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DataBlockHashIndex: fix comment in NumRestarts() (#4286)

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Summary:
Improve the description of the backward compatibility check in NumRestarts()
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4286

Differential Revision: D9412490

Pulled By: fgwu

fbshipit-source-id: ea7dd5c61d8ff8eacef623b729d4e4fd53cca066
This commit is contained in:
Fenggang Wu 2018-08-21 17:06:44 -07:00 committed by Facebook Github Bot
parent 4f12d49daf
commit 640cfa7c33
1 changed files with 9 additions and 6 deletions
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15
table/block.cc
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@ -725,13 +725,16 @@ uint32_t Block::NumRestarts() const {
uint32_t block_footer = DecodeFixed32(data_ + size_ - sizeof(uint32_t));
uint32_t num_restarts = block_footer;
if (size_ > kMaxBlockSizeSupportedByHashIndex) {
// We ensure a block with HashIndex is less than 64KiB in BlockBuilder.
// Therefore the footer cannot be encoded as a packed index type and
// In BlockBuilder, we have ensured a block with HashIndex is less than
// kMaxBlockSizeSupportedByHashIndex (64KiB).
//
// Therefore, if we encounter a block with a size > 64KiB, the block
// cannot have HashIndex. So the footer will directly interpreted as
// num_restarts.
// Such check can ensure legacy block with a vary large num_restarts
// i.e. >= 0x10000000 can be interpreted correctly as no HashIndex.
// If a legacy block hash a num_restarts >= 0x10000000, size_ will be
// much large than 64KiB.
//
// Such check is for backward compatibility. We can ensure legacy block
// with a vary large num_restarts i.e. >= 0x80000000 can be interpreted
// correctly as no HashIndex even if the MSB of num_restarts is set.
return num_restarts;
}
BlockBasedTableOptions::DataBlockIndexType index_type;