Re: lvm volume like support

[Alex Elsayed <eternaleye@gmail.com>]

Roman Mamedov wrote:

> On Wed, 27 Feb 2013 13:23:23 +1100
> "Fajar A. Nugraha" <list@fajar.net> wrote:
> 
>> Not to mention the hassle in accessing the data if it resides on a
>> partition inside the file (e.g. you need losetup + kpartx to access it,
>> and you must remember to do the reverse when you're finished with it).
> 
>> 
>> In zfsonlinux it's very easy to do so since a zvol is treated pretty
>> much like a disk, and whenever there's a partition inside a zvol, a
>> coressponding device noed is also created automatically.
> 
> So I'd say what you (we) need is a generic Linux kernel framework that
> would allow treating any regular file pretty much like a disk. Not some
> filesystem-specific block device emulation kludge.
> 
> Btw some years ago there was a patchset adding proper automatic partition
> support to 'loop'; but it seems like that went nowhere, and I have no idea
> why something this useful did not end up being added into the mainline
> kernel.
> 

I mentioned it in another branch of the thread, but the SCSI Target stack 
*does* this. If you use the FILEIO backend and the 'loopback' transport, you 
get a file that is visible to the local system as Just Another SCSI Disk. 
There are patches on the scsi target list to add UNMAP and WRITE SAME 
w/UNMAP=1 support to the FILEIO backend, at which point it will do 
FL_PUNCH_HOLE operations appropriately.

The commands are:

---cut---

# $IDX = a preferably-unique index
# $NAME = a useful name for humans
# $IMGFILE = a disk image
# $SIZE = the size of $IMGFILE in bytes
# $UUID = a random v4 UUID
# ${NAA[x]} = a valid naa WWN, targetcli generates this as
#     "naa.6001405" + the first 10 digits of a random v4 UUID
#     (not the same uuid as $UUID). When x varies, it's a different
#     WWN, since more than one gets used.

# Load the scsi target core and backends
modprobe target_core_mod

# Tell it where the file is and how big it is
tcm_node --establishdev "fileio_$IDX/$NAME" \
    "fd_dev_name=$IMGFILE,fd_dev_size=$SIZE"

# Give it a unique serial
tcm_node --setunitserialwithmd fileio_$IDX/$NAME $UUID

# Load the local scsi frontend transport
modprobe tcm_loop
mkdir -p /sys/kernel/config/target/loopback

# Some setup so you have a place to put LUNs
mkdir -p /sys/kernel/config/target/loopback/${NAA[1]}/tpgt_1
echo ${NAA[2]} > /sys/kernel/config/target/loopback/${NAA[1]}/tpgt_1/nexus

# Create a fresh LUN...
mkdir -p /sys/kernel/config/target/loopback/${NAA[1]}/tpgt_1/lun/lun_$IDX

# ...and map the file to it.
ln -s /sys/kernel/config/target/core/fileio_$IDX/$NAME \
    /sys/kernel/config/target/loopback/${NAA[1]}/tpgt_1/lun/lun_$IDX

---cut---

This could be pretty easily put into a shell script that uses du -b and 
manually pokes configfs instead of calling tcm_node, and it'd be able to run 
without any nonstandard userspace dependencies.