[linux-lvm] LVM newbie question.

[linux-lvm] LVM newbie question.

[Melinda Taylor]

Hello :)

I am about to setup LVM for the first time on a new server I have just
installed redhat on. And I have a few questions.

When I do a pvcreate on say /dev/sda1 does that destroy all the existing
data on that partition?

Which is better for pvcreate to be used on a whole disk (pvcreate
/dev/sda) and destroy all the underlying data or for me to merge all my
existing paritions over to PV to combine in a volume group? Is there any
advantage over either?

I've read the LVM how-to and am stilll alittle confused having never done
it in practice. One part of the how-to said converting a root system to
LVM is not recommended. SHould your OS directories /var / /boot
etc all not be included in the LVM? That was the main idea behind me
implementing LVM so that specific partitions (say var) could be resized at
a later date if needed.

Will I have any trouble restoring my system (which I backed up using dd
with a BS of 1024k) Do I have to make the LV the same size as my original
partition were for this to work?

Many Thanks for any help. I am just a bit confused as to where to step to
start.

Thanks,

melinda

Re: [linux-lvm] LVM newbie question.

[lembark]

Have you read the FAQ?

> When I do a pvcreate on say /dev/sda1 does that destroy all the existing
> data on that partition?

Why would you run pvcreate on a non-empty partition? The
only thing you'd do after that would be creating LV's 
which will overwrite any data on the partition.

> Which is better for pvcreate to be used on a whole disk (pvcreate
> /dev/sda) and destroy all the underlying data or for me to merge all my
> existing paritions over to PV to combine in a volume group? Is there any
> advantage over either?

Depends on how many disks you have. If have only a 
single disk then you're better off w/ a root partition
and some swap outside LVM in case you have to recover
from a situation in which LVM has problems. At that 
point you can parition a disk with, say, 120MB, 64MB,
2GB, <the rest> w/ types 83, 82, 83, and 8e. This 
allows you to install the system w/ the 3rd partition
as /usr if your distribution doesn't support LVM out of
the box or use it for scratch space. If /usr is a mount
point then the root can comfortably be 128MB.

> I've read the LVM how-to and am stilll alittle confused having never done
> it in practice. One part of the how-to said converting a root system to
> LVM is not recommended. SHould your OS directories /var / /boot
> etc all not be included in the LVM? That was the main idea behind me
> implementing LVM so that specific partitions (say var) could be resized at
> a later date if needed.

If /usr is a mount point then having /boot as a separate
mount is extraneous. If the machine isn't a 7x24 server
you can probably get by with /var on LVM; for a hard-core
system having /var on a partition is useful in case LVM
fries. Make /var 320MB and use a separate /scratch partition
for building new code, &c, and the only thing you'll need
on /var is spooling and logs, which will be unlikely to 
outrun a 320MB partition.

> Will I have any trouble restoring my system (which I backed up using dd
> with a BS of 1024k) Do I have to make the LV the same size as my original
> partition were for this to work?

You cannot perform backups with dirty dump (dd). Use dump or
cpio to back the system up before touching LVM. At the 
simplest:

    find / |
    egrep -v '/(proc|tmp|cache)/' |
    cpio -ov -Hcrc --io-size=$((80*1024*1024)) --file=$TAPE 2>&1 |
    tee /var/log/backup.log;

would give a full system backup to the tape device.

For a full backup using cpio -it --io-size=$((1024*1024*80)) --file=$TAPE
to check the archive is highly recommended.

Pick up a copy of Frisch, Essential System Administration, (3rd ed,
O'Reilly Press, 2002) for recommendations on this; Nemeth, et al, 
Unix System Administrator's Handook (3rd ed) is also an excellent
read.


--
Steven Lembark                              2930 W. Palmer
Workhorse Computing                      Chicago, IL 60647
                                           +1 800 762 1582

Re: [linux-lvm] LVM newbie question.

[bscott]

On Tue, 23 Jul 2002, at 4:49pm, Melinda Taylor wrote:
> When I do a pvcreate on say /dev/sda1 does that destroy all the existing
> data on that partition?

  Yes.

> Is there any advantage over [creating a PV on a partition vs a whole
> disk]?

  Not really.  Do whatever makes the most sense for your situation.

> One part of the how-to said converting a root system to LVM is not
> recommended.  SHould your OS directories /var / /boot etc all not be
> included in the LVM?

  Basically, to keep things simple, it is recommended that the boot and root
partition(s) not be placed on LVM.  This is because many boot/rescue/install
environments do not support LVM, making recovery from a boot failure
difficult to impossible.

  By placing / and /boot outside of LVM, you can repair them from just about
any boot/rescue/install set, and/or boot without LVM working.  Once you're
booted from the HDD in single-user mode, you can use the LVM utilities (in
/sbin) to get the rest of the system (on LVM) working.

> That was the main idea behind me implementing LVM so that specific
> partitions (say var) could be resized at a later date if needed.

  The system will typically boot with / mounted read-only.  Nothing else is
mounted, so if /var or /tmp or /usr or whatever are separate filesystem(s),
that is fine.  The system boot scripts (e.g., rc.sysinit) should start the
LVM and mount any needed filesystems.

> Will I have any trouble restoring my system (which I backed up using dd
> with a BS of 1024k)

  Using "dd" for backups is a bad idea.  Use a backup tool, such as "dump",
"cpio", or GNU "tar".

-- 
Ben Scott <bscott@ntisys.com>
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