VMware Cloud Community
haripadmam
Hot Shot
Hot Shot
‎02-10-2009 01:09 AM

What is snapshot LUN?

Hi Guys,

I'm running ESX-3.5 in production environment. A day before I noticed that a datastore which we are using for testing VM is missing from the storage lists on all ESX servers and is mapped recently. Also a warning message is displayed on all ESX server console as below:

"<some code> cpu14 : 1049 LVM:.......... :1 may be snapshot: disabling access. See resignaturing section..."

I have googled through vmware communities and made the following change in my ESX server configuration: ESX configuration > advanced > lvm > disallow snapshot lun=0. Previously it was 1. After rebooting ESX I find that warning message is not displayed and that LUN which was missing is also now become visible on that ESX host too.

Here the thing is that I did this changes to my ESX as very blind. Now I just want to know about what was happened and how that LUN gone invisible and what was the setting "disallow snapshot lun"?

Is snapshot lun is the technology in VMware ESX? or its with SAN configuration?

Thanks a lot..

Hari.

0 Kudos
2 Replies
AmaliaNL
Contributor
Contributor
‎02-10-2009 02:15 AM

Hi Hari,

when you create a vmfs datastore on a lun some additional information is written to it (LVM header). This LVM header section contains information about the underlying storage, such as hardware vendor and lun id/lun number. The "maybe a snapshot" warning indicates that the ESX/vCenter noticed a change in the vmfs datastore so that it is no longer matching the LVM header info. This mechanism is in place to prevent the ESX host from becoming confused in case a lun is cloned. Suppose you use san tools to clone (or snapshot as it is sometimes called in the san world) a lun and present the clone to the ESX and the original lun as well. If the LVM header mechanism was not functional, the ESX host would be confused and it wouldn't be capable of determining which lun is a clone and which lun is the original. This can cause data loss, ESX would then probably write randomly to both luns or something like that.

I hope you have some understanding about why this happens. I experienced the "maybe snapshot" once when an existing lun was given a new lun number from within the san tooling. I think in your case, if there is enough free space on your san, the easiest solution is to use Storage VMotion and move the vm's away from this lun. Delete the lun when it is empty and then recreate it. And somewhere in between use the rescan option.

For some additional reading or a in place solution have a look at:

Cheers.

0 Kudos
Franciosi
Contributor
Contributor
‎11-21-2011 02:44 PM

Great post, thank you AmaliaNL. More information about it,


VMFS Volume Can Be Erroneously Recognized as a Snapshot

http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&docTypeID=DT_KB_1_1&e...


vSphere handling of LUNs detected as snapshot

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=101138...



Sds,

André

0 Kudos