[gentoo-user] LVM : pros & cons

[gentoo-user] LVM : pros & cons

[Philip Webb]

Does anyone have advice based on experience using LVM ?
I sb partitioning a new  320 GB  hard drive soon for a simple desktop box.
That is  8 times  the size of the HDD in my present machine,
which I haven't exhausted by any means.  LVM seems more professional
& allows flexibility for unforeseen storage needs,
but it adds a layer of complexity & potential problems arising therefrom.
I wonder whether LVM slows down disk access
& whether there's a disaster lurking unseen if anything goes wrong with LVM:
a bad package update, a damaged config file or file storing LVM's layout
would seem to risk losing everything on the HDD & require re-installation.

-- 
========================,,============================================
SUPPORT     ___________//___,  Philip Webb : purslow@chass.utoronto.ca
ELECTRIC   /] [] [] [] [] []|  Centre for Urban & Community Studies
TRANSIT    `-O----------O---'  University of Toronto
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Re: [gentoo-user] LVM : pros & cons

[Dan Johansson]

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On Sunday 07 October 2007, Philip Webb wrote:
> Does anyone have advice based on experience using LVM ?
> I sb partitioning a new  320 GB  hard drive soon for a simple desktop box.
> That is  8 times  the size of the HDD in my present machine,
> which I haven't exhausted by any means.  LVM seems more professional
> & allows flexibility for unforeseen storage needs,
> but it adds a layer of complexity & potential problems arising therefrom.
> I wonder whether LVM slows down disk access
> & whether there's a disaster lurking unseen if anything goes wrong with
> LVM: a bad package update, a damaged config file or file storing LVM's
> layout would seem to risk losing everything on the HDD & require
> re-installation.

I'm using LVM for all my (linux-)computers (Servers, Desktops and Notebooks). 
The only filesystems not on LVM are / and /boot (I know that I can put / on a 
LVOL as well but I don't like to use initrd if I can avoid it). This is for 
example how my desktop looks like:

/dev/hde1		/boot			30MB
/dev/hde2		swap			4GB
/dev/hde3		/			500MB
/dev/hde4		LVM-vg00
/dev/vg00/lvol01	/usr			4GB
/dev/vg00/lvol02	/var			10GB
/dev/vg00/lvol03	/opt			2GB
/dev/vg00/lvol04	/home/dan		4GB
/dev/vg00/lvol05	/home/ulle		4GB
/dev/vg00/lvol06	/tmp			1GB
/dev/vg00/lvol07	/var/vmware/WinXP	28GB
/dev/vg00/lvol08	/usr/portage		3GB

The main reason for me for using LVM is that I can easily extend a filesystem 
on the fly or add a new one if necessary.
-- 
Dan Johansson, <http://www.dmj.nu>
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Re: [gentoo-user] LVM : pros & cons

[William Kenworthy]

Never had a problem with LVM.

On a mythTV box with LVM, started with 80GB IDE, then added in a 300Gb
IDE, then a few months later another 300mb, sata this time.  Basicly
transparent to Myth

I have avoided LVM on laptops before now because its extra complexity
and I'll never add multiple disks.

But, on a sony laptop without LVM - I have discovered that when I
partitioned it in a rush ~18 months ago, I made the diagnostic partition
5 GBytes instead of 5 mbytes - and now I need the space.  My options are
to format it and symlink it into the tree, wipe and start again or
image, repartition and restore.  All have big disadvantages because this
is my primary, work, not owned by me laptop (!).  

It would have been so much easier if I had used LVM ...

