add documentation on imagestore and add a warning if set

docs/containers-storage.conf.5.md

@@ -56,10 +56,13 @@ $ restorecon -R -v /NEWSTORAGEPATH
 A common use case for this field is to provide a local storage directory when user home directories are NFS-mounted (podman does not support container storage over NFS).
 
 **imagestore**=""
+  image storage path (default is assumed to be same as `graphroot`).
   Path of imagestore different from `graphroot`, by default storage library stores all images in `graphroot` but if `imagestore` is provided it will store newly pulled images in provided `imagestore` but will keep using `graphroot` for everything else. If user is using `overlay` driver then images which were already part of `graphroot` will still be accessible ( Internally storage library will mount `graphroot` as an `additionalImageStore` to allow this behaviour ).
 
 A common use case for this field is for the users who want to split the file-system in different parts i.e disk which stores images vs disk used by the container created by the image.
 
+Imagestore must be different from `graphroot`.
+
 **runroot**=""
   container storage run dir (default: "/run/containers/storage")
 Default directory to store all temporary writable content created by container storage programs. The rootless runroot path supports environment variable substitutions (ie. `$HOME/containers/storage`)

storage.conf

@@ -27,6 +27,11 @@ runroot = "/run/containers/storage"
 # restorecon -R -v /NEWSTORAGEPATH
 graphroot = "/var/lib/containers/storage"
 
+# Optional value for image storage location
+# If set must be different than graphroot.
+
+# imagestore = 
+
 
 # Storage path for rootless users
 #

storage.conf-freebsd

@@ -22,6 +22,10 @@ runroot = "/var/run/containers/storage"
 # Primary Read/Write location of container storage
 graphroot = "/var/db/containers/storage"
 
+# Optional value for image storage location
+# If set must be different than graphroot.
+
+# imagestore = 
 
 # Storage path for rootless users
 #

types/options.go

@@ -179,6 +179,10 @@ func defaultStoreOptionsIsolated(rootless bool, rootlessUID int, storageConf str
 		storageOpts.RootlessStoragePath = storagePath
 	}
 
+	if storageOpts.ImageStore != "" && storageOpts.ImageStore == storageOpts.GraphRoot {
+		return storageOpts, fmt.Errorf("imagestore if set must be a different location than graphroot")
+	}
+
 	return storageOpts, nil
 }