Subject: Re: [PATCH 1/2] mapped-devices: Allow target to be list of strings

From: Mikhail Tsykalov <[email-masked]>
Date: Sat, 6 Jun 2020 23:16:07 +0300

Hi,

Thanks for the report, guess that's what I get by assuming instead of 
testing.

> I’ve tried the patches, but `guix system reconfigure` fails(?) with
>
> 	building /gnu/store/cy6d2a4b8bcqcjiaz8bm367j7vbdrslc-upgrade-shepherd-services.scm.drv...
> 	guix system: error: exception caught while executing 'start' on service 'device-mapping-disk3-data':
> 	error: <>: unbound variable

Can you test with the attached patch, it seems like `cut` macros is 
undefined while shepherd service is expanded(?). I fixed it by replacing 
it with a plain lambda. I managed to mount a flash disk with LVM using 
this patch, so I think it should be working now.

> I’m also curious why sources is a list of VG’s as opposed to a list of device
> nodes (i.e. /dev/sdX), like the other mappings. Does `pvscan --cache` not work
> here?

I always thought that it wasn't possible to mount LVM by PV's, but 
looking at man, `pvscan --cache -aay` seems to do the thing. Still, I 
think that using VG's instead of PV's is better, since you must `pvscan` 
all PV's in a VG in order to activate it, so you'll still need to think 
in terms of VG's. This also can be a source of errors, for example 
adding a device to a VG will result in entire VG not being activated 
until the configuration is changed and the system is reconfigured. Also 
I think that LVM doesn't put that much importance on individual PV's as 
it does on VG's as a place where LV's live.

All of this goes out of the window if you use LVM on only one PV, so it 
may be possible to have a heuristic that will check if source is a 
device node as opposed to a VG name and perform appropriate actions, but 
I'm not sure if the resulting complexity is worth it.

Mikhail

>From e9a2b441190fe18ebc4f5a8c73707edab3459d58 Mon Sep 17 00:00:00 2001
From: Mikhail Tsykalov <[email-masked]>
Date: Sat, 9 May 2020 03:27:13 +0300
Subject: [PATCH 2/2] mapped-devices: Add 'lvm-device-mapping'

---
 gnu/system/mapped-devices.scm | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/gnu/system/mapped-devices.scm b/gnu/system/mapped-devices.scm
index 3339e509e0..641cddc146 100644
--- a/gnu/system/mapped-devices.scm
+++ b/gnu/system/mapped-devices.scm
@@ -34,7 +34,7 @@
   #:autoload   (gnu build linux-modules)
                  (missing-modules)
   #:autoload   (gnu packages cryptsetup) (cryptsetup-static)
-  #:autoload   (gnu packages linux) (mdadm-static)
+  #:autoload   (gnu packages linux) (mdadm-static lvm2-static)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-34)
@@ -59,7 +59,8 @@
             check-device-initrd-modules           ;XXX: needs a better place
 
             luks-device-mapping
-            raid-device-mapping))
+            raid-device-mapping
+            lvm-device-mapping))
 
 ;;; Commentary:
 ;;;
@@ -269,4 +270,26 @@ TARGET (e.g., \"/dev/md0\"), using 'mdadm'."
    (open open-raid-device)
    (close close-raid-device)))
 
+(define (open-lvm-device source target)
+  #~(begin
+       (use-modules (srfi srfi-1))
+       (and
+        (zero? (system* #$(file-append lvm2-static "/sbin/lvm")
+                        "vgchange" "--activate" "ay" #$source))
+        (zero? (system* #$(file-append lvm2-static "/sbin/lvm")
+                         "vgscan" "--mknodes")) ; make /dev/mapper nodes when in initrd
+        (every file-exists? (map (lambda (file) (string-append "/dev/mapper/" file))
+                                 (let ((t '#$target))
+                                   (if (list? t) t (list t))))))))
+
+
+(define (close-lvm-device source target)
+  #~(zero? (system* #$(file-append lvm2-static "/sbin/lvm")
+                    "vgchange" "--activate" "n" #$source)))
+
+(define lvm-device-mapping
+  (mapped-device-kind
+   (open open-lvm-device)
+   (close close-lvm-device)))
+
 ;;; mapped-devices.scm ends here
-- 
2.26.2