[gentoo-user] LVM: Pro and Contra ?

[gentoo-user] LVM: Pro and Contra ?

[Meino Christian Cramer]

Hi,

 Soon I will change me system from PATA (80GB) to SATA (200GB). I am
 playing with installing LVM for all parts of the filesystem with the
 execption of the / -filesystem. May be I will convert this later,
 too, but one step after the other...

 But I have no idea about the Pros and Contras of using LVM instead of
 the classical fixed size partition scheme.

 Can someone point me to a text or something which explains this
 aspect of LVM ?

 I tried google but there far too many "Pro and Contra" texts about
 other things.....

 Thanks a lot in advance for any help in advance!
 mcc

 
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Re: [gentoo-user] LVM: Pro and Contra ?

[kashani]

Meino Christian Cramer wrote:
> Hi,
> 
>  Soon I will change me system from PATA (80GB) to SATA (200GB). I am
>  playing with installing LVM for all parts of the filesystem with the
>  execption of the / -filesystem. May be I will convert this later,
>  too, but one step after the other...
> 
>  But I have no idea about the Pros and Contras of using LVM instead of
>  the classical fixed size partition scheme.
> 
>  Can someone point me to a text or something which explains this
>  aspect of LVM ?
> 
>  I tried google but there far too many "Pro and Contra" texts about
>  other things.....
> 
>  Thanks a lot in advance for any help in advance!
>  mcc

On a home system I'd be tempted to have one big partition where LVM 
would buy you nothing.

Pros: However at work I run a number of dev boxes. You can tell a 
programmer many things, but you can't tell him where to put his files. 
Using LVM I allocated 40GB of an 80GB drive and now can grow /var /tmp 
/opt and /home as needed. I run the whole thing on top of a software 
RAID 1.

Cons: I briefly blew up LVM by not rebuilding lvm after device-mapper 
changed... or it might have been the other way around. Had to chroot, 
figure out how to mount LVM, and then rebuild a few packages. Took maybe 
60 minutes or so since I had to manually setup the RAID 1 first.
	I guess the con is more stuff to remember to build into your kernel and 
watch for updates on.

kashani
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Re: [gentoo-user] LVM: Pro and Contra ?

[Meino Christian Cramer]

From: kashani <kashani-list@badapple.net>
Subject: Re: [gentoo-user] LVM: Pro and Contra ?
Date: Tue, 08 Aug 2006 09:51:58 -0700

> Meino Christian Cramer wrote:
> > Hi,
> > 
> >  Soon I will change me system from PATA (80GB) to SATA (200GB). I am
> >  playing with installing LVM for all parts of the filesystem with the
> >  execption of the / -filesystem. May be I will convert this later,
> >  too, but one step after the other...
> > 
> >  But I have no idea about the Pros and Contras of using LVM instead of
> >  the classical fixed size partition scheme.
> > 
> >  Can someone point me to a text or something which explains this
> >  aspect of LVM ?
> > 
> >  I tried google but there far too many "Pro and Contra" texts about
> >  other things.....
> > 
> >  Thanks a lot in advance for any help in advance!
> >  mcc
> 
> On a home system I'd be tempted to have one big partition where LVM 
> would buy you nothing.
> 
> Pros: However at work I run a number of dev boxes. You can tell a 
> programmer many things, but you can't tell him where to put his files. 
> Using LVM I allocated 40GB of an 80GB drive and now can grow /var /tmp 
> /opt and /home as needed. I run the whole thing on top of a software 
> RAID 1.
> 
> Cons: I briefly blew up LVM by not rebuilding lvm after device-mapper 
> changed... or it might have been the other way around. Had to chroot, 
> figure out how to mount LVM, and then rebuild a few packages. Took maybe 
> 60 minutes or so since I had to manually setup the RAID 1 first.
> 	I guess the con is more stuff to remember to build into your kernel and 
> watch for updates on.
> 
> kashani
> -- 
> gentoo-user@gentoo.org mailing list
> 

Thanks a lot for your reply, kashani ! :)

 Hrrrmmm...I am not a native English speaker...I am not whether I
 understood your first sentence correctly...

 Did you mean: On a home system it is no advantage to use LVM or did
 you mean: On a home system there is no disadvantage when using LVM
 (or did you mean something totally different ???)

 I will use LVM "strictly home"...

 And yes: "You can tell a programmer many things, but you can't tell
 him where to put his files." TRUE! I am a programmer (but the system 
 with or without LVM I have in mind is not for my profession but for
 my hobby...) and know that your are right :) X-}

 What's about fragmentation of data, when using LVM ?

 Keep hacking!
 mcc
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Re: [gentoo-user] LVM: Pro and Contra ?

[Richard Fish]

On 8/8/06, Meino Christian Cramer <Meino.Cramer@gmx.de> wrote:
>  What's about fragmentation of data, when using LVM ?

There are very few disadvantages.  If you put root on LVM, then you
will require an initramfs to boot, and as kashani said, if the LVM
packages break somehow, you will be back to a livecd.  However, I
think keeping a backup initramfs and kernel in /boot should be
sufficient for most cases.

The biggest advantage is that you can easily add more space to a
volume group if you run out.  So say you only allocate the 80G you are
currently using to your logical volumes initially, you can add space
to them as required from the unallocated space.

LVM has no effect on fragmentation however, other than that you can
easily use more and smaller filesystems to keep the effects of
fragmentation managable.  When you consider how the different areas of
a gentoo directory tree are used, it makes sense to use different
filesystems for /, /var, /usr/portage, and /home.  I also recommend
separate filesystems for /usr/portage/packages and distfiles, just to
keep those large archives out of the same filesystem that contains the
ebuilds.

-Richard
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Re: [gentoo-user] LVM: Pro and Contra ?

[Meino Christian Cramer]

From: "Richard Fish" <bigfish@asmallpond.org>
Subject: Re: [gentoo-user] LVM: Pro and Contra ?
Date: Tue, 8 Aug 2006 10:59:02 -0700

> On 8/8/06, Meino Christian Cramer <Meino.Cramer@gmx.de> wrote:
> >  What's about fragmentation of data, when using LVM ?
> 
> There are very few disadvantages.  If you put root on LVM, then you
> will require an initramfs to boot, and as kashani said, if the LVM
> packages break somehow, you will be back to a livecd.  However, I
> think keeping a backup initramfs and kernel in /boot should be
> sufficient for most cases.
> 
> The biggest advantage is that you can easily add more space to a
> volume group if you run out.  So say you only allocate the 80G you are
> currently using to your logical volumes initially, you can add space
> to them as required from the unallocated space.
> 
> LVM has no effect on fragmentation however, other than that you can
> easily use more and smaller filesystems to keep the effects of
> fragmentation managable.  When you consider how the different areas of
> a gentoo directory tree are used, it makes sense to use different
> filesystems for /, /var, /usr/portage, and /home.  I also recommend
> separate filesystems for /usr/portage/packages and distfiles, just to
> keep those large archives out of the same filesystem that contains the
> ebuilds.
> 
> -Richard
> -- 
> gentoo-user@gentoo.org mailing list
> 

Hi Richard !

 Thanks a lot for your explanations! :) 

 Yes, the variable size of a LVM was that feature what attracts my
 attention first.

 Another question came up in my mind: Is a logical volumes (that
 "beast" where I put the filesystem on finally) behaviour identical to
 that of an ordinary old fashioned partion?

 Is it possible (for example) to feed such an lv again through the
 device mapper for encryption purposes ?

 Is the performance impact via LVM (not encryption) noticeable or
 negliable? 

 Keep hacking!
 mcc

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Re: [gentoo-user] LVM: Pro and Contra ?

[brettholcomb]

LVM let's you add space to volumes without having to create new partitions, move the data over, and then do something with the old one.
> 
> From: Meino Christian Cramer <Meino.Cramer@gmx.de>
> Date: 2006/08/08 Tue PM 12:27:53 EDT
> To: gentoo-user@lists.gentoo.org
> Subject: [gentoo-user] LVM: Pro and Contra ?
> 
> 
> Hi,
> 
>  Soon I will change me system from PATA (80GB) to SATA (200GB). I am
>  playing with installing LVM for all parts of the filesystem with the
>  execption of the / -filesystem. May be I will convert this later,
>  too, but one step after the other...
> 
>  But I have no idea about the Pros and Contras of using LVM instead of
>  the classical fixed size partition scheme.
> 
>  Can someone point me to a text or something which explains this
>  aspect of LVM ?
> 
>  I tried google but there far too many "Pro and Contra" texts about
>  other things.....
> 
>  Thanks a lot in advance for any help in advance!
>  mcc
> 
>  
> -- 
> gentoo-user@gentoo.org mailing list
> 
> 

-- 
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Re: [gentoo-user] LVM: Pro and Contra ?

[Alexander Skwar]

Meino Christian Cramer schrieb:
> Hi,
> 
>  Soon I will change me system from PATA (80GB) to SATA (200GB). I am
>  playing with installing LVM for all parts of the filesystem with the
>  execption of the / -filesystem.

/boot should also not be on LVM.

>  Can someone point me to a text or something which explains this
>  aspect of LVM ?

Check out the EXCELLENT HowTo at http://www.tldp.org/HOWTO/LVM-HOWTO/

>  I tried google but there far too many "Pro and Contra" texts about
>  other things.....

What Cons are there reg. LVM?

Hm:

- Only Linux supports LVM - ie. if you need to access the data
   from ANY other OS, LVM cannot be used
- / on LVM can sometimes be hard
- The howto should be read and understood (OTOH, howtos reg. old
   fashioned partitioning also have to be read and understood).

Pro:
- Flexibility - it's *extremely* easy to resize "partitions"
   and file systems
- Snapshot - *very* useful for backups
- Great scalability: If additional space is required, put in
   new discs and extend the existing filesystems to also use
   those discs


Alexander Skwar
-- 
falsie salesman, n:
	Fuller bust man.
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Re: [gentoo-user] LVM: Pro and Contra ?

[Alexander Skwar]

Meino Christian Cramer schrieb:
> From: kashani <kashani-list@badapple.net>

>> On a home system I'd be tempted to have one big partition where LVM 
>> would buy you nothing.
>> 
>> Pros: However at work I run a number of dev boxes. You can tell a 
>> programmer many things, but you can't tell him where to put his files. 
>> Using LVM I allocated 40GB of an 80GB drive and now can grow /var /tmp 
>> /opt and /home as needed. I run the whole thing on top of a software 
>> RAID 1.
>> 
>> Cons: I briefly blew up LVM by not rebuilding lvm after device-mapper 
>> changed... or it might have been the other way around. Had to chroot, 
>> figure out how to mount LVM, and then rebuild a few packages. Took maybe 
>> 60 minutes or so since I had to manually setup the RAID 1 first.
>> 	I guess the con is more stuff to remember to build into your kernel and 
>> watch for updates on.
>> 
>> kashani
>> -- 
>> gentoo-user@gentoo.org mailing list
>> 
> 
> Thanks a lot for your reply, kashani ! :)
> 
>  Hrrrmmm...I am not a native English speaker...I am not whether I
>  understood your first sentence correctly...
> 
>  Did you mean: On a home system it is no advantage to use LVM

Yes, because he'd give the (IMO *extremely* *bad*) advice to use
just one huge filesystem for everything.

I very much disagree with that. IMO, even on home systems it makes
a lot of sense to use multiple file systems.

>  What's about fragmentation of data, when using LVM ?

It doesn't have anything to do with LVM.

You're mentioning a "problem" of LVM: With LVM, the Logical Volumes,
and thus the filesystems on top of those LVs can be fragmented (and
very often will be). This happens, when a LV will be resized. Reason:
Suppose two LVs are created. Those 2 LVs are adjacent. Then the 1st
LV (at the beginning of the VG) is to be extended (made bigger). This
will cause the first part of the 1st LV to be in front of the 2nd
LV and the 2nd part of the 1st LV will be after the 2nd LV.

Alexander Skwar
-- 
Never commit yourself!  Let someone else commit you.
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Re: [gentoo-user] LVM: Pro and Contra ?

[Alexander Skwar]

Meino Christian Cramer schrieb:

>  Another question came up in my mind: Is a logical volumes (that
>  "beast" where I put the filesystem on finally) behaviour identical to
>  that of an ordinary old fashioned partion?

In how far?

>  Is it possible (for example) to feed such an lv again through the
>  device mapper for encryption purposes ?

Yes.

>  Is the performance impact via LVM (not encryption) noticeable or
>  negliable? 

I don't know of any benchmarks. Guts feeling: Not noticeable.


Alexander Skwar
-- 
Half a mind is a terrible thing to waste!
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Re: [gentoo-user] LVM: Pro and Contra ?

[Richard Fish]

Moving this back to gentoo-user...after I accidentally replied off-list...

On 8/8/06, Meino Christian Cramer <Meino.Cramer@gmx.de> wrote:
>  (hopefully) last questions on LVM:
>
>  Here:
>    http://www.gentoo.org/doc/en/lvm2.xml
>  I read of the LVM-installation and usage.
>
>  The text say:
>
>     When you reach the end of the installation part of the handbook,
>     don't forget to umount all your LVM2 logical volumes as well and
>     for a good measure run the following command before you reboot:
>
>     Code Listing 2.13: Shutting down LVM2
>     # vgchange -a n
>
>  ...this "vgchange -a n" is done on alrteady installed systems
>  automagically, isn't it ? Or ?

Yes, by /lib/rcscripts/addons/lvm-stop.sh.

>  Is there any text, which describes how to recover from LVM-damages for
>  example by utility-changes ?

Well, I'm not aware of anything.

Actually it looks like the lvm utilities are now built as static
executables by default (unless you have USE=nolvmstatic!).  So I think
the most dangerous case is already solved.  I do remember there was
one version that didn't work with existing metadata or some such
thing, but it only survived a day or so ~x86!  So if you want to be
even more safe, don't accept the ~x86 keyword for sys-fs/lvm2.  And it
is always a good idea to keep a liveCD around, regardless of whether
you are using lvm or not.

-Richard
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