Adding disks

About adding disks  

When you add disks to a filer, you need to insert them in a disk shelf according to the disk shelf manufacturer instructions. They then become hot spare disks. You also add the disks to a volume with the vol add command.

You can use disks of various sizes in a volume according to the following rules:

• If an added disk replaces a failed data disk, its capacity is limited to that of the failed disk.
• If an added disk is not replacing a failed data disk and it is not larger than the parity disk, its full capacity (subject to rounding) is available as a data disk.

When to add disks  

You add disks when

• You want to add storage capacity to the filer
• You are running out of hot spare disks

Considerations when adding disks to the filer  

Original disk configuration

Network Appliance recommends initial RAID group sizes of 8 to 14 disks. The default RAID group size for Data ONTAP 6.x is 8 disks. Network Appliance recommends adding RAID groups and/or disks before the file system is 80 to 90% full.

The number of drives and RAID groups at initial filer configuration affects read and write performance. A greater number of drives means a greater number of independently seeking disk-drive heads reading data. Write performance can also benefit from more drives; however, the difference can be masked by the effect of NVRAM and the manner in which WAFL manages write operations.

Larger RAID groups increase the probability of a failed disk and increase disk reconstruction time. For best performance, you need to determine the tradeoff between improved performance when all disks are working and performance degradation while a disk is being reconstructed.

Adding more disks to the filer

Additional RAID groups in multiples of 8-14 are recommended for overall improved performance. However, as more disks are configured, the performance increase levels off. Performance is affected more with the fewer disks you add until the striping across all the disks levels out. The more disks you add, the less the performance is affected until the striping across all the disks levels out. When the striping levels out, there is an increase in operations per second and a reduced response time.

Adding more drives will not significantly improve performance if the applied load is not placing stress on the drives in a filer's array. However, this is the time to add more drives to meet future storage requirements.

Adding hot spare disks to the filer  

You should periodically check the number of hot spares you have on your filer and add disks if you have fewer than one per shelf. If there are no hot spare disks, you should add disks to disk shelves so they become available as hot spares.

For more information, see Multiple Volumes and Multiple RAID Groups on NetApp Filers, http://www.netapp.com/tech_library/3027.html.

Determining the number of hot spare disks  

To determine how many hot spare disks you have on your filer, complete the following step.

vol status -s

Spare disks for block or zoned checksum volumes

RAID Disk   HA.ID    HA SHELF BAY    CHAN    Used (MB/blks)   Phys(MB/blks)

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

spare       7.17     7    2    1     FC:A    0                 17046/34910224

spare       7.26     7    3    2     FC:A    0                 34209/70061520

...

Spare disks for zoned checksum volumes only

RAID Disk   HA.ID    HA SHELF BAY    CHAN    Used (MB/blks)   Phys(MB/blks)

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

spare       7.21     7    2    5     FC:A    0                 17366/35566480

spare       7.19     7    2    3     FC:A    0                 17366/35566480

Rules for adding disks to a volume  

You add disks to a volume according to the following rules:

• You can add only hot spare disks to a volume.
• You must specify the volume to which you are adding disks.
• You can add block checksum disks to zoned checksum volumes; you can add only block checksum disks to block checksum volumes.
• Disks must come from the same spare disk pool (if using mirrored volumes).

Parity disk determination  

If an added disk is larger than the existing parity disk, the added disk becomes the parity disk, and the former parity disk becomes a data disk. This enforces a Data ONTAP rule that the parity disk must be at least as large as the largest data disk in a RAID group.

Adding new disks to the filer  

To add new disks to the filer, complete the following steps.

disk swap

vol status -s

Adding new disks to a volume  

To add new disks to a volume, complete the following steps.

vol status -s

vol add vol-name ndisks[@disk-size] | -d disk-name... 

vol add thisvol 4@36

vol add thisvol -d 7.17 7.26 

Note
When adding disks to a volume, Data ONTAP attempts to distribute I/O loads, such as alternating host adapters, to improve reconstruction performance. You can override this behavior by specifying the disks you want.

Adding new disks to a RAID group  

If a volume has more than one RAID group, you can specify the RAID group to which you are adding disks. To add new disks to a specific RAID group of a volume, complete the following step.

vol add vol0 -g rg0 2

Note
The number of disks you can add to a specific RAID group is limited by the RAID group size. See Chapter 4, RAID Group Management, for more information.

Forcibly adding disks to volumes  

The following are situations that would fail if you tried to add disks to a volume:

• The disks specified in the vol add command would cause the disks on an unmirrored volume to span adapters.
• The disks specified in the vol add command add disks to a RAID group other than the last RAID group, thereby making it impossible for the file system to revert to a version prior to Data ONTAP 6.4.

To force Data ONTAP to add disks in these situations, complete the following step.

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Part No. 210-00322
File: disks7.htm
Last updated: 01/22/03 16:17:03