[gpfsug-discuss] migrate policy vs restripe

[Marc A Kaplan]

Doubtless there are subtleties and special cases, but my should-work-well 
advice is:

A) Use mmapplypolicy ... -I defer  -P 
policy-to-set-pool-assignments-and-replication-factor  ...
      where the policy rules file contains rules like

    RULE 'adjust' MIGRATE  [FROM POOL 'x']  TO POOL 'y'  REPLICATE({1|2})  
WHERE decide to which file this rule shall apply
 
   The '-I defer' option says just mark the inodes, do not actually move 
the data

B) Use mmrestripefs ...  -b  ... -N nodeclass   The move data to correct 
pool assignments and rebalance the entire file system, also fix 
ill-replicated and ill-placed files.  (keep reading...)

C) "restripe only works within the same pool"  -- I believe there is a 
misunderstanding.  For any given file, all the data blocks are supposed to 
be in the one pool indicated by pool assignment which is recorded in the 
file's inode.  If the pool assignment changes but not all of the blocks 
are migrated to the new pool, then the file is considered "ill-placed".  I 
believe mmrestripefs endeavors to move blocks so as to "fix" ill-placed 
files.   Failure groups are a different concept that is associated with 
replication.  The replicas of a given data or meta data block should be in 
different failure groups.  If not, the file is considered "ill-replicated" 
 and mmrestripefs endeavors to fix that also.



From:   "J. Eric Wonderley" <eric.wonderley at vt.edu>
To:     gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Date:   10/04/2016 09:39 AM
Subject:        [gpfsug-discuss] migrate policy vs restripe
Sent by:        gpfsug-discuss-bounces at spectrumscale.org



We have the need to move data from one set of spindles to another.

Are there any performance or availability considerations when choosing to 
do either a migration policy or a restripe to make this move?  I did 
discover that a restripe only works within the same pool...even though you 
setup two pools in different failure groups.
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