Tiering and Unified Namespace¶
The DAOS tier can be tightly integrated with the Lustre parallel filesystem, in which DAOS containers will be represented through the Lustre namespace.
The current state of work can be summarized as follows :
DAOS integration with Lustre uses the Lustre foreign file/dir feature (from LU-11376 and associated patches).
Each time a DAOS POSIX container is created using the
daosutility and its '--path' UNS option, a Lustre foreign file/dir of 'symlink' type is created with a specific LOV/LMV EA content that will allow the DAOS pool and containers UUIDs to be stored.
The Lustre Client patch for LU-12682 adds DAOS specific support to the Lustre foreign file/dir feature. It allows for the foreign file/dir of
symlinktype to be presented and act as an
<absolute-prefix>/<pool-uuid>/<container-uuid>symlink to the Linux Kernel/VFS.
<absolute-prefix>can be specified as the new
foreign_symlink=<absolute-prefix>Lustre Client mount option, or also through the new
llite.*.foreign_symlink_prefixLustre dynamic tuneable. Both
<container-uuid>are extracted from foreign file/dir LOV/LMV EA.
To allow for symlink resolution and transparent access to the DAOS container content, it is expected that a DFuse/DFS instance/mount of DAOS Server root exists on
<absolute-prefix>, presenting all served pools/containers as
daosforeign support is enabled at mount time with the
symlink=option present or dynamically, through the
Building and using a DAOS-aware Lustre version¶
As indicated before, a Lustre Client patch (for LU-12682) has been developed to allow for the application's transparent access to the DAOS container's data from a Lustre foreign file/dir.
This patch can be found at https://review.whamcloud.com/35856 and has been landed onto master but is still not integrated with an official Lustre version. This patch must be applied on top of the selected Lustre version's source tree.
After any conflicts are resolved, Lustre must be built and the generated RPMs installed on client nodes by following the instructions at https://wiki.whamcloud.com/display/PUB/Building+Lustre+from+Source.
The Lustre client mount command must use the new
foreign_symlink=<absolute_path> option to set the prefix to be used in
front of the
<pool-UUID>/<cont-UUID> relative path, based on pool/container
information being extracted from the LOV/LMV foreign symlink EAs. This can
be configured by dynamically modifying both
parameters for each Lustre client mount, using the
lctl set_param llite/*/foreign_symlink_[enable,prefix]=[0|1,<path>] command.
The Dfuse instance will then use this prefix to mount/expose all
DAOS pools, or use
<prefix>/<pool-UUID>[/<cont-UUID>] to mount a
To allow non-root/admin users to use the llapi_set_dirstripe()
API (like the
daos cont create command with
--path option), or the
lfs setdirstripe command, the Lustre MDS servers configuration must
be modified accordingly by running the
lctl set_param mdt/*/enable_remote_dir_gid=-1 command.
Additionally, there is a feature available to provide a customized format
of LOV/LMV EAs, apart from the default
<pool-UUID>/<cont-UUID>, through the
llite/*/foreign_symlink_upcall tunable. This provides the path
of a user-land upcall, that will indicate where to extract
<cont-UUID> in the LOV/LMV EAs, using a series of [pos, len]
tuples and constant strings.
lustre/utils/l_foreign_symlink.c is a helper
example in the Lustre source code.
Migration to/from a POSIX filesystem¶
A dataset mover tool is under development to move a snapshot of a DAOS POSIX container or DAOS HDF5 container to a POSIX filesystem and vice versa. The copy will be performed at the POSIX or HDF5 level. (The MPI-IO ROMIO ADIO driver for DAOS also uses DAOS POSIX containers.) For DAOS HDF5 containers, the resulting HDF5 file in the POSIX filesystem will be accessible through the native HDF5 connector with the POSIX VFD.
The POSIX data mover was released with DAOS v1.2 and supports data migration to/from a POSIX filesystem. Parallel data migration is available through mpiFileUtils, which contains a DAOS backend. Serial data migration is supported through the daos filesystem copy utility. The first version of the data mover tool that contains support for HDF5 containers is scheduled for release in DAOS v2.2.
The mover tool supports the ability to serialize and deserialize a DAOS container to a set of POSIX files that can be stored or “parked” in an external POSIX filesystem. This transformation is agnostic to the data model and container type and retains most DAOS internal metadata. The serialized file(s) are written to a POSIX filesystem in an HDF5 file format. A preview of the serialization and deserialization tools is available in DAOS v2.0 through mpiFileUtils, and they will be officially released in DAOS v2.2.
More details and instructions on data mover usage can be found at: https://github.com/daos-stack/daos/blob/master/docs/user/datamover.md