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Is a SAN defragmentation worth it?.

Posted on 2009-12-24
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Hi,

We have a two node Windows cluster which is attached to a SAN. We've made a RAID 5 on that SAN. The case is that Microsoft has suggested us doing a SAN defragmentation and we're hesitant. I'm quite sure that a defragmentation is very useful on a laptod, for instance, but not that much with a SAN system.

In the case that we do the defragmentation, what's the best application for that purpose?. Taking into account that we have over 5 TB of data and 1 TB of free space, how long will take to do the defragmentation?, do we have to stop any activity?.

Thank you very much.
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Question by:JorgeSimarroVillar
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by:jaynir
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ID: 26119520
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by:nhidalgo
ID: 26119710
what type of SAN?

I would check with you vendor for their best practice on it. In my case with the SAN i have the data is virtualized accross all disks, rather than just a group of disk in a raid group.
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by:andyalder
ID: 26122285
Be very careful, if your SAN uses thin provisioning a defrag may cause a thin partition to become fat.
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by:thribhu
ID: 26122479
SAN degrag is not a good idea, will not save much out of it.
You may need to do it if you are going to gain lot space from the defreg.
comming to your question, 5 TB degrag may take more than 72 hours.
Yes, perforemance will degrade in Raid5 , when you do defrag, no need to stop any apps .No down time required .

In worst case if a disk failed during the defrag , it may write the errors to the rest of the disks, that cause multi disk failure.
Defrag is best in thin provision, than in thick provision.

Lastly, What type of storage you use?.You may not be able to stop the defrag in some vendor's storage arrays once you start it , so follow the storage vendor recommendations for defrag than Microsofts.

 
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by:JorgeSimarroVillar
ID: 26130346
Hi,

Sorry for the delay, I was on holidays.

We have a EMC Clarrion CX500.

There is only a two node Windows 2003 cluster server attached to the SAN. We also have another server but it's only giving service as a print spooler with a bit of activity and it's going to disappear in a few weeks.

There are two RAID 5 storage groups, the first one is attached to the print spooler, and the second one is attached to the cluster.

The cluster storage group is made of one Meta LUN and three mote LUNS. The Luns has been added as needed.

For thos who say that defrag is not recommenden, I've read some articles even from EMC saying just the other way around.

I don't want to do a defragmentation to get free space, I only want to get better performance. But before that, I'm doing some tests with Iometer. Does anybody have some benchmarks with this tool?.

Thank you.
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by:JorgeSimarroVillar
ID: 26307760
Any answer?.
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David earned 600 total points
ID: 26313429
In your case a defrag (running the standard windows gui defrag program of the mounted volume) is correct and appropriate.  There is no higher risk to your data breaking than any other I/O.

Will it make any difference?    Well if it is extremely fragmented, to the point where most of your I/Os are now random, then it might save you a few milliseconds per I/O.  

It also won't hurt.  Just do it, but performance will suffer slightly if the EMC is already saturated.  If you have slow usage windows like during evenings and week-ends, then start the defrag on a Friday night, and kill it on Monday AM.  

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by:JorgeSimarroVillar
ID: 26313513
I guess the defrag process won't end up on Monday morning. If I kill the defrag job on Monday morning, Will I lose all the work done or I'll be able to continue from that point?.

Thank you.
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by:andyalder
andyalder earned 200 total points
ID: 26313813
It won't continue from where you left off but it'll still be quicker becuase some of the disk isn't defragmented. Since the CX-500 isn't thin provisioned at least defrag isn't going to make it fat.
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by:dovidmichel
ID: 26317226
my 2 cents

worked with a client that also had a 5tb volume with 4tb of data.
data was almost all 50k jpgs created by an imaging application
backup throughput was worse than awful
tried everything finally under Microsoft's recommendation we defrag'd
had to stop it a few times but it took a month yes a month before it was finally done
result was way better than we expected

so based off some real world experience I gotta join the yes to defrag camp
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by:David
David earned 600 total points
ID: 26317311
There are some alternatives to MSFT's defrag.   sysinternals.com (Now a microsoft company) has a program called contig that makes selected files contiguous (or use it on an entire directory or disk).   You can run an experiment and choose a file that you KNOW is all broken up, just make that one file contiguous (if there is enough space to do so) and bench it.   Also there are some 3rd party products like diskeeper, which has a free 30day download, last time I checked.   That product is superior as they based an entire company around this one-trick wonder program.

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by:dovidmichel
ID: 26317581
a good thing about contig is it can be used to do selective defrag
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by:David
David earned 600 total points
ID: 26317638
And you can use contig selectively with defrag.  Defrag does a good job making free space contiguous, but it has hard problem making things like outlook ost files contiguous, so use defrag to free up space then make it contiguous.   contig is command-line, so you can automate maintenance.  It is also free and written using the same API that MSFT provides to developers, so it is just as "safe" as defrag.

(the microsoft sysinternals site  also has freebie downloadable utilities to fix up page files and other things that defrag won't touch)
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Author Comment

by:JorgeSimarroVillar
ID: 26321010
Thank you so much for that quite interesting and useful posts.

I installed diskeeper a few days ago, so I'll try first with diskeeper and later I'll do some tests with sysinternals tool.

By now, My intention is only to better disk performance, I'm not interested on get free space as there is a lot of it left.

I'll give you news about the defrag during the weekend.

Thank you again.
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Author Comment

by:JorgeSimarroVillar
ID: 26511695
I have received an alert from this question as it was abandoned, but I'm still waiting for an expert answer.

I'd like to close this question asap.

Thank you.
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by:David
ID: 26511702
So what is wrong with response #26313429 ?

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by:David
ID: 26511729
Or if you are concerned about defrag, run the contig program to see which files are the most fragmented.  Choose a rather large and fragmented file, benchmark I/O thoroughly, then use contig to just fix that one file, and run benchmarks again.  If it is faster, defrag everything.  If it is slower or no change, leave things alone.
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