[Discuss] LVM Re: A really interesting chain of functionality

[matt at mattshields.org (Matt Shields)]

On Tue, Sep 27, 2011 at 8:10 AM, Edward Ned Harvey <blu at nedharvey.com>wrote:

> > From: Mark Woodward [mailto:markw at mohawksoft.com]
> >
> > I don't think this is right. Running nagios on a snapshot would do
> > nothing. A snapshot is protected from change. Typically, what you would
> > do is this:
> >
> > Create a volume, monitor it, create a snapshot to get a "point in time"
> > image of the volume, backup the snapshot, and then remove the snapshot.
> >
> > Pretty much the same model as the other things.
>
> My memory is similar to what Matt wrote.  Suppose you have a 400G volume,
> and you use a 100G volume for snapshots.  You create a snapshot, and then
> the 400G is frozen, while all new changes get written to the 100G.  When
> 100G runs out, the snapshot disappears.  I don't know if you have to
> monitor
> available usage using df on the pool, df on the snapshot, or lvdisplay or
> some other command, but I'm sure there's a command that will let you
> monitor
> the amount of space remaining in your snapshot device.  It is not allocated
> or resized dynamically.  If you want to make it reallocate dynamically,
> you're doing some pretty crazy scripting which is not necessary on other
> snapshot systems (zfs etc)
>
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>

See previous comment.  vgdisplay and the second to last line is "Free PE /
Size".  Here's one of my desktops below.  See the bottom section, on this
150GB drive, it has 4753 x 32MB extents, I've allocated all of them to the
volume, and I have 0 free extents.  If I was building this for snapshots I
wouldn't use all the extents.

[root at mattlinux matt]# vgdisplay
  --- Volume group ---
  VG Name               vg_mattlinux
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  4
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                3
  Open LV               3
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               148.53 GiB
  PE Size               32.00 MiB
  Total PE              4753
  Alloc PE / Size       4753 / 148.53 GiB
  Free  PE / Size       0 / 0
  VG UUID               dwP9d1-YYaJ-GisZ-8Lm4-1hiz-g1sc-XcTRMt

Matthew Shields
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