the installer does not recognise the meta devices and
  insists on working with the underlying disk partitions.

  The kernel of the Live CD recognises and starts the meta devices just
  fine.

  I would prefer the installer to offer the use of the meta devices,
  since using the underlying partitions makes it tricky to encapsulate
  the partitions later in order to get the mirror structure back. Since
  Ubuntu aims for the corporate environment, it is imperative to support
  mirrored setups, even for the desktop! Many organisations, especially
  in the financial sector prefer it that way.

  If you decide to fix this bug, please consider to adjust the grub
  setup as well, to allow for automatic boot from the mirror disk in
  case the root disk fails.

  Attached you will find the disk layout as used on the system (taken
  from the installed Debian Sid). Additional information is available
  upon request.

  # /sbin/sfdisk -l /dev/hda

  Disk /dev/hda: 158816 cylinders, 16 heads, 63 sectors/track
  Units = cylinders of 516096 bytes, blocks of 1024 bytes, counting from 0

     Device Boot Start     End   #cyls    #blocks   Id  System
  /dev/hda1   *      0+  31001   31002-  15624976+  fd  Linux raid autodetect
  /dev/hda2      31002   38751    7750    3906000   fd  Linux raid autodetect
  /dev/hda3      38752   79441   40690   20507760   fd  Linux raid autodetect
  /dev/hda4      79442  158815   79374   40004496   fd  Linux raid autodetect

  # /sbin/sfdisk -l /dev/hdc

  Disk /dev/hdc: 158816 cylinders, 16 heads, 63 sectors/track
  Units = cylinders of 516096 bytes, blocks of 1024 bytes, counting from 0

     Device Boot Start     End   #cyls    #blocks   Id  System
  /dev/hdc1   *      0+  31001   31002-  15624976+  fd  Linux raid autodetect
  /dev/hdc2      31002   38751    7750    3906000   fd  Linux raid autodetect
  /dev/hdc3      38752   79441   40690   20507760   fd  Linux raid autodetect
  /dev/hdc4      79442  158815   79374   40004496   fd  Linux raid autodetect

  # cat /proc/mdstat
  Personalities : [raid1]
  md3 : active raid1 hda4[0] hdc4[1]
        40004416 blocks [2/2] [UU]

  md2 : active raid1 hda3[0] hdc3[1]
        19591168 blocks [2/2] [UU]

  md1 : active raid1 hda2[0] hdc2[1]
        3903680 blocks [2/2] [UU]

  md0 : active raid1 hda1[0] hdc1[1]
        15623104 blocks [2/2] [UU]

  unused devices: <none>

  # swapon -s
  Filename                                Type            Size    Used    Priority
  /dev/md1                                partition       3903672 0       -1

  # mount
  /dev/md0 on / type ext3 (rw,errors=remount-ro)
  proc on /proc type proc (rw)
  sysfs on /sys type sysfs (rw)
  usbfs on /proc/bus/usb type usbfs (rw)
  tmpfs on /dev/shm type tmpfs (rw)
  devpts on /dev/pts type devpts (rw,gid=5,mode=620)
  /dev/md2 on /export/home type ext3 (rw)
  /dev/md3 on /export/scratch type ext3 (rw)
  tmpfs on /tmp type tmpfs (rw,mode=1777,size=2G)
  tmpfs on /dev type tmpfs (rw,size=10M,mode=0755)
  automount(pid6343) on /net type autofs (rw,fd=4,pgrp=6343,minproto=2,maxproto=4)
  automount(pid6351) on /home type autofs (rw,fd=4,pgrp=6351,minproto=2,maxproto=4)
  nfsd on /proc/fs/nfsd type nfsd (rw)
  binfmt_misc on /proc/sys/fs/binfmt_misc type binfmt_misc (rw)
  /export/home/dominik on /home/dominik type none (rw,bind)

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