On Sun, 2007-10-07 at 07:01 -0400, Philip Webb wrote:
> Does anyone have advice based on experience using LVM ?
> I sb partitioning a new  320 GB  hard drive soon for a simple desktop box.
> That is  8 times  the size of the HDD in my present machine,
> which I haven't exhausted by any means.  LVM seems more professional
> & allows flexibility for unforeseen storage needs,
> but it adds a layer of complexity & potential problems arising therefrom.
> I wonder whether LVM slows down disk access
> & whether there's a disaster lurking unseen if anything goes wrong with LVM:
> a bad package update, a damaged config file or file storing LVM's layout
> would seem to risk losing everything on the HDD & require re-installation.
> 
> -- 
> ========================,,============================================
> SUPPORT     ___________//___,  Philip Webb : purslow@chass.utoronto.ca
> ELECTRIC   /] [] [] [] [] []|  Centre for Urban & Community Studies
> TRANSIT    `-O----------O---'  University of Toronto
-- 
William Kenworthy <billk@iinet.net.au>
Home in Perth!
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Re: [gentoo-user] LVM : pros & cons

[Florian Philipp]

Philip Webb schrieb:
> Does anyone have advice based on experience using LVM ?
> I sb partitioning a new  320 GB  hard drive soon for a simple desktop box.
> That is  8 times  the size of the HDD in my present machine,
> which I haven't exhausted by any means.  LVM seems more professional
> & allows flexibility for unforeseen storage needs,
> but it adds a layer of complexity & potential problems arising therefrom.
> I wonder whether LVM slows down disk access
> & whether there's a disaster lurking unseen if anything goes wrong with LVM:
> a bad package update, a damaged config file or file storing LVM's layout
> would seem to risk losing everything on the HDD & require re-installation.
> 


Simple pros and cons?

Pro:
More flexible, especially when using a special setup [1], can easily be 
expanded to RAID1 and/or RAID0 (even both at the same time!).

Cons:
- Can't be used with Windows. Even worse, Windows sees the partition as 
unformatted rather than unknown and therefore might have no problem 
formatting it without warning.
- Can't be used for /boot, usage for / needs initrd (my advice: keep 
/boot, /bin, /sbin and /etc on a normal partition).
- It's one more thing that can break.
- You can't use *parted to resize the underlying partitions.
- I had trouble mounting my lvm /home partition (mirrored) with Ubuntu, 
possibly a missing kernel module in the default kernel or missing use 
flag. I didn't try to fix it, though.
- You loose bit performance, but not much, you won't feel it without 
benchmarks.

[1]
Some simple advices: Create small logical volumes. It's easier to expand 
them than to shrink them (expanding reiserfs takes less than 5 minutes).
If you are not shure wether you might need more space on ordinary dos 
partitions later on, don't create one big partition for lvm but several 
smaller. That makes management a bit more complicated (especially when 
you are unfamiliar with the tools and/ or have several physical disks) 
but you can simply reformat one of them if the need arises (see "man 
pvmove" for more information).
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Re: [gentoo-user] LVM : pros & cons

[Alan McKinnon]

On Sunday 07 October 2007, Florian Philipp wrote:
> Philip Webb schrieb:
> > Does anyone have advice based on experience using LVM ?

<snip>

> Simple pros and cons?
>
> Pro:

<snip>

> - You loose bit performance, but not much, you won't feel it without
> benchmarks.

I very much doubt this. LVM is one extra layer between the filesystem 
and the physical disk and it basically consists of a mapping between 
the extents in the VG and exactly where they are on the volume. This is 
nothing more than an elementary lookup table; on a 500G VG using 32M 
extents this consists of precisely 15,625 entries, it can all be stored 
in RAM and can consist of one pointer plus precisely one calculation to 
determine the offset from the start of the table where the desired 
extent lies.

Considering that RAM runs at many orders of magnitude faster than the 
disk you are trying to get the data off of, the extra fraction of a % 
overhead is not even worth trying to measure, let alone benchmark it. 
Moving the heads just once more because of file fragmentation probably 
takes longer than the entire LVM lookup

alan

-- 
Optimists say the glass is half full,
Pessimists say the glass is half empty,
Developers say wtf is the glass twice as big as it needs to be?

