| Article Index |
|---|
| RAID 101 - Introduction to RAID |
| RAID 0 - Striping |
| RAID 1 - Mirroring |
| RAID 3 - Striping with Dedicated Parity |
| RAID 5 - Striping with Distributed Parity |
| Wrap-Up |
| All Pages |
RAID 1 - Mirroring
RAID 1, or Mirroring, by definition requires exactly 2 disks, although some advanced controllers, or some fancy manual configuration, can make a multiple mirror set from 3 or more disks too.
The RAID 1 set is created using the same amount of space on each disk, and the total size is the same as the size used on each disk. If we had a 250GB disk and a 500GB disk, the RAID 1 set would use 250GB on each disk, and the total size for the virtual disk would also be 250GB.
RAID 1 offers data protection but with a significant cost - half the disk space is lost for redundancy. Furthermore, when reading data, the disk controller can read from either of the disks, which is faster than a single disk, but has to write the data to both disks at the same time, which is slower. RAID 1 can therefore be slower than a single disk for some tasks.
RAID 1 takes the data on the computer and lays it out across the disks so that each part of the data is stored in the same place on each disk.
Here’s our example data - a simple pattern of 3 ones followed by 3 zeroes for simplicity: 111000111000111000111000111000111000111000111000
Let's store the following sentence on a RAID 1 set of 2 disks. RAID 1 sets do not break the data into stripes or chunks - it is simply written to each disk in the same place on each:
| Disk 1 | Disk 2 |
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|
Now let's look at what happens if we lose one of the disks and try to read our data back from the remaining good disk:
| Disk 1 | Disk 2 |
 |
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We get 111000111000 - none of the data is lost! We can continue using the computer, although it might be prudent to arrange to replace the failed drive sooner rather than later:
In summary, RAID 1 offers redundancy for the data, but at the cost of buying 2GB of disk space for every 1GB of data.
Where Does RAID 1 Make Sense
RAID 1 is a common RAID type, perhaps the most common of all, due to its use for the operating system drive on servers.
RAID 1 is perfect for small data sets, where the amounts of data written are small or in sequence, the amounts of data read are large, and reliability is crucial. Examples are the disks for an operating system, disks for database and email logs, web servers and most importantly, the Windows paging file or Linux swap space.
Let's move on and take a look at RAID 3, Striping with Dedicated Parity.
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|192.101.136.xxx |2009-05-28 06:46:55 doublemint - homebuilt NAS ownerI'd also like to know what the recovery process is like, especially if the RAID controller fails. Can a failed RAID controller be replaced with one of a different brand and still recover? As a novice, the RAID BIOS is a bit confusing, some pointers would be helpful. I had a motherboard with RAID fail and assumed I could take one of my RAID 1 drives, plug it into another PC and read it however I couldn’t, why?
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|70.17.178.xxx |2009-05-28 13:53:44 zaphod - replyIm pretty sure youd need both HDs for that to work... not just one.
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|134.211.129.xxx |2009-05-29 17:13:22 David Rawling - Your Mileage Will VaryBasically, the answer is no - you need the same model of RAID controller to replace a failed one.
This applies from the virtually-free Intel RAID controllers on the motherboard to the thousand (and multiple-thousand) dollar add-in cards.
Each controller has its own way of marking the disks as being part of a RAID set - the disk "signature". One brand might write "RAID1-1" to the first disk and "RAID1-2" to the second. Others might use numbers with the brand (MYRAID-716825).
I guess it comes back to the same comment I made before. RAID is not backup. It only protects you from disk failure.
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|71.198.81.xxx |2009-06-01 02:54:12 doublemint - homebuilt NAS ownerDavid,
While unfortunate there isn't more standardization, I understand why this might be. I can't however understand why this would apply to RAID 1 (mirroring). Why do these drives need to be treated any different than non-RAID drives? Just write the same data two places rather than just one.
