Understanding disks on the filer

About disks

Removable disks and disks that you add are in disk shelves that are connected to a filer. The filer supports SCSI disks in SCSI disk shelves and fibre channel disks in fibre channel disk shelves.

Kinds of disks

There are three kinds of disks, as shown in the following table.

Disk	Description
Data	Holds data stored on behalf of clients and data generated about the state of the filer as a result of a malfunction.
Hot spare	Does not hold usable data, but is available for addition to a volume.
Parity	Stores data reconstruction information.

Disk formats

Disks are formatted for the following two types of RAID parity checksums:

Block checksums: You can use block checksum disks for block checksum volumes and zoned checksum volumes.
Zoned checksums: You can use zoned checksum disks only for zoned checksum volumes.

You cannot change the format of a disk.

Disk addressing

You identify a disk by its address, which is listed in the Device column of the sysconfig -d output. You can also use the storage show or storage show disk -p command to view the disk addresses. Disk addresses are represented in the following format:

path_id.device_id

path_id is generally the slot number on the filer where the host adapter is attached. However, certain exceptions exist. For example, the path_id for the onboard fibre channel card is 0a, and the path_id for the dual channel adapter is represented by slot_number a|b.

device_id is a protocol-specific identifier for the attached disks. For SCSI, the device_id is an integer from 0 to 15. For FC-AL, the device_id is an integer from 0 to 126.

SCSI disk addresses

For a SCSI disk, path_id is the disk's SCSI adapter number and device_id is the disk's SCSI ID number. For example, disks attached to the disk adapter 7a are numbered 7a.0 through 7a.14.

Fibre channel disk addresses

For a fibre channel disk, path_id is the disk's host adapter number and device_id is the disk's ID. To identify the disk ID, see the fcstat device_map command output on your filer.

Available space on new disks

When you add a new disk, Data ONTAP reduces the amount of disk space available for user data by rounding down to maintain compatibility across manufacturers. The available disk space listed by informational commands such as sysconfig is, therefore, less for each disk than its rated capacity.

The available disk space on a disk is rounded down as shown in the following table.

Disk class	Available space	Available blocks
4-GB disks	4 GB	8,192,000
9-GB disks	8.6 GB	17,612,800
18-GB disks	17 GB	34,816,000
35-GB disks (block checksum disks)	34 GB	69,632,000
36-GB disks (zoned checksum disks)	34.5 GB	70,656,000
72-GB disks	68 GB	139,264,000