On 09/07/2012 08:21 AM, M. Mohan Kumar wrote:
On Thu, 6 Sep 2012 18:59:19 -0400 (EDT), Ayal Baron <aba...@redhat.com> wrote:

----- Original Message -----
----- Original Message -----
From: "M. Mohan Kumar" <mo...@in.ibm.com>
To: vdsm-devel@lists.fedorahosted.org
Sent: Wednesday, July 25, 2012 1:26:15 PM
Subject: [vdsm] [RFC] GlusterFS domain specific changes

We are developing a GlusterFS server translator to export block
as regular files to the client. Using block devices to serve VM
gives performance improvements, since it avoids some file system
bottlenecks in the host kernel. Goal is to use one block device(ie
at the client side) per VM image and feed this file to QEMU to get
performance improvements. QEMU will talk to glusterfs server
using libgfapi.

Currently we support only exporting Volume groups and Logical
Volumes. Logical volumes are exported as regular files to the

Are you actually using LVM behind the scenes?
If so, why bother with exposing the LVs as files and not raw block devices?


The idea is to provide a FS interface for managing block devices. One
can mount the Block Device Gluster Volume and create a LV and size it
just by
  $ touch lv1
  $ truncate -s5G lv1

And other file commands can be used to clone LVs, snapshot LVs
  $ ln lv1 lv2 # clones
  $ ln -s lv1 lv1.sn # creates snapshot

By enabling this feature GlusterFS can directly export storage in
SAN. We are planning to add feature to export LUNs also as regular files
in future.

In GlusterFS terminology a volume capable of exporting block
devices is
created by specifying the 'Volume Group' (ie VG in Logical Volume
management). Block Device translator(BD xlator) exports this volume
group as a directory and LVs under it as regular files. In the
mount point creating a file results in creating a logical volume,
removing a file results in removing logical volume etc.

When a GlusterFS volume enabled with BD xlator is used, directory
creation in that gluster mount path is not supported because
maps to Volume groups in BD xlator. But it could be an issue in
environment when a new VDSM volume is created for GlusterFS domain,
mounts the storage domain and creates directories under that and
files for vm image and other uses (like meta data).

Is it possible to modify this behavior in VDSM to use flat
instead of creating directories and VM images and other files
it? ie for GlusterFS domain with BD xlator VDSM will not create any
directory and only creates all required files under the mount point
directory itself.

 From your description I think that the GlusterFS for block devices is
actually more similar to what happens with the regular block domains.
You should probably need to mount the share somewhere in the system
then use symlinks to point to the volumes.

Create a regular block domain and look inside
you'll probably get the idea of what I mean.

That said we'd need to come up with a way of extending the LVs on the
gluster server when required (for thin provisioning).

Why? if it's exposed as a file that probably means it supports sparseness.  
i.e. if this becomes a new type of block domain it should only support 
'preallocated' images.

For start using the LVs we will always do truncate for the required
size, it will resize the LV. I didn't get what you are mentioning about
thin-provisioning, but I have a dumb code using dm-thin targets showing
BD xlators can be extended to use dm-thin targets for thin-provisioning.

so even though this is block storage, it will be extended as needed? how does that work exactly?
say i have a VM with a 100GB disk.
thin provisioning means we only allocated 1GB to it, then as the guest uses that storage, we allocate more as needed (lvextend, pause guest, lvrefresh, resume guest)

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