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

[Tomasz Chmielewski <mangoo@wpkg.org>]

Stuart D. Gathman schrieb:
> On Mon, 3 Sep 2007, Tomasz Chmielewski wrote:
> 
>> 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.
> 
> I may have an answer after all.  I suspect that the chunksize parameter
> to lvcreate -s will affect memory use.  A larger chunksize should use
> less memory.
> 
>> So, as a rule of thumb, we can say that for each *used* GB of snapshot, 
>> ~3 MB of RAM is needed.
> 
> On my snapshots on Centos-5, the chunksize defaults to 8k.  With your
> observation of 3MB per Gig of snapshot, that comes to 24 bytes per chunk.
> Quite reasonable.  If you make your chunksize 512K (the maximum supported),
> memory use should change to 48K per Gig.
> 
> lvdisplay will show the chunksize for a snapshot.

Indeed, it was 8 kB.

What is the meaning of the chunksize in snapshots?

With 512 kB chunksize, I guess the snapshot will fill faster than with 8 
KB chunksize, given that the same amount of data changes?


Here is a quick test that shows it.

I created two 20 GB snapshots of a 20GB volume, one with 8kB, and the 
other with 512 kB chunksize.

Then, I created a 50 MB file on the original volume (out of /dev/zero):

   build1                  san1 owi-ao  20.00G
   build1-512              san1 swi-a-  20.00G build1            0.09
   build1-8                san1 swi-a-  20.00G build1            0.08

Copying /bin, /boot and /lib (71 MB in total) into a new directory makes 
that difference bigger:

   build1                  san1 owi-ao  20.00G
   build1-512              san1 swi-a-  20.00G build1            0.76
   build1-8                san1 swi-a-  20.00G build1            0.62

Copying a 1.2 GB /usr into a new directory shows this:

   build1-512              san1 swi-a-  20.00G build1            7.40
   build1-8                san1 swi-a-  20.00G build1            7.18


And finally, create 1000000 empty files:

   build1                  san1 owi-ao  20.00G
   build1-512              san1 swi-a-  20.00G build1            8.22
   build1-8                san1 swi-a-  20.00G build1            7.91


The difference is not that big, usually it should be below 5%.


-- 
Tomasz Chmielewski
http://wpkg.org