Alan McKinnon
alan at linuxholdings dot co dot za
+27 82, double three seven, one nine three five
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Re: [gentoo-user] LVM : pros & cons

[Alan McKinnon]

On Sunday 07 October 2007, Philip Webb wrote:
> Does anyone have advice based on experience using LVM ?

The majority of folks around here will have used LVM :-)

> I sb partitioning a new  320 GB  hard drive soon for a simple desktop
> box. That is  8 times  the size of the HDD in my present machine,
> which I haven't exhausted by any means.  LVM seems more professional
> & allows flexibility for unforeseen storage needs,

this is it's main benefit on desktop class machines - the ability to 
resize volumes when you realize that you guessed wrong. There are 
others too :-)

> but it adds a layer of complexity & potential problems arising
> therefrom. 

A total non-issue in my experience. I've never had an LVM problem yet, 
but maybe I'm just lucky. The one thing you do need to be aware of it 
that you require LVM support at boot time or shortly thereafter. So 
either compile it into the kernel, or make sure it's in the initrd.

For a gentoo system using roll-your-own kernels, the consensus seems to 
be a regular / volume of 500M-1G is plenty and everything else is on 
LVM. That way you avoid the issues of not having the required support 
to be able to mount /. We don't build distro kernels that must boot on 
everything out there, we have the luxury of customizing everything

> I wonder whether LVM slows down disk access 

No. See my other mail.

> & whether there's a disaster lurking unseen if anything goes wrong
> with LVM: a bad package update, a damaged config file or file storing
> LVM's layout would seem to risk losing everything on the HDD &
> require re-installation.

Not true. You *already* have many layers of software between user space 
and disk, any one of which can go wrong at any time. LVM metadata is 
stored in text files and it maintains many historical copies of 
previous configs and it's easy to fix if it ever goes wrong. I've never 
seen a *real* LVM error, but I have matched myself do some really dumb 
things and I could fix them every time.

Seriously, the problem LVM solves has been known about for decades and 
the method used was worked out about the same time. It's a mature 
technology that is *very* well understood, completely the opposite of 
drivers to support some latest new-fangled chipset. I would be much 
more worried about that code trashing your disk than LVM. Just to put 
it all into perspective...

Extra benefits of LVM: You won't need this right now for your simple 
desktop with one drive, but it's good to know what else LVM can do:

Snapshots. You can "freeze" the state of a filesystem at any time and 
LVM will track the changes since then until you release the snapshot. 
This is a lifesaver if your job is to perform backups of 4TB databases 
that can never be taken down for backups.

Huge volumes: LVM is the only way on Linux to be able to get local 
volumes bigger than any single disk. Again, on servers, 2TB+ databases 
are becoming commonplace.

If you need any more convincing, IBM mainframes and HP machines running 
HP-UX have required you to use LVM for years now - you can't get to the 
disks without using LVM. If it was risky, do you think those hardware 
vendors would have gone down that route?

alan




-- 
Optimists say the glass is half full,
Pessimists say the glass is half empty,
Developers say wtf is the glass twice as big as it needs to be?

Alan McKinnon
alan at linuxholdings dot co dot za
+27 82, double three seven, one nine three five
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Re: [gentoo-user] LVM : pros & cons

[Mark Kirkwood]

Alan McKinnon wrote:
> On Sunday 07 October 2007, Florian Philipp wrote:
>   
>
>> - You loose bit performance, but not much, you won't feel it without
>> benchmarks.
>>     
>
> I very much doubt this. LVM is one extra layer between the filesystem 
> and the physical disk and it basically consists of a mapping between 
> the extents in the VG and exactly where they are on the volume. This is 
> nothing more than an elementary lookup table; on a 500G VG using 32M 
> extents this consists of precisely 15,625 entries, it can all be stored 
> in RAM and can consist of one pointer plus precisely one calculation to 
> determine the offset from the start of the table where the desired 
> extent lies.
>
> Considering that RAM runs at many orders of magnitude faster than the 
> disk you are trying to get the data off of, the extra fraction of a % 
> overhead is not even worth trying to measure, let alone benchmark it. 
> Moving the heads just once more because of file fragmentation probably 
> takes longer than the entire LVM lookup
>
>
>   
A year or two ago it was possible to measure a performance hit - for 
instance we had some Supermicro PCI-X based machines with 3Ware RAID 
cards where we could get (quoting from memory here as it was a while 
ago) uncached sequential scan rates of about 1Gb/s without LVM and 
somewhere in the region of 800Mb/s with it.

