Current BD xlator (block backend) has a few limitations such as
* Creation of directories not supported
* Supports only single brick
* Does not use extended attributes (and client gfid) like posix xlator
* Creation of special files (symbolic links, device nodes etc) not
supported
Basic limitation of not allowing directory creation is blocking
oVirt/VDSM to consume BD xlator as part of Gluster domain since VDSM
creates multi-level directories when GlusterFS is used as storage
backend for storing VM images.
To overcome these limitations a new BD xlator with following
improvements is suggested.
* New hybrid BD xlator that handles both regular files and block device
files
* The volume will have both POSIX and BD bricks. Regular files are
created on POSIX bricks, block devices are created on the BD brick (VG)
* BD xlator leverages exiting POSIX xlator for most POSIX calls and
hence sits above the POSIX xlator
* Block device file is differentiated from regular file by an extended
attribute
* The xattr 'user.glusterfs.bd' (BD_XATTR) plays a role in mapping a
posix file to Logical Volume (LV).
* When a client sends a request to set BD_XATTR on a posix file, a new
LV is created and mapped to posix file. So every block device will
have a representative file in POSIX brick with 'user.glusterfs.bd'
(BD_XATTR) set.
* Here after all operations on this file results in LV related
operations.
For example opening a file that has BD_XATTR set results in opening
the LV block device, reading results in reading the corresponding LV
block device.
When BD xlator gets request to set BD_XATTR via setxattr call, it
creates a LV and information about this LV is placed in the xattr of the
posix file. xattr "user.glusterfs.bd" used to identify that posix file
is mapped to BD.
Usage:
Server side:
[root@host1 ~]# gluster volume create bdvol host1:/storage/vg1_info?vg1 host2:/storage/vg2_info?vg2
It creates a distributed gluster volume 'bdvol' with Volume Group vg1
using posix brick /storage/vg1_info in host1 and Volume Group vg2 using
/storage/vg2_info in host2.
[root@host1 ~]# gluster volume start bdvol
Client side:
[root@node ~]# mount -t glusterfs host1:/bdvol /media
[root@node ~]# touch /media/posix
It creates regular posix file 'posix' in either host1:/vg1 or host2:/vg2 brick
[root@node ~]# mkdir /media/image
[root@node ~]# touch /media/image/lv1
It also creates regular posix file 'lv1' in either host1:/vg1 or
host2:/vg2 brick
[root@node ~]# setfattr -n "user.glusterfs.bd" -v "lv" /media/image/lv1
[root@node ~]#
Above setxattr results in creating a new LV in corresponding brick's VG
and it sets 'user.glusterfs.bd' with value 'lv:<default-extent-size'
[root@node ~]# truncate -s5G /media/image/lv1
It results in resizig LV 'lv1'to 5G
New BD xlator code is placed in xlators/storage/bd directory.
Also add volume-uuid to the VG so that same VG can't be used for other
bricks/volumes. After deleting a gluster volume, one has to manually
remove the associated tag using vgchange <vg-name> --deltag
<trusted.glusterfs.volume-id:<volume-id>>
Changes from previous version V5:
* Removed support for delayed deleting of LVs
Changes from previous version V4:
* Consolidated the patches
* Removed usage of BD_XATTR_SIZE and consolidated it in BD_XATTR.
Changes from previous version V3:
* Added support in FUSE to support full/linked clone
* Added support to merge snapshots and provide information about origin
* bd_map xlator removed
* iatt structure used in inode_ctx. iatt is cached and updated during
fsync/flush
* aio support
* Type and capabilities of volume are exported through getxattr
Changes from version 2:
* Used inode_context for caching BD size and to check if loc/fd is BD or
not.
* Added GlusterFS server offloaded copy and snapshot through setfattr
FOP. As part of this libgfapi is modified.
* BD xlator supports stripe
* During unlinking if a LV file is already opened, its added to delete
list and bd_del_thread tries to delete from this list when a last
reference to that file is closed.
Changes from previous version:
* gfid is used as name of LV
* ? is used to specify VG name for creating BD volume in volume
create, add-brick. gluster volume create volname host:/path?vg
* open-behind issue is fixed
* A replicate brick can be added dynamically and LVs from source brick
are replicated to destination brick
* A distribute brick can be added dynamically and rebalance operation
distributes existing LVs/files to the new brick
* Thin provisioning support added.
* bd_map xlator support retained
* setfattr -n user.glusterfs.bd -v "lv" creates a regular LV and
setfattr -n user.glusterfs.bd -v "thin" creates thin LV
* Capability and backend information added to gluster volume info (and
--xml) so
that management tools can exploit BD xlator.
* tracing support for bd xlator added
TODO:
* Add support to display snapshots for a given LV
* Display posix filename for list-origin instead of gfid
Change-Id: I00d32dfbab3b7c806e0841515c86c3aa519332f2
BUG: 1028672
Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Reviewed-on: http://review.gluster.org/4809
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>