INTERVIEW

Vault drives:

• 1st 4 or 5 drives 
• It contains 
• Vault area, 
• PSM Lun, 
• Flare database Lun 
• and Operating System. 
• The Vault:
• The vault is a ‘save area’ across the first five disks
• to store write cache from the Storage Processors when power failure
• The PSM Lun:
• The Persistent Storage Manager Lun
• stores the configuration of the Clariion
• Such as Disks, Raid Groups, Luns, Access Logix information, SnapView configuration, MirrorView and SanCopy configuration as well.
• Operating System:
• The Operating System of the Storage Processors is stored to the first five drives of the Clariion.

Reserved LUN Pool

• RLP works with Replication software
• while replication stores the data/information required to complete a replication
• RLP contains one or more Private LUNS.

Private LUN

• The LUN become Private LUN, when it is added to RLP
• These LUNs cannot belong to any storage group & a server cannot perform I/O to them.
• 
  

VNX SNAPSHOTS

• VNX snapshots is a feature in VNX for Block OE release 32.
• created to improve the existing Snapview snapshot 
• by better integrating with pools
• vnx snapshots can only be used with pool luns
• bcz this technology require pool space
• VNX snapview &Snapview Snapshots can coexist on the same pool LUN
• Classic LUNs, LUNs that are created on physical RAID groups.
• support only Snapview Snapshots.
• VNX snapshots support 256 writable snaps per pool LUN.
• support branching, also called Snap of Snap
• a snap of snap hierarchy cannot exist 10 levels
• no restriction on no.of branching
• limit is 256 which is the hard limit
• consistency groups are also supported with the feature.
• several pool LUNs can be combined into a consistency group and snapped cocurrently.

HOW snapshots work

• VNX snapshots use Redirect On Write(ROW) technology.
• ROW redirects new writes destined for the primary LUN to a new location in the storage pool.
• which is different from Copy On First Write(COFW) used in Snapview

Diffrence

• VNX snapshots technology writes the new data to a new area within a pool as show in bellow figure
• without the need to read/write to the old data block.
• this improve overall performance compared to snapview
• Similarly, during a read from a snapshot all data read from source lun
• ths snapshot's data is not constructed from 2 different places

VNX snapshots point at unchanged blocks, primary lun is using new blocks for new i/o writes 

Snapshot granularity

• Every VNX snapshot has 8KB block granularity.
• i.e that every write occupies at least 8kb on the pool