Effects of using both md raid1 and raid0 on same two disks?

[mhenry676]

I know, another RAID question, but I'm having a hard time finding out this particular question.

So, I have 2 WD Caviar Black 750GBs. I also have a ASUS A8N32-SLI Deluxe, overclocked Athlon X2 4200+ @ 2.75 and 4GB DDR OC'd to DDR500 (don't worry, quite stable) and currently have the drive setup to use the nforce4 RAID1.

I'm thinking of doing this and using the md software raid. XP installs not raided

sda1 sdb1 - md0 - raid1 - /boot - 256MB
sda2 ---- - XP64 - 20GB to 30GB (for gaming etc.)
---- sdb2 - NTFS - 20 to 30GB (for XP gaming etc.)
sda3 sdb3 - md1 - raid1 - / - 40GB
sda4 sdb4 - ext part
sda5 sdb5 - md2 - raid1 - swap - 4GB
sda6 sdb6 - md3 - raid1 - /home - 300GB
sda7 sdb7 - md4 - raid? - /extra - rest of disks

So the question is for md4, would it be ok to have this raid0? this will be mostly for media encoding, and other stuff I don't care if I lose. Is mixing raid0 and raid1 on the same set of disk OK?

Just a side question: Is it best to have the 'extra' as
/extra
/mnt/extra
/media/extra

Thanks

[ken_yap]

No problem at all. RAID builds on top of partitions and partions are distinct areas of the disk.

[BenderBendingRodriguez]

I'd put extra as /media/extra (it will at least show up in sysinfo )

I guess there are no problems in putting both RAID 0 and RAID 1 on the same disk(at least linux kernel is flexible enough to handle that). The other thing is putting /boot on RAID, my informations may be out of date but it is not possible to have /boot on raid since it won't boot? Or is that case valid only with /boot on RAID-0?

[mhenry676]

Actually, I have had /boot work with md raid1 with opensuse 11.1 and CentOS 5.x.

A few years ago, I used Gentoo, and had to make /boot non-raid and then do dd if=/dev/sda1 of=/dev/sdb1 to get the same effect of 'raid1'. Also, needed to make changes to grub to make sure the /boot was either sda1 or sdb1 according to the drive that grub would boot. Just had to remember to re-run the command if you made changes to /boot.

[ken_yap]

There's no problem with RAID-1ing /boot but of course when booting from the BIOS, only one of the disks is accessed. Provided that each component contains a complete filesystem, it will boot. Which is why you can't (software) RAID-5 /boot.

You can even put a bootloader on each disk and if the BIOS will try the secondary if the primary fails to read, it can provide a backup boot. For this you probably have to run grub manually, I'm not sure if the YaST bootloader setup is smart enough. It simple enough:

grub> device (hd0) /dev/sda
grub> root (hd0,0) # or ,1 or wherever /boot is
grub> setup (hd0)

and again for sdb

grub> device (hd0) /dev/sdb
grub> root (hd0,0)
grub> setup (hd0)

Notice that sdb is still addressed as hd0, the first disk. This is the case should the primary disk be unreadable, the secondary becomes the primary disk.

In practice though this fallback feature is very rarely needed so don't fret if you only put the bootloader on sda.