Raid 1 with reading speed of Raid0, explain please!

dgtL · January 31, 2013

From wikipedia. http://en.wikipedia.org/wiki/Standard_RAID_levels

[h=3]RAID 1 performance[/h]

Since all the data exists in two or more copies, each with its own hardware, the read performance can go up roughly as a linear multiple of the number of copies. That is, a RAID 1 array of two drives can be reading in two different places at the same time, though not all implementations of RAID 1 do this.^[4] To maximize performance benefits of RAID 1, independent disk controllers are recommended, one for each disk. Some refer to this practice as splitting or duplexing (for two disk arrays) or multiplexing (for arrays with more than two disks). When reading, both disks can be accessed independently and requested sectors can be split evenly between the disks. For the usual mirror of two disks, this would, in theory, double the transfer rate when reading. The apparent access time of the array would be half that of a single drive. Unlike RAID 0, this would be for all access patterns, as all the data are present on all the disks. In reality, the need to move the drive heads to the next block (to skip blocks already read by the other drives) can effectively mitigate speed advantages for sequential access. Read performance can be further improved by adding drives to the mirror. Many older IDE RAID 1 controllers read only from one disk in the pair, so their read performance is always that of a single disk. Some older RAID 1 implementations read both disks simultaneously to compare the data and detect errors. The error detection and correction on modern disks makes this less useful in environments requiring normal availability. When writing, the array performs like a single disk, as all mirrors must be written with the data. Note that these are best case performance scenarios with optimal access patterns.

RAID 1 has many administrative advantages. For instance, in some environments, it is possible to "split the mirror," declare one disk as inactive, do a backup of that disk, then "rebuild" the mirror. This is useful in situations where the file system must be constantly available. This requires that the application supports recovery from the image of data on the disk at the point of the mirror split. This procedure is less critical in the presence of the "snapshot" feature of some file systems, in which some space is reserved for changes, presenting a static point-in-time view of the file system. Alternatively, a new disk can be substituted so that the inactive disk can be kept in much the same way as traditional backup. To maintain redundancy during the backup process, some controllers support adding a third disk to an active pair. After the third disk rebuild completes, it is made inactive and backed up as described above.

So yes in most arrays it will increase read speeds.

MsTrMind · January 31, 2013

So, on the RAID video I commented something about RAID1 with reading speeds of RAID0.

My idea was that the principe of RAID1 would be used, so Disk B is an exact copy of Disk A, but the reading would go like this:

- bit 0 read @ disk A

- bit 1 read @ disk B

- bit 2 read @ disk A

- bit 3 read @ disk B

- ...

Could this work? Would this increase reading speeds?

If no, why not?

LinusTech · January 31, 2013

Yep it does. I've observed real world differences in system performance using RAID 1.

With that said, the purpose of these "As fast as possible" segments is to give the viewer a basic understanding without having to sit through 5 minutes of "blah blah blah". The performance improvement is not really the reason most people run a RAID 1 array.