View Full Version : Defragging RAID 1 System: pro's and con's?

Billy T
19-05-2013, 10:36 PM
I've read up about this on the Web, and there are diverse opinions, but the thrust I tend to agree with is that the RAID 1 array is a mirror system, it acts like a single disk, and since it maintains consistency across both drives, defragging RAID 1 is as straightforward as for a single disk system.

However, I get a touch of the tata's when I think about cluster variations? between disks (not sure on the terminology), so the rewriting of data to the two disks might then require slightly different locations on each disk for the same data. Maybe the system is intelligent enough to deal with that scenario anyway, as it could well strike variations in accessibility between disks in the normal course of events.

Just thought I'd ask before I either make a monumental mistake, or lose sleep over nothing!


Billy 8-{) :help:

20-05-2013, 12:40 PM
The drive is a logical drive, you are not defraging a drive as such, just the virtual drive.

20-05-2013, 02:13 PM
I'm pretty sure windows 7 does some defragging in the backroud on idle making it largely unecessary. Also some real world tests I've read indicate there is little to no noticeable improvement most of the time on modern hard drives using a modern OS. There are still people who swear by it and do it regularly though. If the drive is badly fragmented it can be a good thing. I remember when I used to do it though, looking at the results of the analyse stage and realising the badly fragmented files were mostly unimportant and temporary files of no real impact to my system performance.

As to whether it's a good Idea on a RAID 1 array, I don't know the answer but I'd assume it's not really any different than on a single drive given that one just duplicates the other.

Personally I never Defrag any more.

20-05-2013, 03:43 PM
Unless you editing large videos on your PC, I wouldn't bother.

Fragmentation can cause problems on high-performance servers, in particular ones that do a long sequential read.

The classic example is database servers, where there's a good chance that when you read a sector, you're likely to read nearby sectors. This is sometimes called pre-fetching or read-ahead.

Typically DBA's get around fragmentation by pre-allocating largely empty large files, set to not increase in size. To defragment a multi-GB file, a DBA may copy a file to a new file, delete the old, and rename the new.

By the way, for certain servers there's a definite advantage to disk partition alignment; it helps SQL servers and Exchange servers in particular.


Like most things, there are exceptions: SAN volumes, or SSD drives used by servers

Billy T
20-05-2013, 06:18 PM
Hmmm.........diverse opinions :confused:

What brought it to mind was that I recently cleared out the temp files etc on Billy Junior's computer (XP Pro) then ran defraggler and it is now very noticeably faster on boot, shutdown and on all other functions. My computer is also XP Pro so I figured.....................

DBA's are of no relevance to me, apart from the use of RAID 1 and separate data and OS/program disks my computer is bog standard.

So, I am not really any further ahead on the safety or otherwise of defragging RAID 1 arrays. I think my Intel Matrix Storage was updated last time I took it in for the boot problem, because the icon in the system tray has changed and my access icon for another RAID function ceased to work because the target file no longer existed.

Is it, or is it not, potentially dangerous?


Billy 8-{)

21-05-2013, 08:29 AM
Unless you have a specific need, "if it ain't broke, don't fix it"

21-05-2013, 11:05 AM
Is it, or is it not, potentially dangerous?

If you arnt 100% sure, simply dont do it. Possibly safe, probably will have no worthwhile gain.
I saw at least 1 raid controller manufacturer claimed that systems with their controllers CANT be defragged .

21-05-2013, 01:20 PM
I used to defrag my RAID 1 when I had XP on it, it doesn't damage it. I used to do it all the time as the mirror kept on breaking (so now I have about 300 years of bad luck to endure) and it used to take hours to rebuild. It is good house keeping to defrag. RAID 1 does slow your data access a little though, I found it was a bit of a pointless pain in the backside and after about 18 months ditched it.