NetApp reallocation : Can you tell me more

This blog covers the various reallocation tools available in ONTAP
1) reallocate command
2) Continous reallocation features:

• Options on the volume

• read_realloc

4) aggregate option free_space_realloc

Reallocate command: 

The command can be used to optimize layout of a volume, LUN, file and aggregates. The reallocate command

1. First measures the layout and if above the threshold then proceed to step2.
2. Perform physical or traditional reallocation 
1. Physical : The physical data blocks are moved while retaining the logical data blocks. In this method, no additional capacity is required by snapshot copies.
2. Traditional: Scans the file system and rewrites the data blocks on the active file system. The blocks used by volume snapshot is not changed and therefore there is more capacity required to store the same data.
3. repeats steps a and b , until the layout is less than the threshold. 

Notes: 

• Reallocate start without any parameters causes reallocate to run once daily 

• You can use the reallocate command to only measure the volume allocation. 

             reallocate measure -o -vserver XXX -path /vol/volxxx 

•  Use the reallocate start command to start a reallocation. You can change the threshold value using the -t option.  

             reallocate start -t 5 -vserver XXX -path /vol/volxxx

• Use the -n flag to start a reallocate without a measurement. 

• Use the -p flag to perform reallocation using the Physical method.  

• A forced reallocation scan optimizes all the data in the volume, LUN, or file regardless of current optimization level.Forced reallocation is recommended to be performed after adding disks to an aggregate so that existing data is spread onto the new disks, reducing the time required to balance the load across all the disks

• reallocate with the flag -A to reallocate blocks at the aggregate level. This will create a contiguous space on the aggregate.  

 Continous reallocation features:

Volume read reallocation: 

 Read reallocation uses the normal workload reads along with the read ahead engine to determine the current layout optimization. If the read was less than optimal, the data will be reallocated to improve the next read of this data. Read reallocate offers both the traditional and physical reallocation methods associated with the reallocate command 

volume modify -vserver xx -volume xxx -read_realloc on/space_optimized  [on is traditional & space_optimized is physical] 

Free space reallocation : 

Free space reallocation is an aggregate option, introduced in Data ONTAP 8.1.1, which performs opportunistic free space reallocation to maintain an optimal free space layout. When enabled, if Data ONTAP detects that free space is not optimal, it will physically move data to produce areas of contiguous free space.Optimized free space improves the efficiency of WAFL and can reduce overall disk utilization. 

storage aggregate modify -aggregate aggr1 –free-space-realloc on