Risk in defrag

Nope
The LUN doesn't change at all
In my experience if you set the defrag speed to high on a CX300, all the work is done in about 5 hours or so. So if a slight performance impact isn't that important after office hours, start the defrag at high speed when you go home at 5PM.

24 hours ? What kind of drives and at what priority did you defrag ? I've been defragging on CX300' quite a bit and with high prio on 146GB drives in R5(4+1) it shouldn't take more than 4 or 5 hours.

I believe the time it takes is dependend on drive size (73G, 300G, etc), drive speed (10k, 15k), drive type (FC, SATA), and LUN size. If your LUNS are small it may only take a few hours. I have defragmented a few raid groups on some CX700s that took more than 24 hours.

I'll be honest, it has been a while since I have done one, so I can't say for sure. But, I had some RAID groups on my CX700s that were 8+8 and 8+1. Maybe I ran it at a lower priority than high too, I cannot say for sure.

I wonder if it runs at the same rate as LUN migrations or rebuilds. There is some good information in the latest Best Practices guide showing the rebuild rates for varios raid group types and how it affects application workloads.

Sorry I cannot be more specific.

I know about this paper: it's been mentioned in another thread last month, so the different speeds that come with the different setting are explained there. It was a paper called something like: performance impact.. something..

Do you have to stop all access to the datas before executing the defrag ?

not at all ..it's an online process. I typically pick very low activity time so not to cause too much contention with host access.

The title of the paper RRR mentioned above is "The Influence of Priorities on CLARiiON LUN Management Operations - Applied Technology".

Also, reminder that the defrag operation performed by the array only acts to move LUNs within a RAID Group, in order to maximize contiguous free space available on the RAID Group - it has no effect/impact on files (as the storage system is not filesystem aware). How long it will take defrag to run, also depends on how many LUNs need to be moved. For a RAID Group that has never had a LUN deleted, there will be nothing to move.

Regards,
DGM

"The Influence of Priorities on CLARiiON LUN Management Operations - Applied Technology" did not speak to defrags on a raid group though it does have other interesting information on binding disks, Rebuilds, and LUN migrations. Is defragging a RG less invasive than a these other operations?

Thanks,

Mike

A defrag can be set to low, medium and high as well and it depends on the "gaps" in your raid group how long it takes for a complete defrag to run. If the position of a LUN in a raid group doesn't matter you could use the following trick where you have for example a 10GB gap in the beginning, but a 10GB LUN in the end of the raid group: create an new empty LUN that will automatically be created in the gap I just mentioned. Now start a migration from the 10GB LUN in the end to the new empty LUN. Only the data in that last LUN will be moved from the back to the front and the defrag will be ready much faster. For the rest, I know in a 5 disk RAID5 raid group with 146GB 10k drives a full defrag with a gap in the beginning will take about 3 to 6 hours, depending on the load on that particular raid group.
I'd say: try starting a defrag a high speed and if you don't notice any impact, let it run at that speed. Should you notice an impact, lower the speed to medium or low. Once started, you cannot stop it.

There is no risk in doing a LUN defrag of a RAID group. All the advice you have received here is good. I particularly like that little shortcut trick that was just explained.

The duration of a defrag is dependent on the size of the RAID group, the degree of fragmentation (how many and how big are the holes), the RAID type, and the drive type.

A defrag is not fast. Using RAID 4+1, with Fibre Channel drives, FLARE 26 on a CX3-80, cache off with a 1 thread 8kb 2:1 R:W load and a medium fragmentation the following rates were observed:

Load and High: 16 MB/s
Load and Medium: 1.5 MB/s
Load and Low: 3.5 MB/s

The abnormality with Medium load is noted. YMMV.

There is a summary description of defrag in "EMC CLARiiON Storage System Fundamentals for Performance and Availability" available on PowerLink. However, it does not contain much more (with exception of the short-cut) than the contents of this thread.

We are currently planning an update of the "The Effect of Priorities on LUN Management Operations" paper to include a detailed discussion of RAID group defrag. The information in that paper will be summarized in "EMC CLARiiON Best Practices for Performance and Availability, FLARE Revision 29" (or 30). Both of these updated documents will be available on Powerlink.

null

I wouldn't say the risk is zero as there is always some risk whenever you make a change.

I have experienced a defrag getting stuck and unable to complete. I was told this was caused by a trespass occurring during the defrag.The advice I was given at the time was a reboot of the sps was need to clear the fault. In the end I managed to clear the problem by moving off one or all of luns from the Raid Group.

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clarriion