openstack/swift
Code Issues Proposed changes

Clean up EC overview docs a bit

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Change-Id: I3bab2c015c63f32dcd6e4beefbcd0fcf22e91eec
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Tim Burke 2017-01-30 23:30:35 +00:00
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doc/source/overview_erasure_code.rst
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@ -601,7 +601,7 @@ The Reconstructor
The Reconstructor performs analogous functions to the replicator:
#. Recovery from disk drive failure.
#. Recovering from disk drive failure.
#. Moving data around because of a rebalance.
#. Reverting data back to a primary from a handoff.
#. Recovering fragment archives from bit rot discovered by the auditor.
@ -612,14 +612,14 @@ of the key elements in understanding how the reconstructor operates.
Unlike the replicator, the work that the reconstructor does is not always as
easy to break down into the 2 basic tasks of synchronize or revert (move data
from handoff back to primary) because of the fact that one storage node can
house fragment archives of various indexes and each index really /"belongs/" to
house fragment archives of various indexes and each index really \"belongs\" to
a different node. So, whereas when the replicator is reverting data from a
handoff it has just one node to send its data to, the reconstructor can have
several. Additionally, its not always the case that the processing of a
particular suffix directory means one or the other for the entire directory (as
it does for replication). The scenarios that create these mixed situations can
be pretty complex so we will just focus on what the reconstructor does here and
not a detailed explanation of why.
several. Additionally, it is not always the case that the processing of a
particular suffix directory means one or the other job type for the entire
directory (as it does for replication). The scenarios that create these mixed
situations can be pretty complex so we will just focus on what the
reconstructor does here and not a detailed explanation of why.
Job Construction and Processing
===============================
@ -627,29 +627,31 @@ Job Construction and Processing
Because of the nature of the work it has to do as described above, the
reconstructor builds jobs for a single job processor. The job itself contains
all of the information needed for the processor to execute the job which may be
a synchronization or a data reversion and there may be a mix of jobs that
a synchronization or a data reversion. There may be a mix of jobs that
perform both of these operations on the same suffix directory.
Jobs are constructed on a per partition basis and then per fragment index basis.
Jobs are constructed on a per-partition basis and then per-fragment-index basis.
That is, there will be one job for every fragment index in a partition.
Performing this construction \"up front\" like this helps minimize the
interaction between nodes collecting hashes.pkl information.
Once a set of jobs for a partition has been constructed, those jobs are sent off
to threads for execution. The single job processor then performs the necessary
actions working closely with ssync to carry out its instructions. For data
actions, working closely with ssync to carry out its instructions. For data
reversion, the actual objects themselves are cleaned up via the ssync module and
once that partition's set of jobs is complete, the reconstructor will attempt to
remove the relevant directory structures.
The scenarios that job construction has to take into account include:
Job construction must account for a variety of scenarios, including:
#. A partition directory with all fragment indexes matching the local node
index. This is the case where everything is where it belongs and we just
need to compare hashes and sync if needed, here we sync with our partners.
#. A partition directory with one local fragment index and mix of others. Here
we need to sync with our partners where fragment indexes matches the
local_id, all others are sync'd with their home nodes and then deleted.
need to compare hashes and sync if needed. Here we simply sync with our
partners.
#. A partition directory with at least one local fragment index and mix of
others. Here we need to sync with our partners where fragment indexes
matches the local_id, all others are sync'd with their home nodes and then
deleted.
#. A partition directory with no local fragment index and just one or more of
others. Here we sync with just the home nodes for the fragment indexes that
we have and then all the local archives are deleted. This is the basic
@ -688,13 +690,13 @@ basic reconstruction which, at a high level, looks like this:
* Update the etag and fragment index metadata elements of the newly constructed
fragment archive.
* Establish a connection to the target nodes and give ssync a DiskFileLike class
that it can stream data from.
from which it can stream data.
The reader in this class gathers fragments from the nodes and uses PyECLib to
reconstruct each segment before yielding data back to ssync. Essentially what
this means is that data is buffered, in memory, on a per segment basis at the
node performing reconstruction and each segment is dynamically reconstructed and
delivered to `ssync_sender` where the `send_put()` method will ship them on
delivered to ``ssync_sender`` where the ``send_put()`` method will ship them on
over. The sender is then responsible for deleting the objects as they are sent
in the case of data reversion.