What i want from a raid1:

notiy if a disk fails

This is done by mdadm

boot degraded with only one disk

boot degraded does not work with ubuntu, because mdadm does not start a degraded raid :-(

my workaround is to add a bootmenue entry in grub

title           Ubuntu, kernel 2.6.20-16-generic (raid defect)
root            (hd0,1)
kernel          /boot/vmlinuz-2.6.20-16-generic root=/dev/md1 ro raid_degraded
initrd          /boot/initrd.img-2.6.20-16-generic

and make an initrd (update-innitramfs -u) with /etc/initramfs-tools/scripts/init-premount/raid_degraded

#!/bin/sh

set -eu

PREREQ="udev"

prereqs()
{
        echo "$PREREQ"
}

case ${1:-} in
  prereqs)
    prereqs
    exit 0
    ;;
  *)
    . /scripts/functions
    ;;
esac

if [ -e /scripts/local-top/md ]; then
  log_warning_msg "old md initialisation script found, getting out of its way..."
  exit 1
fi

MDADM=$(command -v mdadm)
[ -x $MDADM ] || exit 0

if grep raid_degraded /proc/cmdline 2>/dev/null; then
  echo "MD_DEGRADED_ARGS=' '" >> /conf/md.conf
fi

exit 0

reported to ubuntu https://bugs.launchpad.net/ubuntu/+source/mdadm/+bug/120375

easy change disk if one disk fails

Threrefore i made a script doing partitioning, adding to raid and installing bootloader of the new disk

#!/bin/sh

if grep -q sda /proc/mdstat; then
  SRC_DISK=/dev/sda
  DST_DISK=/dev/sdb
else
  SRC_DISK=/dev/sdb
  DST_DISK=/dev/sda
fi

if ! sfdisk -l 2>/dev/null | grep -q $SRC_DISK; then
  echo $SRC_DISK not found
  exit 1
fi

if ! sfdisk -l 2>/dev/null | grep -q $DST_DISK; then
  echo $DST_DISK not found
  exit 1
fi

sfdisk -d $SRC_DISK | sfdisk $DST_DISK

mdadm /dev/md0 -a ${DST_DISK}1
mdadm /dev/md1 -a ${DST_DISK}2

cat /proc/mdstat

/usr/sbin/grub --batch --device-map=/dev/null <<EOF
device (hd0) $DST_DISK
root (hd0,1)
setup (hd0)
quit
EOF

exit 0

set uuid

ubuntu cannot boot if any uuid of any disk is different - solution set uuid:

mdadm --stop /dev/md0
madmd --assemble --verbose /dev/md0 /dev/hda1 /dev/hdb1 \
--update=uuid --uuid=xxxxxxxx:xxxxxxxx:xxxxxxxx:xxxxxxxx

newer versions of mdadm should be able to set uuid with --create

why is my raid1 so slow ?

Reading from my raid1 is as fast as a single sata disc:

hdparm -t /dev/sda1

/dev/sda1:
Timing buffered disk reads: 172 MB in 3.00 seconds = 57.27 MB/sec

hdparm -t /dev/sdb1

/dev/sdb1:
Timing buffered disk reads: 172 MB in 3.01 seconds = 57.20 MB/sec

hdparm -t /dev/md0

/dev/md0:
Timing buffered disk reads: 172 MB in 3.00 seconds = 57.29 MB/sec

i tried

* different motherboard with a different sata onboard controller but also the same results

* different ubuntu kernels: 2.6.17, 2.60.20 and from gutsy 2.6.22rc

* different images:
linux-image-2.6.20-16-generic 2.6.20-16.29
linux-image-2.6.20-16-lowlatency 2.6.20-16.29
linux-image-2.6.20-16-server 2.6.20-16.29

* also a raid1 with PATA Disks

reported to ubuntu https://bugs.launchpad.net/ubuntu/+source/mdadm/+bug/120378

can anybody explain me why?

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