what does "Wiping cache of LVM-capable devices" mean in pvs?

GenericLinuxUser · 04-27-2012, 09:59 AM

Hi, I am trying to find the meaning behind "Wiping cache of LVM-capable devices" in pvs. As I understand it (and as man says) pvs is a report about physical volumes. When you do a verbose listing for all (pvs -av) you get this message at the beginning. There is very little info that I can find on the LVM cache, what exactly is the LVM caching? Is it data regarding the LVM's and each "cache flush" rebuilds some sort of database? Am I affecting my volumes using pvs?

polpak · 04-28-2012, 10:35 PM

removed oops.

GenericLinuxUser · 04-29-2012, 08:04 AM

Nothing but an oops so far? Its great that my question is #2 on Google for "Wiping cache of LVM-capable devices", now perhaps we can get some information?

polpak · 04-30-2012, 05:29 AM

The wikipedia artile:

http://en.wikipedia.org/wiki/Logical...er_%28Linux%29

includes reference tp :

http://www.markus-gattol.name/ws/lvm.html

This seems provide good explanation of LVM, with various images and examples :-)

Quote:

LVM Components

With Linux, the LVM is structured in three elements:

Volumes: physical and logical volumes and volume groups
Extents: physical and logical extents
Device mapper: the Linux kernel module

Quote:

vgscan - Scan all disks for volume groups and rebuild caches.

Quote:

Lines 99 to 103 show another quite interesting/important command — vgscan scans all disks for volume groups and rebuilds caches. Here is the rationale for line 99: I am using an external USB (Universal Serial Bus) HDD (Hard Disk Drive) for this article i.e. what can be seen in line 13 to 15 are physical partitions on this HDD. Since I turn this HDD off over night (I went to bed between lines 98 and 99 i.e. turned off the external HDD and subnotebook), I need to issue vgscan in order to rescan for VGs — otherwise only vg0 on my subnotebook would be known to the system.

Am NON-technical, so my guess, reads as though cache refers to data-block addresses stored in cache=memory needing be refreshed from time to time so as to keep track of where everything is. Add a new computer with HD's required LVM learning where in new pc all the components were, then storing their address details in memory to enable speedy access.

Guess, wiping the cache means replacing current LV Memory using new scan results of where everything is stored.

.

GenericLinuxUser · 04-30-2012, 01:09 PM

thanks polpak, I appreciate the effort but I too came to the same guess more or less, I was mostly hoping for confirmation that I am indeed not affecting the LVM's, seeing a "wiping" message make some people nervous