Unsolved
This post is more than 5 years old
20 Posts
0
1318
June 16th, 2014 11:00
Migrating from thick to thin on AIX using migratepv
We are in the process of migrating to a VPLEX/VMAX on AIX using migratepv. At the same time we are going from thick (fully pre-allocated) to thin devices.
Primus emc242709 states that "migratepv will never recover free space, as it will copy every logical extent from the source volume to the target volume regardless of the contents - forcing (unnecessary) free space to be allocated to the thin device". This makes sense but it says the two workarounds are using PPME or creating a new filesystem on the thin LUN and then copying the files using favorite file copy utility. Isn't another workaround to perform the migration with migratepv and run a reclaim on the device once complete?
0 events found
PedalHarder
4 Apprentice
•
465 Posts
0
June 16th, 2014 16:00
What PPME and filesystem copy do is only copy allocated files. migratepv can be less granular than that, copying at the physical partition(PP) level. Since a PP can be quire large (and some folks may not check this) I suspect this is one reason why it isn't listed as a thick to thin migration technique in the KB. There may be other factors that limit the benefit with migratepv as well (striped volumes?)
As you suggest it will work from a migration perspective, but may not achieve the same thinning results as the other techniques.
pdxschverigs
20 Posts
0
June 17th, 2014 08:00
Thank you! Can you please explain this "There may be other factors that limit the benefit with migratepv as well (striped volumes?)"?
PedalHarder
4 Apprentice
•
465 Posts
0
June 17th, 2014 19:00
I am suggesting the primary reason is likely to be the lack of granularity migrating at the PP level. There may be other factors I am not aware of. Striping is one possibility... since you are striping across PP's, you may have more non-empty PP's in a striped config.
Another consideration would be that JFS2 is not thin friendly as such... deleted data does not get zeroed. If you have a 1 TB volume for example that got 99% full at some time, but is currently 10% full. The PP migration will copy 1 TB of data where a file level copy will copy 100GB. Quite a difference!
PedalHarder
4 Apprentice
•
465 Posts
0
June 17th, 2014 21:00
Actually, let me correct myself on the last sentence above. A migratepv will only move a PP if there is active data in it.