topic Re: LUN Theory in Disk Enclosures

LUN Theory

Jeff Hagstrom — Mon, 03 Mar 2008 16:48:42 GMT

What is a LUN and why is used? Is it better to have more luns or less? I have a rp7410 with 15 73gig drives running hpux 11.11. On my other RP7410 I have 2 luns, 1 for each volume group. Should I create smaller LUNs or larger? What is the purpose of the LUN 0 or 10gig? Why do I have to create it?

Re: LUN Theory

Torsten. — Mon, 03 Mar 2008 16:58:21 GMT

You need to add you have an va7410 ...

Why not continue your other thread?

Anyway, a LUN is a logical unit, see
http://en.wikipedia.org/wiki/Logical_Unit_Number

Regarding the setup you should read the manual. Get it from here

http://h20000.www2.hp.com/bizsupport/TechSupport/SupportTaskIndex.jsp?lang=en&cc=us&taskId=101&prodClassId=-1&contentType=SupportManual&docIndexId=64255&prodTypeId=18964&prodSeriesId=64112

Re: LUN Theory

Torsten. — Mon, 03 Mar 2008 17:00:19 GMT

Regarding LUN0 on a VA7xxx:

(from the manual)

Many operating systems rely on the presence of LUN 0 to establish a
communication path to all LUNs on the array. Consequently, it is
recommended that the array always have LUN 0 present.
When LUN 0 is created, the array automatically creates an entry in the LUN
security table granting Write & Configure access to all hosts. This entry ensures
that all Command View SDM hosts will be able to manage the array. It is
recommended that this entry not be deleted from the table.
Because all hosts will have access to LUN 0, you may want to limit its size (10
MB) and not use it to store any critical data.

Re: LUN Theory

Carsten.Cimander — Mon, 03 Mar 2008 17:59:20 GMT

Hello Jeff,

LUN 0 is used for communication to the storage array only/normally. Normally it's used to have a working scsi device in place for each storage-array-controller (-port). Don't assign it as a "usable" LUN and don't put it under any Volume Manager control (do not do a pvcreate or a vxdisksetup on it).

Which LUN is the right size? If you use a "midrange storage array" like HP EVA or EMC Clariion it might be useful to use only few and large LUNs: the number of LUNs which can be presented to one and the same server is limited to e.g. 256 LUNs. Keep in mind that your Volume Manager (LVM) does not support dynamic growth of the underlying PVs (physical volumes): unless you "format" them again (loosing all data on them) you cannot resize the PV (take it off any LV, take it off the VG, do a pvcreate on it). So to grow your filesystem you have to create an additional LUN an add it to the VG, enlarge your LV by using it and then enlarge your filesystem. You will need another (free) LUN-Number for it ...
Assuming the Diskgroup-Layout on the Storage-Array is done in that way that there are only few disk groups (2 to 4) each holding a preferably large number of disks, the performance of one single LUN will be almost as good as concating 2 LUNs from 2 Diskgroups together. So I would start with as few LUNs as possible (not knowing any further parameters).

For High-End Storge like HP XP or EMC DMX the limitation of LUN-Numbers is much higher. But for other reasons it might be necessary to use more LUNs than one because of the creation policy used for small LUNs or (bigger) (Loose-)Volumes.

Regards,
Carsten

Re: LUN Theory

krusty — Wed, 05 Mar 2008 01:25:38 GMT

A LUN have two meaning in the computer world. The first is comes from the architecture of SCSI, the Bus, the Target, and the LUN (Logical Unit Number). Over time is also became used for a chuck of space presented from a disk array. It is also know as a VDisk on an EVA.

As for HP-UX, architecturally it is typically beneficial to have more than one LUN in a Volume Group. By default, HP-UX gives each LUN (actually the device file) a "queue depth" of 8. If you have more LUNs, they are additive in nature in the VG. So, if you have 8 LUNs in a VG, you would have a queue depth of 64. Whether you need that or it would help, that up to your application. As they say your mileage may vary. But, it typically doesn't hurt. There are ways around this with some tuning, but this is the basic concept.

Cheers,

Curt

Re: LUN Theory

Nguyen Anh Tien — Wed, 05 Mar 2008 02:51:44 GMT

Hope my slide will address your concern
tienna