However that was then, and this is now - LVM and hardware have no doubt 
improved , so it would be interesting to do the test again (if we do 
I'll let you know).

I would think that for the OP's use case (i.e 1 disk on a desktop box) 
there will be no measurable difference at all.

Cheers

Mark

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Re: [gentoo-user] LVM : pros & cons

[felix]

On Sun, Oct 07, 2007 at 09:26:33PM +0200, Alan McKinnon wrote:

> A total non-issue in my experience. I've never had an LVM problem yet, 
> but maybe I'm just lucky. The one thing you do need to be aware of it 
> that you require LVM support at boot time or shortly thereafter. So 
> either compile it into the kernel, or make sure it's in the initrd.

I had a problem once, scared me to pieces until I got past it.  An
emerge had removed a library which was still used by the lvm command.
"ldd lvm" showed "not found".  Boot didn't like that.  I got lucky; a
rescue disk allowed me to create a bogus symlink to the old library so
that boot could proceed, at which point I remerged the broken command
and no longer needed the bogus symlink.

I have since written a perl program which looks for such broken libs.
I have no idea why gentoo's revdep-rebuild didn't find it.

I still use LVM and even gentoo, but I am also increasingly wary of
gentoo updates.

-- 
            ... _._. ._ ._. . _._. ._. ___ .__ ._. . .__. ._ .. ._.
     Felix Finch: scarecrow repairman & rocket surgeon / felix@crowfix.com
  GPG = E987 4493 C860 246C 3B1E  6477 7838 76E9 182E 8151 ITAR license #4933
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[gentoo-user] Re: LVM : pros & cons

[Alexander Skwar]

Alan McKinnon <alan@linuxholdings.co.za> wrote:

> Extra benefits of LVM: You won't need this right now for your simple
> desktop with one drive, but it's good to know what else LVM can do:
> 
> Snapshots.

Well, I "disagree". This feature is also very useful on a 
single drive setup. Reason why: Backup. You can easily create
snapshot(s) and then backup those snapshot volumes. And at
the same time, you can keep on working on the normal filesystems.

> This is a lifesaver if your job is to perform backups of 4TB databases
> that can never be taken down for backups.

IMO it's also good for smaller setups.

For huge setups, it's sort of a must, exactly as you wrote.

> If you need any more convincing, IBM mainframes and HP machines running
> HP-UX have required you to use LVM for years now - you can't get to the
> disks without using LVM.

Not true. With HP-UX 11.11, you could also choose *NOT* to use LVM.
But nobody in a right state of mind would do that :) (Well, generally
speaking at least. There will certainly be some corner cases, I suppose.)

Alexander Skwar

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Re: [gentoo-user] Re: LVM : pros & cons

[Alan McKinnon]

On Monday 08 October 2007, Alexander Skwar wrote:
> Alan McKinnon <alan@linuxholdings.co.za> wrote:
> > Extra benefits of LVM: You won't need this right now for your
> > simple desktop with one drive, but it's good to know what else LVM
> > can do:
> >
> > Snapshots.
>
> Well, I "disagree". This feature is also very useful on a
> single drive setup. Reason why: Backup. You can easily create
> snapshot(s) and then backup those snapshot volumes. And at
> the same time, you can keep on working on the normal filesystems.

Yes, that's the general process I was referring to. Snapshots are used 
much less often for backups on desktops (but still useful!) than on 
servers. I had assumed that the original poster wasn't going to be 
using snapshots on his desktop anytime soon.

