For a primer on what RAID is, check out my first blog on the topic.

RAID1 is also generally known as Mirroring. For this level of RAID two disks are required and are configured so that the system sees them as a single drive. When any data is written to the RAID1 group the data is written to both drives at the same time. This means that the second drive is an exact duplicate (mirror) of the first drive.

If something happens to either one of the drives, the system will continue operating by using the remaining drive. Most servers support "hot swapping" of drives so the drive can actually be replaced while the system is online and running. This allows the server to avoid a failure due to a single disk fault. Of course, if both drives went bad at the same time (highly unlikely) the server would go offline and need to be restored.

Since the operating system (in software RAID) or the controller (in hardware RAID [what we support at ORCS Web]) needs to write all the data twice, there is a slight performance hit when writing data to the RAID group. This performance hit for write operations is not nearly as bad as other RAID group options though and is generally not a noticeable difference from writing to a single disk. Because there are now two disks with the data, both can be read from at the same time - proving a performance increase for read operations.

So, with RAID1 you get redundancy to keep a system online in a case of a single disk failure and faster read operations but slightly slower write operations.

Perhaps the biggest drawback of RAID1 is the space loss. Since all data is written twice, you lose 50% of the total capacity of the drives in the system. Losing 50% of the space can be costly and can also create challenges supporting applications that require large amounts of store space. There are other RAID options available though that help address this and other issues.

Happy hosting!

~Brad

(Project architecture consulting is just one of the many managed services we provide at ORCS Web that has our customers confirming to us that we are #1 in service and support for Microsoft hosted technologies.)