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Mnesia currently issues an event whenever it detects an overload
condition. It recognizes two different types of overload:
- whenever the message queue of mnesia_tm process grows large
- when a log dump interval triggers before the previous dump is done
These events could be used to trigger a load regulation mechanism
to reduce the load until the condition seizes. The missing piece
is that there is no facility to ask mnesia whether the overload
condition still exists.
This patch implements a couple of functions in mnesia_lib that
can be used to sample the overload status. It has been tested
in a load regulator component being developed by Erlang Solutions.
No mnesia test suites have been run, since they are not available.
No documentation has been updated. The functions in mnesia_lib
are at any rate undocumented (as are all other functions in that
module). A decision would have to be made about whether to provide
a documented API on top of these functions.
The internal state record of mnesia_recover has been modified.
For this reason, a code change hook has been provided.
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* bd/mnesia-activity-subscription:
Add mnesia activity subscription message
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A process that calls mnesia:subscribe(activity) will receive the message:
{mnesia_activity_event, ActivityID, complete}
when any activity that caused a change to a database has finished
committing its changes. This allows a subscriber to collect messages
already available through the mnesia:subscribe({table, ...}) system
to group them as completed transactions.
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invoking mnesia:sync_transaction/[1,2]. Thanks Igor Ribeiro
Sucupira.
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* is/mnesia-send-compressed:
Add option to compress data when copying tables between Mnesia nodes
OTP-8406 Igor Ribeiro Sucupira added the option to compress data when
copying tables between Mnesia nodes.
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Optionally using data compression for copying Mnesia tables allows
the system to be tuned to eliminate network bottlenecks from
deployments where enough CPU is available to use zlib.
With this patch, running erl -mnesia send_compressed <level>
will enable compression for sending tables between nodes. The
compression level can be any integer in [0, 9], with 0 (the
default) meaning no compression (exactly the previous behaviour)
and 9 being the highest compression level.
To set any non-zero compression level at the sender, both nodes
must have the updated Mnesia modules (the receiver will work
according to the sender's configuration).
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