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RAID-1 vs. RAID-5 Explained!

In my line of business, I often meet customers whom asked me questions about RAID (Redundant Array of Independent Disks) as they have been misled and/or confused by IT vendors whom are either out to make the most money from the customer by selling more storage/disks or they don't have an understanding of the customer's applications and storage needs.

Some of the most popular RAID questions I come across are:
  1. I want RAID-5 as it is a better and faster than RAID-1. Is it?
  2. Should I install the operating system (OS) and put data in different partitions of RAID-5?
  3. Is it ok if we divide a RAID into two partitions?
  4. Does RAID-5 take a long time to rebuild?

Is RAID-5 (hardware-based RAID) a better solution than RAID-1?
Based on my experiences with MS Windows, Linux and UNIX-based (Solaris, AIX, HP-UX) operating systems since year 1996, choosing a RAID level actually depends on what application(s) you are implementing.

RAID-1 characteristics
  • One Write or two Reads possible per mirrored pair
  • Twice the Read transaction rate of single disks, same Write transaction rate as single disks
  • 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk
  • Transfer rate per block is equal to that of a single disk
  • Simplest RAID storage subsystem design

RAID-5 characteristics
  • Highest Read data transaction rate
  • Medium Write data transaction rate
  • Low ratio of ECC (Parity) disks to data disks means high efficiency
  • Good aggregate transfer rate

Based on the above characteristics of RAID-1 and RAID-5, I will not recommend using of RAID-5 technology for any servers that is bound to have write-intensive operations as RAID-5 has slower write data transaction rate than RAID-1 due to the many disks and generating of ECC Parity factor.

From my personal experiences, RAID-5 performance is notoriously slow for small-writes, because of the CPU overhead that each write requires. The overhead is worse if the RAID-5 is software-based. Hardware-based RAID-5 is better, but still not as fast as RAID-1.

Of course, RAID-1 requires a great deal of extra disk space (double the usable space you need), so you pay for each approach in different ways. So, if you are implementing an email server with more than 50-mailboxes, you will be expecting a lot of disk I/O as there will be a lot of write-intensive operations. So, if you need performance and you have a backup strategy, I recommend that you use RAID-1 or RAID-10.

Can you put the OS on RAID-5?
Yes, you definitely can. After that, you will find that your server overall performance will suffer from performance degradation.

Can you divide a RAIDed slice into multiple partitions?
It depends. You may be able to do this, depending on the hardware, operating systems and software you use. I think you'd be better served by having two or more separate RAID slices, and assigning each to a separate file system.

Does RAID-5 take a long time to rebuild?
Short answer: YES. Long answer: It depends on how large the RAID-5 slice is, how many disks you striped it across, the I/O speed of the disks involved, the CPU speed of the system(s) involved, how much other work is underway and many other factors. I have seen large, poorly configured RAID-5 slices that take days to rebuild.

How to optimally deploy RAID-5?
I have met some IT Managers whom due to IT budget constraints could not afford to buy more than 3 hard-drives for their RAID-5 implementations. For your knowledge, the optimal configuration for RAID-5 is to have data spread across four to six disks. Fewer than that is not worth it and you may as well deploy RAID-1 as it will be cheaper and a lot faster! More than that invites trouble by increasing the likelihood of multiple disk failures.

Oh, by the way... more disks require more time to rebuild.



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