[linux-lvm] what use is virtualsize -- or how is it to be used?

[linux-lvm] what use is virtualsize -- or how is it to be used?

[Linda A. Walsh]

I just saw it in the man page, and first thing I tried was creating a 
logical volume
by just giving a --virtualsize, arg -- thinking it would 'auto-extend 
it' as needed.

Nep.

Next was looking on the net and seeing an example that gave it some 
small size (-L 1G),
with a virtual size of 2G.   So what is that supposed to do?

I just now thought to copy a bunch of files ( that differed from 
yesterday to today) to a tmp
partition) but had it fail miserably when it REALLY failed upon reaching 
the physical
limit -- not the virtual limit!)....


I was trying to use rsync to copy from a snap from yesturday of /home 
that was mounted on
/ohome to 'home.diff' (the partition that I used 'virtual size' with, 
... I gave it a real
size of 1G, and a virtual size of 1T.

Using rsync, I had --compare-dest=/ohome (where I mount yesturday's 
snap) and copied
from /home to /home.diff (the virtual partition)....   Well the diffs 
were > 1G, so had
hoped /home.diff would expand to it's virtual size...  Anyway, got I/O 
errors, after I
ran out of space.  Ok, so *that*'s not what virtual size is for...either! 

Anyway, I unmounted the now 'corrupt' /home.diff, and tried to remove it...
Got all sorts of i/o errors using lvremove Home.diff. (-f didn't work 
either).

Got it to remove with lvremove -f /dev/PV/Home.diff
That gave I/O errors too, but successfully completed...

So what's virtual size good for, anyway?

Re: [linux-lvm] what use is virtualsize -- or how is it to be used?

[adultsitesoftware@gmail.com]

Virtual size lets you create a 200G file system om a 20G LV and then lvextend as needed, without the need to resize the filesystem. Read about sparse files for ideas of other uses.

It'd like a sparse device in that blocks need not actually exist until they are written to.

It currently has a performance and flexibility impact that prevents us from using it, though we would like to.

"Linda A. Walsh" <lvm@tlinx.org> wrote:

>I just saw it in the man page, and first thing I tried was creating a 
>logical volume
>by just giving a --virtualsize, arg -- thinking it would 'auto-extend 
>it' as needed.
>
>Nep.
>
>Next was looking on the net and seeing an example that gave it some 
>small size (-L 1G),
>with a virtual size of 2G.   So what is that supposed to do?
>
>I just now thought to copy a bunch of files ( that differed from 
>yesterday to today) to a tmp
>partition) but had it fail miserably when it REALLY failed upon
>reaching 
>the physical
>limit -- not the virtual limit!)....
>
>
>I was trying to use rsync to copy from a snap from yesturday of /home 
>that was mounted on
>/ohome to 'home.diff' (the partition that I used 'virtual size' with, 
>... I gave it a real
>size of 1G, and a virtual size of 1T.
>
>Using rsync, I had --compare-dest=/ohome (where I mount yesturday's 
>snap) and copied
>from /home to /home.diff (the virtual partition)....   Well the diffs 
>were > 1G, so had
>hoped /home.diff would expand to it's virtual size...  Anyway, got I/O 
>errors, after I
>ran out of space.  Ok, so *that*'s not what virtual size is
>for...either! 
>
>Anyway, I unmounted the now 'corrupt' /home.diff, and tried to remove
>it...
>Got all sorts of i/o errors using lvremove Home.diff. (-f didn't work 
>either).
>
>Got it to remove with lvremove -f /dev/PV/Home.diff
>That gave I/O errors too, but successfully completed...
>
>So what's virtual size good for, anyway?
>
>
>
>_______________________________________________
>linux-lvm mailing list
>linux-lvm@redhat.com
>https://www.redhat.com/mailman/listinfo/linux-lvm
>read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

-- 
Sent from my Android phone with K-9 Mail. Please excuse my brevity.

Re: [linux-lvm] what use is virtualsize -- or how is it to be used?

[Mike Snitzer]

On Fri, Jul 08 2011 at  7:17pm -0400,
adultsitesoftware@gmail.com <support@bettercgi.com> wrote:

> Virtual size lets you create a 200G file system om a 20G LV and then lvextend as needed, without the need to resize the filesystem. Read about sparse files for ideas of other uses.
> 
> It'd like a sparse device in that blocks need not actually exist until they are written to.
> 
> It currently has a performance and flexibility impact that prevents us from using it, though we would like to.

Please have a look at the DM thinp target that was just posted to
dm-devel.

We don't have lvm2 support for this thinp target yet, we are working on
it, but thinp is a much richer solution that should address your needs.

Mike

Re: [linux-lvm] what use is virtualsize -- or how is it to be used?

[Linda A. Walsh]

Ah...of course...

When I create the FS, it will create it the size of the existing FS, 
which is less
efficient that calling "<fs>_extend" after each lv_extend...(presuming 
one is using
a fs with that ability)....(do most these days?  I usually just use xfs 
which does)...