VxFS defrag on VVR volumes

[MortenSeeberg]

I suspect that defragging a VxFS file system on a VVR volume would result in LOTS of traffic on the link and lots of blocks requiring sync?

This would risk filling up the SRL and generate lots of sync traffic. Anyone try this or has any insight on this?

[mikebounds]

Defrags are done online so I don't imagine there is that much I/O, otherwise it would impact the application too much, so unless you have very limited bandwidth I wouldn't think the SRL would fill up that much and you could always stop the defrag if you do find the SRL is filling up and resume defrag once the SRL has drained.

Mike

[MortenSeeberg]

Just because it's done online it could still move a lot of blocks around :) I seem to remember some I/O penalty back in the old days when I used to do this on VxFS, and it could run for a very long time.

But your expectation is the same as mine, that all I/O changed as part of a defrag would be VVR replicated then?

[mikebounds · Accepted solution]

VVR replicates at block level, so yes, all blocks moved as part of a defrag will be replicated.  My assumption, but I could be wrong, is that the defrag is throttled so that I/O for filesytem usage will be slow (so you should defrag in quiet I/O periods), but not near unusable if the defrag I/O was maxed out.  In terms of how much is moved, this will depend on how badly you filesystem is fragemented and how often you defrag, so if SRL does start filling up too much, then should defrag more frequently, but if you are already in a position when fragmentation is bad you could limit the defrag to 1 pass (rather than default of 5 passes), by using "-p 1" flag to fsadm.

Mike

[MortenSeeberg]

Thanks Mike, good idea actually to run it with 1 pass the first time.