HP P4000 9.0 HP StorageWorks P4000 SAN Solution User Guide - Page 202

Category Usable space Provisioned space Description Space available for storage after RAID has been configured. Amount of space allocated for volumes and snapshots. 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 44 Common native file systems OS Windows Linux Netware Solaris VMWare File System Names NTFS, FAT EXT2, EXT3 NWFS UFS VMFS 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. However, the file system does not inform the block device underneath (the SAN/iQ volume) that there is freed-up space. In fact, no mechanism exists to transmit that information. There is no SCSI command 202 Provisioning storage