Re: LVM support

[Tomáš Čech]

On Tue, Apr 21, 2015 at 05:52:33PM +0200, Ludovic Courtès wrote:
> Tomáš Čech <address@hidden> skribis:
>
> > On Thu, Apr 16, 2015 at 02:47:52PM +0200, Ludovic Courtès wrote:
> > > Tomáš Čech <address@hidden> skribis:
> > >
> > > > On Wed, Apr 15, 2015 at 02:32:14PM +0200, Ludovic Courtès wrote:
> > >
> > > [...]
> > >
> > > > > Sorry I’m not really familiar with LVM.
> > > >
> > > > It's implemented using device mapper but instead of mapping one block
> > > > device to another you map one block device to whole group (like
> > > > playground where you can do anything).
> > >
> > > What do you mean by “whole group”?  A tree under /dev/mapper?
> >
> > From device node POV it generates
> > /dev/<volume_group_name>/<logical_volume_name> and it also creates
> > /dev/mapper/<volume_group_name>-<logical_volume_name> and
> > /dev/dm-<number>.
>
> OK.
>
> > From block device perspective it adds another level of "partitioning"
> > to "physical volume" partitions. You gather block devices (can be
> > partitions, disks, anything), create volume group to join the space
> > into one entity and then create logical volumes without caring where
> > it really is. Logical volumes are useful for resizing, adding and
> > removing filesystems - it has always the same device node.
>
> Yes, that part I knew.  ;-)
>
>
> [...]
>
> > --- a/gnu/system.scm
> > +++ b/gnu/system.scm
> > @@ -41,6 +41,7 @@
> >    #:use-module (gnu packages compression)
> >    #:use-module (gnu packages firmware)
> >    #:autoload   (gnu packages cryptsetup) (cryptsetup)
> > +  #:autoload   (gnu packages linux) (lvm2/static)
> >    #:use-module (gnu services)
> >    #:use-module (gnu services dmd)
> >    #:use-module (gnu services base)
> > @@ -86,7 +87,9 @@
> >              %base-packages
> >              %base-firmware
> >
> > -            luks-device-mapping))
> > +            luks-device-mapping
> > +            lvm-mapping
> > +            lvm-mapping-used?))
> >
> >  ;;; Commentary:
> >  ;;;
> > @@ -208,6 +211,27 @@ file."
> >     (open open-luks-device)
> >     (close close-luks-device)))
> >
> > +(define (logical-volume-group-activate source target)
> > +  #~(zero? (system* (string-append #$lvm2/static "/sbin/lvm.static")
> > +                         "vgchange" "--activate" "y" #$target)))
> > +
> > +(define (logical-volume-group-deactivate source target)
> > +  #~(zero? (system* (string-append #$lvm2/static "/sbin/lvm.static")
> > +                    "vgchange" "--activate" "n" #$target)))
> > +
> > +(define (lvm-mapping-used? devices)
> > +  (not
> > +   (null? (filter
> > +           (lambda (md)
> > +             (eq? (mapped-device-type md)
> > +                  lvm-mapping))
> > +           devices))))
> > +
> > +(define lvm-mapping
> > +  (mapped-device-kind
> > +   (open  logical-volume-group-activate)
> > +   (close logical-volume-group-deactivate)))
>
> This looks good to me!
>
> So I would declare
>
>  (mapped-device
>    (source "/dev/sda")
>    (target "volume_group_name-logical_volume_name")
>    (kind lvm-device-mapping))
>
> and that would give me
> /dev/mapper/volume_group_name-logical_volume_name, right?

Volume group can be on multiple block devices. For now I rely on autodetect
abilities of LVM.

So you would declare:

(mapped-device
 (source "") ; irrelevant for LVM
 (target "volume_group_name")
 (type lvm-mapping))

and that would give you
/dev/mapper/volume_group_name-some_volume
/dev/mapper/volume_group_name-other_volume
...

and more conveniently
/dev/volume_group_name/some_volume
/dev/volume_group_name/other_volume
...

> >  (define (other-file-system-services os)
> >    "Return file system services for the file systems of OS that are not marked
> >  as 'needed-for-boot'."
> > @@ -267,7 +291,10 @@ from the initrd."
> >          (file-systems (operating-system-file-systems os)))
> >     (filter (lambda (md)
> >               (let ((user (mapped-device-user md file-systems)))
> > -               (and user (file-system-needed-for-boot? user))))
> > +               (or
> > +                (and user (file-system-needed-for-boot? user))
> > +                (and (eq? (mapped-device-type md)
> > +                          lvm-mapping)))))
> >             devices)))
>
> I don’t think it’s necessary: if a ‘file-system’ object has
> "/dev/mapper/volume_group_name-logical_volume_name" has its ‘device’
> field, then this device mapping will automatically be recognized as
> needed-for-boot, won’t it?

Yes, you're right, this chunk shouldn't be needed at all. Good catch!

> > --- a/gnu/system/linux-initrd.scm
> > +++ b/gnu/system/linux-initrd.scm
> > @@ -25,6 +25,7 @@
> >                  #:select (%store-prefix))
> >    #:use-module ((guix derivations)
> >                  #:select (derivation->output-path))
> > +  #:use-module (gnu system)
> >    #:use-module (gnu packages cpio)
> >    #:use-module (gnu packages compression)
> >    #:use-module (gnu packages linux)
> > @@ -212,6 +213,9 @@ loaded at boot time in the order in which they appear."
> >                    file-systems)
> >              (list e2fsck/static)
> >              '())
> > +      ,@(if (lvm-mapping-used? mapped-devices)
> > +            (list lvm2/static)
> > +            '())
> >        ,@(if volatile-root?
> >              (list unionfs-fuse/static)
> >              '())))
> > @@ -241,7 +245,19 @@ loaded at boot time in the order in which they appear."
> >
> >           (boot-system #:mounts '#$(map file-system->spec file-systems)
> >                        #:pre-mount (lambda ()
> > -                                    (and address@hidden))
> > +                                    (and address@hidden
> > +                                         ;; If we activated any volume group, 
> > we
> > +                                         ;; need to ensure that device nodes 
> > are
> > +                                         ;; created.  Add code here to call it
> > +                                         ;; once for all activations.
> > +                                         #$(when (lvm-mapping-used? 
> > mapped-devices)
> > +                                             #~(zero?
> > +                                                (system* (string-append
> > +                                                          #$lvm2/static
> > +                                                          "/sbin/lvm.static")
> > +                                                         "vgscan"
> > +                                                         "--mknodes")))))
>
> So ‘lvm vgchange --activate y’ does not create /dev nodes?

Right.

> Would it be possible to change the command returned by
> ‘logical-volume-group-activate’ to somehow create the nodes?  That would
> be ideal.

There are two actions needed to be taken:
1] volume group activation
2] creation of nodes

This design choice does as many 1] as needed and 2] once in the end.

I could do always 1] and 2] for every volume group, but I didn't find it nice,
since previous 2] calls are useless only slowing down the process. Do you
really think I should change it?

Thanks for your review.

S_W

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