What LVM support is missing?

What LVM support is missing?

[Simon Josefsson]

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The manual is quite clear that LVM support is missing:

  https://guix.gnu.org/manual/en/html_node/Limitations.html

This held me back from using guix as the OS for my virtualization
servers since I use LVM for virtual machines.  However, one evening I
was curious how difficult it would be to fix the above limitation, so I
started with a simple 'guix package -i lvm2' and that allowed me to get
LVM to work and I can't really notice anything missing.

Before producing a patch to correct the manual, could someone explain
what LVM support was intended that was missing?  Does a reference to LVM
as a limitation of Guix still make sense?

I'm thinking that having the root file system on LVM may be unsupported,
so that manual could say that, but I'm not even sure that is true.  I
haven't tried it though.  Perhaps 'lvm2' should be pre-installed on the
default installation image to help move LVM support along.

Another thought may be that constructing LVM partitions through the
mapped-device mechanism is not supported:

http://guix.gnu.org/manual/en/html_node/Mapped-Devices.html

however as far as I can tell, LVM is working just as well as LUKS/RAID
here: LVM just doesn't need any command to start the devices.  The
mapped-device approach appear to require out-of-band device creation for
LUKS and RAID, just like is required for LVM.  A no-op
`lvm-device-mapping' could be added for completeness, and it might run
'pvscan --active ay' or something like that if we really wanted to but
I'm not sure when that would be useful.

Thanks,
/Simon

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Re: What LVM support is missing?

[Marius Bakke]

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Simon Josefsson <simon@josefsson.org> writes:

> The manual is quite clear that LVM support is missing:
>
>   https://guix.gnu.org/manual/en/html_node/Limitations.html
>
> This held me back from using guix as the OS for my virtualization
> servers since I use LVM for virtual machines.  However, one evening I
> was curious how difficult it would be to fix the above limitation, so I
> started with a simple 'guix package -i lvm2' and that allowed me to get
> LVM to work and I can't really notice anything missing.
>
> Before producing a patch to correct the manual, could someone explain
> what LVM support was intended that was missing?  Does a reference to LVM
> as a limitation of Guix still make sense?

What's missing is support in the configuration system: the ability to
declare a LVM mapped device in the OS configuration and expect 'guix
system init', 'guix system reconfigure' etc to work.

LVM 2.03 simplifies things a bit, but I don't know of any recent
attempts to add LVM support to the configuration system.

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Re: What LVM support is missing?

[Simon Josefsson]

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Marius Bakke <mbakke@fastmail.com> writes:

> Simon Josefsson <simon@josefsson.org> writes:
>
>> The manual is quite clear that LVM support is missing:
>>
>>   https://guix.gnu.org/manual/en/html_node/Limitations.html
>>
>> This held me back from using guix as the OS for my virtualization
>> servers since I use LVM for virtual machines.  However, one evening I
>> was curious how difficult it would be to fix the above limitation, so I
>> started with a simple 'guix package -i lvm2' and that allowed me to get
>> LVM to work and I can't really notice anything missing.
>>
>> Before producing a patch to correct the manual, could someone explain
>> what LVM support was intended that was missing?  Does a reference to LVM
>> as a limitation of Guix still make sense?
>
> What's missing is support in the configuration system: the ability to
> declare a LVM mapped device in the OS configuration and expect 'guix
> system init', 'guix system reconfigure' etc to work.
>
> LVM 2.03 simplifies things a bit, but I don't know of any recent
> attempts to add LVM support to the configuration system.

How is that different than RAID or LUKS support not being available
through that mechanism?  I need to run 'mdadm --create' or 'cryptsetup
luksFormat' externally to config.scm too.  It appears to me that the
status of LVM is similar o RAID or LUKS wrt the configuration system.  I
think people generally regard RAID and LUKS as supported and working by
Guix, right?  My point is that the state of LVM support is similar, so
the warning about this limitation may do more harm than good.  There
could be some more examples added on how to setup some LVM partitions
and have them be mounted, or even examples of how to put rootfs on LVM,
and with that the prominent warning about missing LVM could be dropped.

/Simon

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Re: What LVM support is missing?

[Michael Rohleder]

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Hey Simon!

Simon Josefsson <simon@josefsson.org> writes:
> I'm thinking that having the root file system on LVM may be unsupported,
> so that manual could say that, but I'm not even sure that is true.  I
> haven't tried it though.  Perhaps 'lvm2' should be pre-installed on the
> default installation image to help move LVM support along.
>
> Another thought may be that constructing LVM partitions through the
> mapped-device mechanism is not supported:
>
> http://guix.gnu.org/manual/en/html_node/Mapped-Devices.html
>
> however as far as I can tell, LVM is working just as well as LUKS/RAID
> here: LVM just doesn't need any command to start the devices.  The
> mapped-device approach appear to require out-of-band device creation for
> LUKS and RAID, just like is required for LVM.  A no-op
> `lvm-device-mapping' could be added for completeness, and it might run
> 'pvscan --active ay' or something like that if we really wanted to but
> I'm not sure when that would be useful.

I use a root fs on lvm.
What helped me a lot was this posting, where I stole most things:

https://lists.gnu.org/archive/html/bug-guix/2020-02/msg00285.html

What (currrently) isnt working is the lvm device-mapper and (for me more
important) lvmcache...


-- 
Fly Windows NT:
All the passengers carry their seats out onto the tarmac, placing the chairs
in the outline of a plane. They all sit down, flap their arms and make jet
swooshing sounds as if they are flying.

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Re: What LVM support is missing?

[Simon Josefsson]

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Michael Rohleder <mike@rohleder.de> writes:

> Hey Simon!
>
> Simon Josefsson <simon@josefsson.org> writes:
>> I'm thinking that having the root file system on LVM may be unsupported,
>> so that manual could say that, but I'm not even sure that is true.  I
>> haven't tried it though.  Perhaps 'lvm2' should be pre-installed on the
>> default installation image to help move LVM support along.
>>
>> Another thought may be that constructing LVM partitions through the
>> mapped-device mechanism is not supported:
>>
>> http://guix.gnu.org/manual/en/html_node/Mapped-Devices.html
>>
>> however as far as I can tell, LVM is working just as well as LUKS/RAID
>> here: LVM just doesn't need any command to start the devices.  The
>> mapped-device approach appear to require out-of-band device creation for
>> LUKS and RAID, just like is required for LVM.  A no-op
>> `lvm-device-mapping' could be added for completeness, and it might run
>> 'pvscan --active ay' or something like that if we really wanted to but
>> I'm not sure when that would be useful.
>
> I use a root fs on lvm.
> What helped me a lot was this posting, where I stole most things:
>
> https://lists.gnu.org/archive/html/bug-guix/2020-02/msg00285.html

Thanks for pointer!  Maybe some examples like this could be added to the
manual.  I'll experiment a bit too and see if I can propose something...

> What (currrently) isnt working is the lvm device-mapper and (for me more
> important) lvmcache...

What do you expect from a lvm device-mapper?  When I thought about it,
it is mostly a no-op since the kernel handles this automatically, or?

Working lvmcache does not seem required to claim support for LVM to me.
Why doesn't lvmcache work?  Also sometimes the performance improvements
doesn't seem that significant:
http://strugglers.net/~andy/blog/2017/07/19/bcache-and-lvmcache/

/Simon

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