Persistent binding

Persistent binding

Host-based zoning can include WWN or LUN masking, and is typically known as “persistent binding.”

In storage networking, ”persistent binding” is an option of zoning.

Host-based zoning is usually referred to as persistent binding or LUN, [mapping] and is perhaps the least implemented form of zoning. Because it requires the host configuration to be correct in order to avoid zoning conflicts, this form of zoning creates a greater opportunity for administrative errors and conflicting access to targets. Moreover zoning interfaces vary among different host operating systems and [HBA] s — increasing the possibility for administrative errors. If a host is not configured with the zoning software, it can access all devices in the fabric and create an even higher probability of data corruption. Host-based zoning is often used when clusters are implemented to control the mapping of devices to specific [SCSI] target IDs. However, it should never be the only form of zoning. Augmenting host-based zoning with storage- and fabric-based zoning is the only acceptable method to reliably control device access and data security.

Basically, A given LUN has it SCSI id assigned by its RAID device (typically a SAN ). But for some purposes it's useful to have the SCSI id assigned by the host itself: that's persistent binding.

What is Persistent binding for ?

Without persistent binding, after every reboot, the SCSI id of a LUN may change. For Example, under Linux, a LUN bound on /dev/sda could migrate to /dev/sdb after a reboot. The risks augments with multipathing. Based on that, it is obvious that many software may crash without persistent binding.

from http://www.storagesearch.com/datalink-art1.html:"Operating systems and upper-level applications (such as backup software) typically require a static or predictable SCSI target ID for their storage reliability and persistent binding affords that happening."

Types of zoning

A zone can include host and LUNS. The LUNS are exported by the DISK ARRAY, the hosts are the clients (servers, computers). Each host in a zone can access each LUNS in the same zone. That's zoning. The zone is usually set on the central point of connection of the hosts and the DISK ARRAY: the FC switch.


=LUN masking=

from Fibre Channel zoning:"Zoning is sometimes confused with LUN masking, because it serves the same goals. LUN masking, however, works on Fibre channel level 4 (i.e. on SCSI level), while zoning works on level 2. This allows zoning to be implemented on switches, whereas LUN masking is performed on endpoint devices - host adapters or disk array controllers."

Here, we are talking about level 4.

This form of zoning has to be augmented by another zoning : RAID ARRAY based or fabric-based(FC switch) to improve security and avoid any errors. Otherwise two hosts may access the same data(LUNS) at the same time and it will result in data corruption.Persistent binding is often used in a cluster environnement, to associate a LUN with a persistent SCSI id and device (example /dev/sda)

References

External links

* http://www.storagesearch.com/datalink-art1.html
* http://www.enterprisestorageforum.com/sans/features/print.php/11188_1431341_2


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