fix ReadaheadRandomAccessFile/iterator prefetch bug #3454
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this is quite a good case for improving |
sagar0
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Summary: `ReadaheadRandomAccessFile` is used by iterators for file reads in several cases, like in compaction when `compaction_readahead_size > 0` or `use_direct_io_for_flush_and_compaction == true`, or in user iterator when `ReadOptions::readahead_size > 0`. `ReadaheadRandomAccessFile` maintains an internal buffer for readahead data. It assumes that, if the buffer's length is less than `ReadaheadRandomAccessFile::readahead_size_`, which is fixed in the constructor, then EOF has been reached so it doesn't try reading further. Recently, d938226 started calling `RandomAccessFile::Prefetch` with various lengths: 8KB, 16KB, etc. When the `RandomAccessFile` is a `ReadaheadRandomAccessFile`, it triggers the above condition and incorrectly determines EOF. If a block is partially in the readahead buffer and EOF is incorrectly decided, the result is a truncated data block. The problem is reproducible: ``` TEST_TMPDIR=/data/compaction_bench ./db_bench -benchmarks=fillrandom -write_buffer_size=1048576 -target_file_size_base=1048576 -block_size=18384 -use_direct_io_for_flush_and_compaction=true ... put error: Corruption: truncated block read from /data/compaction_bench/dbbench/000014.sst offset 20245, expected 10143 bytes, got 8427 ``` Closes #3454 Differential Revision: D6869405 Pulled By: ajkr fbshipit-source-id: 87001c299e7600a37c0dcccbd0368e0954c929cf
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ReadaheadRandomAccessFileis used by iterators for file reads in several cases, like in compaction whencompaction_readahead_size > 0oruse_direct_io_for_flush_and_compaction == true, or in user iterator whenReadOptions::readahead_size > 0.ReadaheadRandomAccessFilemaintains an internal buffer for readahead data. It assumes that, if the buffer's length is less thanReadaheadRandomAccessFile::readahead_size_, which is fixed in the constructor, then EOF has been reached so it doesn't try reading further.Recently, d938226 started calling
RandomAccessFile::Prefetchwith various lengths: 8KB, 16KB, etc. When theRandomAccessFileis aReadaheadRandomAccessFile, it triggers the above condition and incorrectly determines EOF. If a block is partially in the readahead buffer and EOF is incorrectly decided, the result is a truncated data block.The problem is reproducible:
Test Plan:
db_benchwith various options that previously failed; now they passmake check -j64