> > This is a lifesaver if your job is to perform backups of 4TB
> > databases that can never be taken down for backups.
>
> IMO it's also good for smaller setups.
>
> For huge setups, it's sort of a must, exactly as you wrote.
>
> > If you need any more convincing, IBM mainframes and HP machines
> > running HP-UX have required you to use LVM for years now - you
> > can't get to the disks without using LVM.
>
> Not true. With HP-UX 11.11, you could also choose *NOT* to use LVM.

Ah, OK, thanks for that. My info came from a long-time HP sysadmin when 
we were comparing notes between HP-UX and Linux. He might have 
generalised a lot though.

This question of "Is LVM a good idea?" keeps cropping up on mailing 
lists. I find this a bit strange as I find huge benefits and have yet 
to find a valid downside for general use. Embedded systems would be an 
exception I suppose but they generally don't need any flexibility at 
all on their volumes.

alan

-- 
Optimists say the glass is half full,
Pessimists say the glass is half empty,
Developers say wtf is the glass twice as big as it needs to be?

Alan McKinnon
alan at linuxholdings dot co dot za
+27 82, double three seven, one nine three five
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Re: [gentoo-user] Re: LVM : pros & cons

[Philip Webb]

071008 Alan McKinnon wrote:
> This question "Is LVM a good idea?" keeps cropping up on mailing lists.
> I find this a bit strange as I find huge benefits
> and have yet to find a valid downside for general use.

If you haven't used it, it looks like a questionable extra complexity,
which could bite your fingers unexpectedly for little real gain.
However, I am grateful for all the replies & may decide to use it,
the comments being generally reassuring as to its stability.
There is a Gentoo doc re how to fit it into the install process.

-- 
========================,,============================================
SUPPORT     ___________//___,  Philip Webb : purslow@chass.utoronto.ca
ELECTRIC   /] [] [] [] [] []|  Centre for Urban & Community Studies
TRANSIT    `-O----------O---'  University of Toronto
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Re: [gentoo-user] Re: LVM : pros & cons

[Neil Bothwick]

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Hello Philip Webb,

> If you haven't used it, it looks like a questionable extra complexity,
> which could bite your fingers unexpectedly for little real gain.

It' one of those things[1] that you put off using because it looks
complicated. then you get round to trying it and wish you'd done so much
earlier.

[1] Screen falls into this category too.


-- 
Neil Bothwick

Borg, James Borg. Vodka martini, Gin is irrelevant.

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Re: [gentoo-user] Re: LVM : pros & cons

[Dale]

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Philip Webb wrote:
> 071008 Alan McKinnon wrote:
>   
>> This question "Is LVM a good idea?" keeps cropping up on mailing lists.
>> I find this a bit strange as I find huge benefits
>> and have yet to find a valid downside for general use.
>>     
>
> If you haven't used it, it looks like a questionable extra complexity,
> which could bite your fingers unexpectedly for little real gain.
> However, I am grateful for all the replies & may decide to use it,
> the comments being generally reassuring as to its stability.
> There is a Gentoo doc re how to fit it into the install process.
>
>   


That's me too.  I checked into it but just have not got the nerve up to
switch.  I have had to redo my partitions a couple times though so I
could use it for sure. 

Maybe one of these days.

Dale

:-)  :-)  :-)

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Re: [gentoo-user] Re: LVM : pros & cons

[Jim Burwell]

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Dale wrote:
> Philip Webb wrote:
>> 071008 Alan McKinnon wrote:
>>   
>>> This question "Is LVM a good idea?" keeps cropping up on mailing lists.
>>> I find this a bit strange as I find huge benefits
>>> and have yet to find a valid downside for general use.
>>>     
>>
>> If you haven't used it, it looks like a questionable extra complexity,
>> which could bite your fingers unexpectedly for little real gain.
>> However, I am grateful for all the replies & may decide to use it,
>> the comments being generally reassuring as to its stability.
>> There is a Gentoo doc re how to fit it into the install process.
>>
>>   
>
>
> That's me too.  I checked into it but just have not got the nerve up
> to switch.  I have had to redo my partitions a couple times though so
> I could use it for sure. 
>
> Maybe one of these days.
>
> Dale
>
> :-)  :-)  :-)
I must be one of the 'crazy' ones.  I run LVM2 for / under linux RAID. 
Even /boot is on a RAID1.  I use genkernel for the proper initramfs. 
You just need to use "dolvm2" and "lvmraid=/dev/mdX ..." on your boot line.



