Please see my other blog for Oracle EBusiness Suite Posts - EBMentors

Search This Blog

Note: All the posts are based on practical approach avoiding lengthy theory. All have been tested on some development servers. Please don’t test any post on production servers until you are sure.

Monday, November 25, 2013

Exadata: Calculating Reserve Space for Cell/Disk Failure Coverage

ASM disk groups in Exadata are defined as either normal or high redundancy. Normal redundancy provides for two copies of file extents while high redundancy provides for three copies of file extents. Each disk is partnered with a set of other disks in other failure groups to ensure that file extent copies are stored in separate failure groups and the disk group can tolerate the loss of one disk (or one cell) in normal redundancy or two disks (or two cells) in high redundancy disk groups.



Failure coverage refers to the amount of space in a disk group that will be used to remirror data in the event of some storage failure - more space kept free means greater failure coverage.


Disk failure coverage (DFC) refers to have enough free space to allow data to be re-mirrored (rebalanced) after a single disk failure in a normal redundancy disk group, or single or dual disk failure in a high redundancy disk group.


Cell failure coverage (CFC) refers to have enough free space to allow data to be re-mirrored after the loss of one entire cell.

Calculating Reserve Space for Failure Coverage
Reserving space in the disk group means that you monitor the disk group to ensure that FREE_MB never goes below the minimum amount needed for disk or cell failure coverage. The below script, executed in an ASM instance, will show the amount of free space needed for each disk group and will indicate if sufficient space exists for disk and cell failure coverage with a PASS/FAIL comment.


Output on my Test environment.

------ DISK and CELL Failure Diskgroup Space Reserve Requirements  ------
This procedure determines how much space you need to survive a DISK or CELL
failure. It also shows the usable space
available when reserving space for disk or cell failure.
Please see MOS note 1551288.1 for more information.
.  .  .
Description of Derived Values:
One Cell Required Mirror Free MB : Required Mirror Free MB to permit successful
rebalance after losing largest CELL regardless of redundancy type
Disk Required Mirror Free MB     : Space needed to rebalance after loss of
single or double disk failure (for normal or high redundancy)
Disk Usable File MB              : Usable space available after reserving space
for disk failure and accounting for mirroring
Cell Usable File MB              : Usable space available after reserving space
for SINGLE cell failure and accounting for mirroring
.  .  .
ASM Version: 11.2.0.3
.  .  .
-------------------------------------------------------------------------
DG Name:                                      DATA
DG Type:                                    EXTERN
Num Disks:                                       6
Disk Size MB:                                  944
.  .  .
DG Total MB:                                 5,664
DG Used MB:                                  2,504
DG Free MB:                                  3,160
.  .  .
One Cell Required Mirror Free MB:            1,038
.  .  .
Disk Required Mirror Free MB:                3,776
.  .  .
Disk Usable File MB:                          -205
Cell Usable File MB:                           707
.  .  .
Enough Free Space to Rebalance after loss of TWO disks: FAIL
Enough Free Space to Rebalance after loss of ONE cell: PASS
-------------------------------------------------------------------------
DG Name:                                       DG1
DG Type:
Num Disks:                                       6
Disk Size MB:                                    0
.  .  .
DG Total MB:                                     0
DG Used MB:                                      0
DG Free MB:                                      0
.  .  .
One Cell Required Mirror Free MB:                0
.  .  .
Disk Required Mirror Free MB:                    0
.  .  .
Disk Usable File MB:                             0
Cell Usable File MB:                             0
.  .  .
Enough Free Space to Rebalance after loss of TWO disks: FAIL
Enough Free Space to Rebalance after loss of ONE cell: WARNING (cell failure is
very rare and high redundancy offers ample protection already)
.  .  .
Script completed.

PL/SQL procedure successfully completed.

SQL>

No comments: