HP LeftHand P4000 Virtual SAN Appliance Software - Manual (Page 152)

Table 41 Information on the System Use tab
Description Category
Host name of the storage system. Name
Total amount of disk capacity on the storage system. Note:
Storage systems with greater capacity will only operate to Raw space
the capacity of the lowest capacity storage system in the
cluster.
RAID level configured on the storage system. RAID configuration
Space available for storage after RAID has been
configured. Usable space
Amount of space allocated for volumes and snapshots. Provisioned space
Measuring disk capacity and volume size
All operating systems that are capable of connecting to the SAN via iSCSI interact with two disk
space accounting systems—the block system and the native file system (on Windows, this is usually
NTFS).
Table 42 Common native file systems
File System Names OS
NTFS, FAT Windows
EXT2, EXT3 Linux
NWFS Netware
UFS Solaris
VMFS VMware
Block systems and file systems
Operating systems see hard drives (both directly connected [DAS] and iSCSI connected [SAN])
as abstractions known as “block devices”: arbitrary arrays of storage space that can be read from
and written to as needed.
Files on disks are handled by a different abstraction: the file system. File systems are placed on
block devices. File systems are given authority over reads and writes to block devices.
iSCSI does not operate at the file system level of abstraction. Instead, it presents the iSCSI SAN
volume to an OS such as Microsoft Windows as a block device. Typically, then, a file system is
created on top of this block device so that it can be used for storage. In contrast, an Oracle
database can use an iSCSI SAN volume as a raw block device.
Storing file system data on a block system
The Windows file system treats the iSCSI block device as simply another hard drive. That is, the
block device is treated as an array of blocks which the file system can use for storing data. As the
iSCSI initiator passes writes from the file system, the SAN/iQ software simply writes those blocks
into the volume on the SAN. When you look at the CMC, the used space displayed is based on
how many physical blocks have been written for this volume.
When you delete a file, typically the file system updates the directory information which removes
that file. Then the file system notes that the blocks which that file previously occupied are now
freed. Subsequently, when you query the file system about how much free space is available, the
space occupied by the deleted files appears as part of the free space, since the file system knows
it can overwrite that space.
152 Provisioning storage