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Re: [gentoo-user] Re: LVM : pros & cons

[Dale]

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Jim Burwell wrote:
> Dale wrote:
>   
>> Philip Webb wrote:
>>     
>>> 071008 Alan McKinnon wrote:
>>>   
>>>       
>>>> This question "Is LVM a good idea?" keeps cropping up on mailing lists.
>>>> I find this a bit strange as I find huge benefits
>>>> and have yet to find a valid downside for general use.
>>>>     
>>>>         
>>> If you haven't used it, it looks like a questionable extra complexity,
>>> which could bite your fingers unexpectedly for little real gain.
>>> However, I am grateful for all the replies & may decide to use it,
>>> the comments being generally reassuring as to its stability.
>>> There is a Gentoo doc re how to fit it into the install process.
>>>
>>>   
>>>       
>> That's me too.  I checked into it but just have not got the nerve up
>> to switch.  I have had to redo my partitions a couple times though so
>> I could use it for sure. 
>>
>> Maybe one of these days.
>>
>> Dale
>>
>> :-)  :-)  :-)
>>     
> I must be one of the 'crazy' ones.  I run LVM2 for / under linux RAID. 
> Even /boot is on a RAID1.  I use genkernel for the proper initramfs. 
> You just need to use "dolvm2" and "lvmraid=/dev/mdX ..." on your boot line.
>
>
>   

But all I need is some glue and feathers.  I worked long and hard to get
my Gentoo install just like it is.  Even the thought of losing it makes
me as nervous as a long tail cat in a room full of rocking chairs. 

You are right though, everything I have read says it is a great thing to
use.  I even read up on EVMS.  Just got to get the nerve up.

Dale, AKA the chicken.  LOL

:-)  :-)

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[gentoo-user] Re: LVM : pros & cons

[Remy Blank]

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Neil Bothwick wrote:
> It' one of those things[1] that you put off using because it looks
> complicated. then you get round to trying it and wish you'd done so much
> earlier.
> 
> [1] Screen falls into this category too.

I can confirm about screen, I use it everywhere now. But I've yet to try
LVM (I am convinced as well, but just haven't had to install a machine
lately).

-- Remy


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Re: [gentoo-user] LVM : pros & cons

[Gabriel Rossetti]

Philip Webb wrote:
> Does anyone have advice based on experience using LVM ?
> I sb partitioning a new  320 GB  hard drive soon for a simple desktop box.
> That is  8 times  the size of the HDD in my present machine,
> which I haven't exhausted by any means.  LVM seems more professional
> & allows flexibility for unforeseen storage needs,
> but it adds a layer of complexity & potential problems arising therefrom.
> I wonder whether LVM slows down disk access
> & whether there's a disaster lurking unseen if anything goes wrong with LVM:
> a bad package update, a damaged config file or file storing LVM's layout
> would seem to risk losing everything on the HDD & require re-installation.
>
>   
Hello,

I chose to use it on my laptop because of it's flexibility. On a desktop
system, it you ever need more room, you can just a a new HD. If you use
LVM on it, you can expand whatever you need to, if you don't, you can
move data around and free up some space where needed. On a laptop,
unless you attach an external drive (and in reality, those are annoying
to carry around all the time), it's hard to free up some space, but with
LVM, such a task is possible. I found it not hard to configure and it's
benefits largely outsize the cons one could find. I would recommend it
on single and multi-HD systems.

Cheers,
Gabriel Rossetti
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