What is REBALANCE or RESILVER or RESYNC or COMPACT?

What is REBALANCE or RESILVER or RESYNC or COMPACT?

All these words look similar in nature of work they perform and everyone can assume all these words represents the similar in ASM diskgroup.

IF we go in deep understanding of each of these activities, each activity has its own roles to be performed.

I will try to explain in this blog in simple words what all these activities means.

When these activities can be seen in ASM disk groups?

--- When disk fails (transit failure or permeant failure)

--- When disk replacement activity happens

--- When flash disk fails

--- When ACFS disk fails

--- When ZFS disk fails

--- and may more scenarios  

SQL> select * from v$asm_operation;

   INST_ID GROUP_NUMBER OPERA PASS      STAT      POWER     ACTUAL  SOFAR EST_WORK EST_RATE EST_MINUTES ERROR_CODE   CON_ID

---------- ------------ ----- --------- ---- ----------  ---------- ----- -------- -------- ----------- -----------  ----------

         1            1 REBAL RESYNC        DONE         12                                                              0

         1            1 REBAL RESILVER     DONE         12                                                              0

         1            1 REBAL REBALANCE WAIT          12                                                              0

         1            1 REBAL COMPACT     WAIT          12               

                                  

          

What is RESYNC or RESILVER or REBALANCE or COMPACT?

Rebalancing:

is something like spreading the data evenly across all the disks in a disk group.

--- It will happen during disk replacement operation.

--- It happens when disk fails.

Resilvering:

means copying of data from one side of the mirror to another. It is like rebuilding.

--- It will happen in EXADATA flash disks

--- It will happen in ACFS disks as well 

--- It will also be seen in ZFS

Resync:

is something like syncronizing the disks with the data that should reside on them. (can be used on transient failure)

--- It occurs in transient failure of a disk.

--- The content present in the failed disk is tracked by other disks and any modification that is made to the content of failed disk is actually made in other available disks. Once we get the disk back and attach it, the data belonging to this disk and which got modified during that time will get resynchronized back again.

Compact:

is de-fragments and compacts extents across Oracle ASM disks

Regards,

Mallik