dannytbecker opened a new pull request, #7427:
URL: https://github.com/apache/hadoop/pull/7427
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### Description of PR
Currently EC Reads are less stable than replication reads because if 4 out
of 9 datanodes in the block group are busy, then the whole read fails. Erasure
Coding reads need to be able to handle ERROR_BUSY signals from DataNodes and
retry after a backoff duration to avoid overloading the DataNodes while
increasing the stability of the read.
Throttling on server side was another proposed solution, but we prefer this
client side backoff for a few main reasons (see
https://msasg.visualstudio.com/DefaultCollection/Multi%20Tenancy/_git/Hadoop/pullRequest/5272897#1739008224):
1. Throttling on the server would use up thread connections which have a
maximum limit.
2. Throttling was originally added only for cohosting scenario to reduce
impact on other services
3. Throttling would use up resources on the DataNode which is already busy.
#### What
The previous implementation followed a 4 phase algorithm to read.
1. Attempt to read chunks from the data blocks
2. Check for missing data chunks. Fail if there are more missing than the
number of parity blocks, otherwise read parity blocks and null data blocks
3. Wait for data to be read into the buffers and handle any read errors by
reading from more parity blocks
4. Check for missing blocks and either decode or fail.
The new implementation now merges phase 1-3 into a single loop:
1. Loop until we have enough blocks for read or decode, or we have too many
missing blocks to succeed
- Determine the number of chunks we need to fetch. ALLZERO chunks count
towards this total. null data chunks also count towards this total unless there
are missing data chunks.
- Read chunks until we have enough pending or fetched to be able to
decode or normal read.
faster.
- Get results from reads and handle exceptions by preparing more reads
for decoding the missing data
- Check if we should sleep before retrying any reads.
2. Check for missing blocks and either decode or fail.
Add two new states to StripingChunk:
- "SLEEPING" to indicate that the node where the chunk is stored has failed
and will be retried in the future
- "READY" to indicate that the node where the chunk is stored is ready to be
attempted
### How was this patch tested?
Add unit test to `TestWriteReadStripedFile`
- Covers RS(3,2) with 1 chunk on failed nodes, 2 chunks on failed nodes, and
3 chunks on failed nodes.
### For code changes:
- [x] Does the title or this PR starts with the corresponding JIRA issue id
(e.g. 'HADOOP-17799. Your PR title ...')?
- [ ] Object storage: have the integration tests been executed and the
endpoint declared according to the connector-specific documentation?
- [ ] If adding new dependencies to the code, are these dependencies
licensed in a way that is compatible for inclusion under [ASF
2.0](http://www.apache.org/legal/resolved.html#category-a)?
- [ ] If applicable, have you updated the `LICENSE`, `LICENSE-binary`,
`NOTICE-binary` files?
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