The ZIO pipeline has been modified to allow for external,
alternative implementations of existing operations to be
used. The original ZFS functions remain in the code as
fallback in case the external implementation fails.
Definitions:
Accelerator - an entity (usually hardware) that is
intended to accelerate operations
Offloader - synonym of accelerator; used interchangeably
Data Processing Unit Services Module (DPUSM)
- https://github.com/hpc/dpusm
- defines a "provider API" for accelerator
vendors to set up
- defines a "user API" for accelerator consumers
to call
- maintains list of providers and coordinates
interactions between providers and consumers.
Provider - a DPUSM wrapper for an accelerator's API
Offload - moving data from ZFS/memory to the accelerator
Onload - the opposite of offload
In order for Z.I.A. to be extensible, it does not directly
communicate with a fixed accelerator. Rather, Z.I.A. acquires
a handle to a DPUSM, which is then used to acquire handles
to providers.
Using ZFS with Z.I.A.:
1. Build and start the DPUSM
2. Implement, build, and register a provider with the DPUSM
3. Reconfigure ZFS with '--with-zia=<DPUSM root>'
4. Rebuild and start ZFS
5. Create a zpool
6. Select the provider
zpool set zia_provider=<provider name> <zpool>
7. Select operations to offload
zpool set zia_<property>=on <zpool>
The operations that have been modified are:
- compression
- non-raw-writes only
- decompression
- checksum
- not handling embedded checksums
- checksum compute and checksum error call the same function
- raidz
- generation
- reconstruction
- vdev_file
- open
- write
- close
- vdev_disk
- open
- invalidate
- write
- flush
- close
Successful operations do not bring data back into memory after
they complete, allowing for subsequent offloader operations
reuse the data. This results in only one data movement per ZIO
at the beginning of a pipeline that is necessary for getting
data from ZFS to the accelerator.
When errors ocurr and the offloaded data is still accessible,
the offloaded data will be onloaded (or dropped if it still
matches the in-memory copy) for that ZIO pipeline stage and
processed with ZFS. This will cause thrashing if a later
operation offloads data. This should not happen often, as
constant errors (resulting in data movement) is not expected
to be the norm.
Unrecoverable errors such as hardware failures will trigger
pipeline restarts (if necessary) in order to complete the
original ZIO using the software path.
The modifications to ZFS can be thought of as two sets of changes:
- The ZIO write pipeline
- compression, checksum, RAIDZ generation, and write
- Each stage starts by offloading data that was not
previously offloaded
- This allows for ZIOs to be offloaded at any point
in the pipeline
- Resilver
- vdev_raidz_io_done (RAIDZ reconstruction, checksum, and
RAIDZ generation), and write
- Because the core of resilver is vdev_raidz_io_done, data
is only offloaded once at the beginning of
vdev_raidz_io_done
- Errors cause data to be onloaded, but will not
re-offload in subsequent steps within resilver
- Write is a separate ZIO pipeline stage, so it will
attempt to offload data
The zio_decompress function has been modified to allow for
offloading but the ZIO read pipeline as a whole has not, so it
is not part of the above list.
An example provider implementation can be found in
module/zia-software-provider
- The provider's "hardware" is actually software - data is
"offloaded" to memory not owned by ZFS
- Calls ZFS functions in order to not reimplement operations
- Has kernel module parameters that can be used to trigger
ZIA_ACCELERATOR_DOWN states for testing pipeline restarts.
abd_t, raidz_row_t, and vdev_t have each been given an additional
"void *<prefix>_zia_handle" member. These opaque handles point to
data that is located on an offloader. abds are still allocated,
but their payloads are expected to diverge from the offloaded copy
as operations are run.
Encryption and deduplication are disabled for zpools with Z.I.A.
operations enabled
Aggregation is disabled for offloaded abds
RPMs will build with Z.I.A.
Signed-off-by: Jason Lee <jasonlee@lanl.gov>
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