Re: [linux-lvm] how much memory does LVM need? oom-killer comes

[Tomasz Chmielewski <mangoo@wpkg.org>]

Tomasz Chmielewski schrieb:

> Ouch - now it went so far that the machine only loops from a kernel 
> panic to a kernel panic - that's really bad:
> 
> (...)
> Out of memory: kill process 2220 (getty) score 24 or a child
> Killed process 2220 (getty)
> Kernel panic - not syncing: Out of memory and no killable processes...
> 
> Rebooting in 20 seconds..

Again, replying to myself, as no one on this list seem to have a clue 
about problems with LVM snapshots... I added RAM, and was able to boot 
again.


Below is a quick try to estimate the memory usage of LVM snapshots; I'm 
using 2.6.22.6.

The machine has 256 MB RAM.

With these volumes and snapshots, almost 200 MB of memory is used:

# lvs
   LV                  VG          Attr   LSize   Origin Snap%  Move Log 
Copy%
   backup              st          owi-ao   1.20T
   backup-snap         st          swi-a- 100.00G backup  31.22
   backup-snap-resized st          swi-a-  30.00G backup  72.73
   backup1             st          -wi-a-   3.00G
   backup1-sync-test   st          -wi-a-   3.00G
   src                 st          -wi-a-   2.00G
   swap                st          -wi-ao   3.00G

# free
              total       used       free     shared    buffers     cached
Mem:        512864     418380      94484          0     216036       5156
-/+ buffers/cache:     197188     315676


After removing a 100 GB snapshot that is over 30% full, we reclaim 
almost 100 MB of memory:

# lvremove /dev/st/backup-snap

Do you really want to remove active logical volume "backup-snap"? [y/n]: y
   Logical volume "backup-snap" successfully removed


# free
              total       used       free     shared    buffers     cached
Mem:        512864     328668     184196          0     220652       7480
-/+ buffers/cache:     100536     412328
Swap:      3145720        220    3145500


After removing yet another snapshot - 30 GB, over 70% full - we reclaim 
70 MB:

# lvremove /dev/st/backup-snap-resized
Do you really want to remove active logical volume 
"backup-snap-resized"? [y/n]: y
   Logical volume "backup-snap-resized" successfully removed

# free
              total       used       free     shared    buffers     cached
Mem:        512864     335500     177364          0     296444       9436
-/+ buffers/cache:      29620     483244
Swap:      3145720        216    3145504


So, as a rule of thumb, we can say that for each *used* GB of snapshot, 
~3 MB of RAM is needed.


-- 
Tomasz Chmielewski
http://wpkg.org