James Thomas created HDFS-6777:
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Summary: Supporting consistent edit log reads when in-progress
edit log segments are included
Key: HDFS-6777
URL: https://issues.apache.org/jira/browse/HDFS-6777
Project: Hadoop HDFS
Issue Type: Sub-task
Components: qjm
Reporter: James Thomas
Assignee: James Thomas
For inotify, we want to be able to read transactions from in-progress edit log
segments so we can serve transactions to listeners soon after they are
committed.
Inotify clients ask the active NN for transactions, and the NN opens
EditLogInputStreams that pull transactions from its various edit repositories
(local directories, individual JournalNodes, etc.). The goal is to send back
only successfully sync’ed transactions. What constitutes a successful sync
varies among the edit repositories. In the case of the JournalNodes, a
successful sync requires a write to a quorum of the JournalNodes. Properly
sync’ed transactions are always applied to the namesystem, and unsync’ed
transactions are never applied. So it is natural that listeners are only
interested in sync’ed transactions.
To achieve this goal, the NN returns to clients only transactions up to the
last txid it has sync'ed.
Some infrastructural changes are required for this policy to prevent any
unsync’ed transactions from reaching the client, and we tackle those in this
JIRA.
The first issue is that the current implementation of selectInputStreams
(inProgressOk = true, fromTxId = X) in FSEditLog (which the NN uses to read
transactions) essentially asks all available edit log repositories for all
in-progress or finalized log segments with transactions after and including X,
and combines log segments starting at the same transaction ID in a
RedundantEditLogInputStream. The problem here is that the
RedundantEditLogInputStream may combine finalized and in-progress segments
starting at the same txid, and may serve unsync’ed transactions from the
in-progress segments. So we simply discard in-progress segments if we have any
finalized segments starting at the same txid -- the modifcation is to
JournalSet.chainAndMakeRedundantStreams.
The second change we need to make is for the case where we only have
in-progress segments starting at a particular txid. In this case, we know we
are at the log segment the NN currently has open, since the various journals
ensure that if there are finalized segments available for a particular starting
txid, we are able to see them (e.g. reads from QJM require a quorum of
JournalNodes to respond, and a finalized segment must be present on at least
one JN in the quorum). Our goal in this case is to return only the in-progress
segments being written by the current NN. This happens trivially in the local
edits directory case and the NFS shared edits case, since there is only a
single “replica” and if the NN is writing to it, it is fully up-to-date. So we
focus on QJM here. The key is that JournalNodes maintain a lastWriterEpoch,
which increases monotonically as new writers arrive. So we keep track of the
lastWriterEpochs of the segments we receive from the JNs and discard any
segments with lastWriterEpochs less than the maximum one seen. We know that
when a new writer contacts the JournalNodes, it finalizes all sync’ed
transactions, so in-progress segments with out-of-date lastWriterEpochs may
contain unsync’ed transactions and any sync’ed transactions they contain will
also be contained in finalized segments on a quorum on JNs. So we can safely
discard them. The segments with the largest lastWriterEpochs may also contain
unsync’ed transactions from a previous writer if the current NN has not yet
sync’ed any transactions, but in this case we will make sure to read only from
finalized segments. The main modification for this change is to
QuorumJournalManager.selectInputStreams.
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