Definition of RAID

[Danilo Piazzalunga]

Vincent Trouilliez wrote:
> I don't understand how RAID 0 can be done in S/W ?

The relevant information (how to put the pieces together) is written into the
raid superblock. See also mdadm(8).

> If Linux handles it, then I guess it done at the kernel level, or just
> above ?

At kernel level. I don't know if booting from a RAID 0 device is supported,
though. I'm using a RAID 1 root partition with a separate /boot, although
booting from a RAID 1 device *is* supported by LILO and works under GRUB.

> Also, how much CPU load does handling the array require ?

I don't have any measurements to show, but I guess that wasting some
microseconds (or less?) is negligible when compared to the milliseconds
required to access the disk.

> More importantly, how to trust S/W for this highly sensitive low-level
> I/O stuff like RAID 0 ? I anything goes wrong in the system (program
> crashing or otherwise misbehaving, bugged etc), how that might impact
> the RAID driver ? If a program misbehaves, anything can happen, and if
> it affects the RAID driver in whatever way, it could cause it to
> ruin/corrupt all your data in a micro second, horror ! :o(((

How to trust software for highly sensitive low-level I/O stuff like sending
the right commands to write data onto the disk and the same commands to read
back your data? Or to manage the file system?

More importantly, how to trust a hard drive which may fail anytime without
notice? Do you know your hd health status? Are you constantly monitoring
SMART values?

If you have valuable data, you are advised to use RAID 1 in the first place
(because it is quite unlikely that two or more disks crash at the same time).
RAID 0 should be used if you care about performance and keep backups
elsewhere, because it is less reliable than using a single drive.

Cheers,
Danilo

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