HP LeftHand P4000 SAN Solutions - Manual (Page 152)

Table 49 Information on the System Use tab (continued)
Description Category
group or not. When a storage system is in a management group, the LeftHand OS reserves space
for data handling.
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 50 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 LeftHand OS 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 LeftHand OS volume)
that there is freed-up space. In fact, no mechanism exists to transmit that information. There is no
SCSI command which says “Block 198646 can be safely forgotten.” At the block device level,
there are only reads and writes.
So, to ensure that our iSCSI block devices work correctly with file systems, any time a block is
written to, that block is forever marked as allocated. The file system reviews its “available blocks”
list and reuses blocks that have been freed. Consequently, the file system view (such as Windows
152 Provisioning storage