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Improve comments to options.allow_mmap_reads (#9936)

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Summary:
It confused users and use that with options.allow_mmap_reads = true, CPU is high with checksum verification. Add a comment to explain it.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9936

Reviewed By: anand1976

Differential Revision: D36106529

fbshipit-source-id: 3d723bd686f96a84c694c8b2d91ad28d9ccfd979
This commit is contained in:
Siying Dong 2022-05-03 16:21:31 -07:00 committed by Facebook GitHub Bot
parent 440c7f6306
commit b82edffc7b
1 changed files with 8 additions and 0 deletions
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8
include/rocksdb/options.h
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@ -767,6 +767,14 @@ struct DBOptions {
// Allow the OS to mmap file for reading sst tables. // Allow the OS to mmap file for reading sst tables.
// Not recommended for 32-bit OS. // Not recommended for 32-bit OS.
// When the option is set to true and compression is disabled, the blocks
// will not be copied and will be read directly from the mmap-ed memory
// area, and the block will not be inserted into the block cache. However,
// checksums will still be checked if ReadOptions.verify_checksums is set
// to be true. It means a checksum check every time a block is read, more
// than the setup where the option is set to false and the block cache is
// used. The common use of the options is to run RocksDB on ramfs, where
// checksum verification is usually not needed.
// Default: false // Default: false
bool allow_mmap_reads = false; bool allow_mmap_reads = false;