Glossary of VMFS terms
CID Chain
For a proper CID chain understanding let's have a look at CID itself. CID stands for Content Identification. It's a 32-bit, randomly created value, that VMware generates every time when it opens the *.WMX file. Along with a new snapshot, Vmware creates a delta.vmdk file and unites it in a chain with the help of CID values. Those values are not constant and are modified every time VM restarts, but the hierarchy of parent-child CID Values in the chain must be kept. In the 3-component CID chain with Parent .vmdk, 1st child. VMDK and 2nd child *.VMDK created after the first and second snapshots respectively the submission must be the following:
child vdisk-000002.vmdk points to the child vdisk-000001.vmdk and vdisk-000001.vmdk points to the parent vdisk.vmdk.
If this CID chain is broken VM won't be able to mount virtual disks assigned to it until the order is reconstructed!
CLI or Command-Line Interface
is a powerful management tool for VMware users to administer VMware ESX/ESXi infrastructures. For those who are used to working from the command line, vSphere Command Line Interface is a strong alternative to such managing tools as vSphere Client. It helps you to run most of the common system administration commands preserving network access to ESX/ESXi systems. vSphere CLI comprises the ESXCLI command set, VICFG- commands, etc., providing more freedom in ESX/ESXi features configuration.
ISCSI
Internet Small Computer System Interface is a transport layer protocol that sends SCSI commands over local-area networks (LANs), wide-area networks (WANs) or the Internet to link data storage devices, ISCSI envelopes commands and bundles the data in packets before sending them over a TCP/IP network, employing a point-to-point connection. After the arrival packets are uncovered, the SCSI commands are separated and the operating system sees the storage as a local SCSI device. So, it allows uniting separate storage into a centralized storage array, while hosts perceive it as locally attached disks. iSCSI allows two hosts e.g. web server and database to exchange commands via IP network.
ISCSI doesn't depend on additional expensive equipment and uses standard Ethernet, not requiring specialized skills to install and configure everything properly.
Despite iSCSI's great popularity there are more expensive alternatives such as Fibre Channel over IP (FCIP), Internet Fibre Channel Protocol (iFCP), Fibre Channel over Ethernet (FCoE), ATA over Ethernet (AoE).
LUN
stands for a Logical Unit Number. It is a number that identifies separate devices (logical units) addressed by SCSI, iSCSI or Fibre Channel protocol. A LUN can be used with any reading/writing devices, but more often this term is used to refer to the logical disk. Notice that one and the same virtual volume in the RAID array can have different LUN values assigned by different hosts. It's also possible to come across identical LUNs in one host, that belong to the different SCSI Target IDs.
NFS
Network File System provides an opportunity for a user on a client computer to view, update, and store files on remote systems over a network as if they were stored locally on the user's own hard drive. The user's system requires an NFS client and the other computer needs the NFS server to be mounted. TCP/IP must be also installed on both computers since NFS uses it for file exchange.
More often NFS is employed to configure a central server where users store all their home directories. Further, they can be accessed by logging into a client anywhere on the network. NFS also allows installing software products on several computers at once from a centralized source.
There is an extended NFS version - WebNFS which is considered to be more efficient when accessing Web pages and other internet content.
vSphere Snapshot
is a copy of the virtual machine disk file that reveals the state and data of the VMDK at a certain point in time. The snapshots are mostly used to provide detailed information about a virtual disk at a given time if there is a need to restore a virtual machine in the case of any failure or system errors. However, the snapshots must not be treated as providing backups. They are not absolute copies of the original VMDK files. If the base disks become unavailable, the snapshots appear to be useless. Snapshot files can't be bigger than the original disk file, but, nevertheless, demand some extra disk space. This space grows when disk writing activity increases. Snapshots are usually deleted within an hour. Those that stay longer are advised to be erased in 24 hours. 72 hours is the deadline of existence for any snapshot to provide normal VM operation. Though VMware operation is calculated to support up to 32 snapshots united in a chain, it's strongly recommended not to use more than 2 or 3 of them for further backup purposes.
ZFS
Zettabyte File System is one of the most advanced combinations of a logical volume manager and properly a filesystem. It features high storage capacities: up to 16 Exabyte file sizes and 256 Zettabyte volume sizes. ZFS provides powerful data compression, protects data from corruption, verifies data for integrity, and performs automatic repair if needed. It also grants such features as snapshots and copy-on-write clones. ZFS consists of the following structural elements:
- vdev – elements making up a pool
- pool – nexus that manifests the underlying storage devices
- dataset – the pool's child filesystem
- volume – the pool's child, functioning as a virtual block device
- snapshot - the state of a dataset or a volume frozen at a point in time
- clone – a new dataset or a volume derived from a snapshot
Being characterized as the last word in filesystems, ZFS is secure, stable, and really fast.
Raidz
Realization or RAID5 architecture on the file system level. RAID-Z is a software RAID based on ZFS which provides a data\parity distribution scheme similar to RAID 5. RAID-Z offers some performance improvements compared to RAID 5.
Actually RAID-Z can act like RAID1, RAID6(RAID-Z2), and RAID7(RAID-Z3), we support RAID-Z with single and double parity versions.