lvcreate: new cache or writecache lv with single command

To create a new cache or writecache LV with a single command:

lvcreate --type cache|writecache
    -n Name -L Size --cachedevice PVfast VG [PVslow ...]

- A new main linear|striped LV is created as usual, using the
  specified -n Name and -L Size, and using the optionally
  specified PVslow devices.
- Then, a new cachevol LV is created internally, using PVfast
  specified by the cachedevice option.
- Then, the cachevol is attached to the main LV, converting the
  main LV to type cache|writecache.

Include --cachesize Size to specify the size of cache|writecache
to create from the specified --cachedevice PVs, otherwise the
entire cachedevice PV is used.  The --cachedevice option can be
repeated to create the cache from multiple devices, or the
cachedevice option can contain a tag name specifying a set of PVs
to allocate the cache from.

To create a new cache or writecache LV with a single command
using an existing cachevol LV:

lvcreate --type cache|writecache
    -n Name -L Size --cachevol LVfast VG [PVslow ...]

- A new main linear|striped LV is created as usual, using the
  specified -n Name and -L Size, and using the optionally
  specified PVslow devices.
- Then, the cachevol LVfast is attached to the main LV, converting
  the main LV to type cache|writecache.

In cases where more advanced types (for the main LV or cachevol LV)
are needed, they should be created independently and then combined
with lvconvert.

Example
-------

user creates a new VG with one slow device and one fast device:

$ vgcreate vg /dev/slow1 /dev/fast1

user creates a new 8G main LV on /dev/slow1 that uses all of
/dev/fast1 as a writecache:

$ lvcreate --type writecache --cachedevice /dev/fast1
    -n main -L 8G vg /dev/slow1

Example
-------

user creates a new VG with two slow devs and two fast devs:

$ vgcreate vg /dev/slow1 /dev/slow2 /dev/fast1 /dev/fast2

user creates a new 8G main LV on /dev/slow1 and /dev/slow2
that uses all of /dev/fast1 and /dev/fast2 as a writecache:

$ lvcreate --type writecache --cachedevice /dev/fast1 --cachedevice /dev/fast2
    -n main -L 8G vg /dev/slow1 /dev/slow2

Example
-------

A user has several slow devices and several fast devices in their VG,
the slow devs have tag @slow, the fast devs have tag @fast.

user creates a new 8G main LV on the slow devs with a
2G writecache on the fast devs:

$ lvcreate --type writecache -n main -L 8G
    --cachedevice @fast --cachesize 2G vg @slow

1 files changed, 1 insertions, 1 deletions

diff --git a/tools/args.h b/tools/args.h
index c69a0ecce..3a7e5d488 100644
--- a/tools/args.h
+++ b/tools/args.h
@@ -126,7 +126,7 @@ arg(cachepool_ARG, '\0', "cachepool", lv_VAL, 0, 0,
 arg(cachevol_ARG, '\0', "cachevol", lv_VAL, 0, 0,
     "The name of a cache volume.\n")
 
-arg(cachedevice_ARG, '\0', "cachedevice", pv_VAL, 0, 0,
+arg(cachedevice_ARG, '\0', "cachedevice", pv_VAL, ARG_GROUPABLE, 0,
     "The name of a device to use for a cache.\n")
 
 arg(cachesize_ARG, '\0', "cachesize", sizemb_VAL, 0, 0,