Using Disks in a ZFS Storage Pool

The most basic element of a storage pool is a piece of physical storage. Physical storage can be any block device of at least 128 Mbytes in size. Typically, this device is a hard drive that is visible to the system in the /dev/dsk directory.

A storage device can be a whole disk (c1t0d0) or an individual slice (c0t0d0s7). The recommended mode of operation is to use an entire disk, in which case the disk does not need to be specially formatted. ZFS formats the disk using an EFI label to contain a single, large slice. When used in this way, the partition table that is displayed by the format command appears similar to the following:

Current partition table (original):
Total disk sectors available: 17672849 + 16384 (reserved sectors)

Part      Tag    Flag     First Sector        Size        Last Sector
  0        usr    wm               256       8.43GB         17672849    
  1 unassigned    wm                 0          0              0    
  2 unassigned    wm                 0          0              0    
  3 unassigned    wm                 0          0              0    
  4 unassigned    wm                 0          0              0    
  5 unassigned    wm                 0          0              0    
  6 unassigned    wm                 0          0              0    
  8   reserved    wm          17672850       8.00MB         17689233 

To use whole disks, the disks must be named by using the /dev/dsk/cXtXdX naming convention. Some third-party drivers use a different naming convention or place disks in a location other than the /dev/dsk directory. To use these disks, you must manually label the disk and provide a slice to ZFS.

ZFS applies an EFI label when you create a storage pool with whole disks. For more information about EFI labels, see EFI Disk Label in System Administration Guide: Devices and File Systems.

A disk that is intended for a ZFS root pool must be created with an SMI label, not an EFI label. You can relabel a disk with an SMI label by using the format -e command.

Disks can be specified by using either the full path, such as /dev/dsk/c1t0d0, or a shorthand name that consists of the device name within the /dev/dsk directory, such as c1t0d0. For example, the following are valid disk names:

• c1t0d0

• /dev/dsk/c1t0d0

• c0t0d6s2

• /dev/foo/disk

Using whole physical disks is the simplest way to create ZFS storage pools. ZFS configurations become progressively more complex, from management, reliability, and performance perspectives, when you build pools from disk slices, LUNs in hardware RAID arrays, or volumes presented by software-based volume managers. The following considerations might help you determine how to configure ZFS with other hardware or software storage solutions:

• If you construct ZFS configurations on top of LUNs from hardware RAID arrays, you need to understand the relationship between ZFS redundancy features and the redundancy features offered by the array. Certain configurations might provide adequate redundancy and performance, but other configurations might not.

• You can construct logical devices for ZFS using volumes presented by software-based volume managers, such as Solaris^TM Volume Manager (SVM) or Veritas Volume Manager (VxVM). However, these configurations are not recommended. While ZFS functions properly on such devices, less-than-optimal performance might be the result.

For additional information about storage pool recommendations, see the ZFS best practices site:

http://www.solarisinternals.com/wiki/index.php/ZFS_Best_Practices_Guide

Disks are identified both by their path and by their device ID, if available. This method allows devices to be reconfigured on a system without having to update any ZFS state. If a disk is switched between controller 1 and controller 2, ZFS uses the device ID to detect that the disk has moved and should now be accessed using controller 2. The device ID is unique to the drive's firmware. While unlikely, some firmware updates have been known to change device IDs. If this situation happens, ZFS can still access the device by path and update the stored device ID automatically. If you inadvertently change both the path and the ID of the device, then export and re-import the pool in order to use it.