Software projects/OS/Slackware/Advanced usage/RAID

WIP WIP WIP HURRAID

• Need to properly rebuild one array while completing this Wiki section.
• For now, don't try this incomplete tutorial unless you have some RAID knowledge
• Once finished and this warning removed, go for it.

RAID

• http://en.wikipedia.org/wiki/RAID
• We will use the robust native Linux software RAID.
• It's a feature that can be used on the Pandora, so let's have fun with it
• I will cover here only a RAID 1 (mirror) on two SD cards, as it should be the more interesting for a server usage because under read load, you should reach about 27MB/s against 16MB/s on one card.
• A SD + USB drive can also work although untested. Same with a two SD + USB drive configuration.
• This array will be mounted as /srv , the standard server folder.

Partitioning

• So you have to partition your system SD card a bit differently than the default SL4P setup.
• In this example, we will use the recommended minimal 8G card for the slot 1.
• You can use a different sized card for the right slot. To keep thing interesting, the minimum size is 4G for this second card, and we will use 4G in this example.
• The RAID 1 partitions must have the same exact size.

Card 1

• Partition that way:

6G ext2 for the system.
256M for the swap.

That's all for now, the third partition will be created under Slackware, and will be:

1.5G RAID.

• We will left 256M free. NEVER fill an entire drive with a RAID array. Drives, ie. all 8G, doesn't necessary offer the same exact capacity, so you can get in trouble if you add a drive 32MB short.

Card 2

256M for the swap. Having swap on two different cards is a good thing.

That's all for now, the other partitions will be created under Slackware, and will be:

1.5G unformated RAID.
2G unused. Do what you want with it.

Install SL4P

• Uncompress the rootfs on the first SD card, first partition, put the card in your OP, and boot it.
• Log in.

Backup /srv

• As root:

mkdir /srv-save
mv /srv/* /srv-save/

Finalizing card 1 setup

• If you don't know how to use cfdisk, please read this first.
• As root:

cfdisk

Finalizing card 2 setup

• As root:

cfdisk

Create the RAID 1 array

• Load module ?
• The RAID devices names are md0, md1, md2 etc..

mdadm --create /dev/md0 --level 1 --raid-devices 2 /dev/mmcblk0p3 /dev/mmcblk1p2

Format the RAID array

• As usual, we choose the Linux extended 2 filesystem.

mkfs.ext2 /dev/md0

Make the RAID configuration permanent

mdadm --scan > /etc/mdadm.conf

Edit /etc/fstab and add these lines:

/dev/mmcblk1p1   swap   swap   defaults           0  0
/dev/md0         /srv   auto   defaults,noatime   1  1

Final step

• As root:

mount/dev/md0
mv /srv-save/* /srv/
rm -rf /srv-save/

• That's it, you have now a RAID 1 array on your Pandora :^).

Some Practice

• If you want to know how to repair a RAID array, let's simulate a failure.

Remove a drive from the array

• It's likely to be the one in the right SD slot.
• Unmount the swap:

swapoff /dev/mmcblk1p1

Repair the array

• Depending the size of your RAID partition and he data on it, it can take a while.
• Mount the swap:

swapon /dev/mmcblk1p1