Could someone explain RAID plz?

[oboe]

I've been able to piece together a theory about it by reading board specs, and I want to see how close I got.

As far as I can tell, you need two harddrives to take advantage of what RAID offers ("Striping" or "Mirroring").

Thanks

[mrsid2]

Mirroring means your computer will write the data to two or more harddrives (copy it)

In case of one harddrive breaking, the data is still safe on the other.

Striping means your raid controller will write the data in small stripes split to both harddrives. It gives you almost double speed but if one of the drives fails, you lose your data.

Striping + mirroring is a combination of both but requires 3 or more harddrives

[Broes]

RAID= Redundant Array Inexpensive Disks

Redundant means here that should one hard disk fail, you data should still be available.

Mirroring: data is actualy written on 2 seperate harddisks. If one fails, the other still has the data. Since data is written to 2 disks in full it is actually a bit slower when compared to writting to one disk.

Striping: A file is written equally distributed amongst all hard disks involved in the stripe set (thus more then one). So if you have 2 disks, each will receive 1/2 the data. This makes it faster since both disks can write at the same time and the more disks involved, the faster it gets. BUT... if one disk fails of the stripe set, you will lose ALL data on the disks in the stripe set and the more disks, the likelier this is to happen. Fast but not redundant...

Raid 5/Stripe with parity: The same as normal striping but one disk in the raid 5 set is used to store a parity number. Should one disk fail in the set, the missing data can be calcutated using the remaining data and the parity number. Redundant since one disk can fail (only one though) and fast since it uses stripping. Almost all modern servers use this type of Raid.

hope this helps,

Broes

[AcId]

Just to add to this there is also RAID5 (ADG) advanced data guarding

Where you have a raid-5 that basicaly writes 2 sets of parity information, theres more to it than that but I want to keep it simple here. In raid-5 you can lose a single dirve and still function....in Raid-5 ADG, you can lose 2 drives and still function.

For best Data protection RAID-1 is the option, it's a 1to1 mirror of all drives. You have 10 drives but can only use the capacity of 5 because each has it's own mirror copy, You can lose half of the total amount of drives in a Raid-1 and still function.


There are other RAID-sets as well but are less efficient thus rarely used.

[ 10-24-2001: Message edited by: AcId ]

[Broes]

The best would be a raid 5 system with several Hot Spare drives. If this system notices a drive failing, a Hot Spare disk will be used to directly recalculate the missing data to and will from that moment on be part of the raid 5 set. Fully automatic and given enough Hot Spare disks available in the set you can go a long time before things get critical and human intervention is needed.

Broes