Release Notes For Plasma
This is version: 0.6 "Rechenknecht". This is a beta release intended
for broader testing.
Changed in 0.6
New features in PlasmaFS:
Implementation improvements in PlasmaFS:
New features in the map/reduce framework:
- New "toolkit" (see
Plasmamr_toolkit). The toolkit is
an abstract layer on top of the job execution engine, and allows it to
formulate map/reduce jobs in a functional way, and to compose
complex jobs from elementary ones.
- Concept of an abstract filesystem. This allows it to run
map/reduce on other filesystems than just PlasmaFS. In this
release, there is support for local filesystems. Especially for
trying map/reduce out, it is now possible to run it without having
a PlasmaFS deployment, although only on a single computer.
The abstract filesystem is defined in
- New fixed-size and variable-size file formats. These are binary
formats, and the records can include any data. This overcomes
the limitation that records cannot include LF bytes when LF is
used as line terminator. There is extensive documentation in
- The sorting criterion can be defined by the user (new
- Support for combiners (new
- Support for map-only jobs, and "map+sort but no reduce" jobs
- New module
Mapred_fields for easy parsing of fields.
- The RPC channel from the job control program to the task servers
is now privacy-protected
- The map/reduce jobs can now collect statistics ("counters")
Implementation improvements in the map/reduce framework:
- The scheduling algorithm includes a number of performance optimizations.
There is now support for "greedy" execution, trying to maximize the number
of runnable tasks for best parallelization.
- The sort and emap tasks write the output files directly to the nodes
where the follow-up tasks will be executed. This way, CPU and network
utilization is better interleaved.
- The record reader (in
Mapred_io) can now read the following blocks
in a separate kernel thread while the current blocks are still being
processed, resulting in better resource utilization.
- The command
start_task_servers automatically kills the still running
old instance of the task server
- Optimized and unified implementation of sorting
- Existing PlasmaFS filesystems for Plasma-0.5 are compatible, and can
- The PlasmaFS protocol is exactly the same as before.
- There are incompatible protocol changes in the map/reduce protocol,
What is working and not working in PlasmaFS
Generally, PlasmaFS works as described in the documentation. Crashes
have not been observed for quite some time now, but occasionally one
might see critical exceptions in the log file.
PlasmaFS has so far only been tested on 64 bit, and only on Linux
as operation system. There are known issues for 32 bit machines,
especially the blocksize must not be larger than 4M, and certain
buffers are restricted to 16M.
Data safety: Cannot be guaranteed. It is not suggested to put valuable
data into PlasmaFS.
Not implemented features:
- There are too many hard-coded constants.
- The file name read/lookup functions should never return
errors. (This has been improved in 0.2, though.)
- Support for checksums
- Support for "host groups", so that it is easier to control which machines
may store which blocks. Semantics have to be specified yet.
- Define how blocks are handled that are allocated but never written.
- Recognition of the death of the coordinator, and restart of the
- Lock manager (avoid that clients have to busy wait on locks)
- Restoration of missing replicas
- Rebalancing of the cluster
- Automated copying of the namenode database to freshly added namenode slaves
- No IPv6 support yet.
What is working and not working in Plasma MapReduce
Generally, Plasma MapReduce works as described in the documentation.
Not implemented features:
- Dynamically extensible task servers
- nice web interface
- restart/relocation of failed tasks
- recompute intermediate files that are no longer accessible due to node
- Speculative execution of tasks
- Support job management (remember which jobs have been run etc.)
What we will never implement:
- Jobs only consisting of
reduce but no
map cannot be supported
due to the task scheme. (Reason: Input files for sort tasks must
sort_limit.) Workaround: Use the identity as