RAID Primer: What's in a number?

RAID 1+0 / 0+1

RAID 1+0 (10) or 0+1 attempts to get the best of all worlds: It generally provides the best read and write performance, as well as offering a level of redundancy for its data when compared to RAID 0.


Both RAID 0+1 and RAID 1+0 are considered "nested" solutions, which is to say they use RAID 0's data striping and RAID 1's mirroring capabilities. The difference between the two is that RAID 1+0 (10) creates a striped set from a series of mirrored drives while RAID 0+1 creates a second striped set to mirror the primary striped set.

In practice, the only reason an administrator would choose either RAID 0+1 or 1+0 (10) is in extremely I/O intensive operations which would bottleneck a RAID 5 or RAID 6 array, and where drive cost is not a major concern. The redundancy provided is in reality very low although RAID 1+0 offers better fault tolerance and rebuild capabilities than 0+1.

In an RAID 1+0 array all but one drive from each RAID 1 set could fail without damaging the mirrored data. However, if the failed drive or drives is not replaced, the last working drive in the set then becomes a single point of failure for the entire array. So if that single hard drive fails, all data stored in the entire array is then lost.

The RAID 0+1 array can operate if one or more drives (greater than 4 drives utilized) fail in the same mirror set, However, if two or more drives fail on either side of the mirroring set, then data on the entire array is lost. Also, once a failed drive is replaced, in order to rebuild its data all the disks in the array must participate in the rebuild. In the case of RAID 1+0, it only has to re-mirror the lost drive so the rebuild process is substantially faster.

Pros:

• Best performance available, as the system disk is essentially a RAID 0 array.

Cons:

• Expensive in terms of drives.
• Usable storage space is only half of the total drive capacity.
• Only minimally fault tolerant.

Conclusion

In the IT world, some level of RAID is virtually guaranteed to be employed on any production server due to the relatively high failure rate of hard disks compared with most other components in the system. For end-users, though, the picture becomes far murkier. Most home computers occupy large amounts of time seeking from small file to small file, with the resulting speed limitation imposed by the physical mechanisms of the drive itself (rotational speed, etc). These limitations are not overcome even by the top-performing RAID 0. The only benefits, therefore, that users can seek in RAID are to increase overall capacity of their single drive, add a level of redundancy for their system, or to improve large-file performance.

The attraction of RAID for users seeking a large single drive is diminishing by the day, due to the massive single drive sizes on the market today. When a capacity conscious user can get a full terabyte of space in a single physical package, the argument becomes one of backing up said data, rather than seeing a 2TB drive on their system.

In the case of redundancy, there is most certainly an argument for taking advantage of the RAID 1 feature found on many motherboards (and even in most operating systems). As stated previously, most users have experienced a hard drive failure at one point in their lives, and as more of our daily work shifts to a computing platform, data integrity is becoming increasingly important. More to the point, however: Should users be more worried about backing up their data to removable media on a periodic basis to protect against the accidental deletion or corruption of data, or in keeping their machine up and running when a complete failure occurs?

This type of question can only be answered by the individual user themselves, and depends on the nature of data being stored on the system. We recently provided a first look at Windows Home Server, which may prove to be a far more compelling backup solution than any form of RAID. That does require the use of an entire computer, but the user-controlled data mirroring, volume shadow copy, and the ability to support multiple systems certainly make it a viable alternative in households with multiple computers.

It also bears mention that redundant storage of data using RAID really isn't a sufficient backup strategy for most businesses, and some form of off-site storage of backups should also be considered. RAID can be useful in making sure that systems remain operational in the event of a hard drive failure, but other catastrophes -- flooding, fire, theft, etc. -- can still claim all of the data on a RAID storage device. If the data is truly important, saving periodic backups to a different medium and storing it at a separate location should be considered.

Large-file performance is likely the most compelling reason to adopt RAID in a home system. For video editing operations, bandwidth in write operations is an absolute must, and RAID 0 fills this need very well. Increasingly, however, hard drives are finding their way into new areas of the home - home theater PCs, PVRs, and home video archival systems are but a few of the "read-often, write-less but always needed" systems which could benefit from a solution like RAID 5 or even the more performance oriented RAID 5+1.

At the end of the day, anyone looking into a more elaborate storage solution owes it to themselves to consider the practical implication of the decisions they make. One size most definitely does not fit all in the world of hard drive storage and RAID, and the wrong choice can certainly be more harmful than helpful in this regard.



We would like to thank Adaptec for providing the charts utilized in our article today.