rocksdb

Commit Graph

Author	SHA1	Message	Date
Peter Dillinger	7a1b0207e6	format_version=6 and context-aware block checksums (#9058 ) Summary: ## Context checksum All RocksDB checksums currently use 32 bits of checking power, which should be 1 in 4 billion false negative (FN) probability (failing to detect corruption). This is true for random corruptions, and in some cases small corruptions are guaranteed to be detected. But some possible corruptions, such as in storage metadata rather than storage payload data, would have a much higher FN rate. For example: * Data larger than one SST block is replaced by data from elsewhere in the same or another SST file. Especially with block_align=true, the probability of exact block size match is probably around 1 in 100, making the FN probability around that same. Without `block_align=true` the probability of same block start location is probably around 1 in 10,000, for FN probability around 1 in a million. To solve this problem in new format_version=6, we add "context awareness" to block checksum checks. The stored and expected checksum value is modified based on the block's position in the file and which file it is in. The modifications are cleverly chosen so that, for example * blocks within about 4GB of each other are guaranteed to use different context * blocks that are offset by exactly some multiple of 4GiB are guaranteed to use different context * files generated by the same process are guaranteed to use different context for the same offsets, until wrap-around after 2^32 - 1 files Thus, with format_version=6, if a valid SST block and checksum is misplaced, its checksum FN probability should be essentially ideal, 1 in 4B. ## Footer checksum This change also adds checksum protection to the SST footer (with format_version=6), for the first time without relying on whole file checksum. To prevent a corruption of the format_version in the footer (e.g. 6 -> 5) to defeat the footer checksum, we change much of the footer data format including an "extended magic number" in format_version 6 that would be interpreted as empty index and metaindex block handles in older footer versions. We also change the encoding of handles to free up space for other new data in footer. ## More detail: making space in footer In order to keep footer the same size in format_version=6 (avoid change to IO patterns), we have to free up some space for new data. We do this two ways: * Metaindex block handle is encoded down to 4 bytes (from 10) by assuming it immediately precedes the footer, and by assuming it is < 4GB. * Index block handle is moved into metaindex. (I don't know why it was in footer to begin with.) ## Performance In case of small performance penalty, I've made a "pay as you go" optimization to compensate: replace `MutableCFOptions` in BlockBasedTableBuilder::Rep with the only field used in that structure after construction: `prefix_extractor`. This makes the PR an overall performance improvement (results below). Nevertheless I'm seeing essentially no difference going from fv=5 to fv=6, even including that improvement for both. That's based on extreme case table write performance testing, many files with many blocks. This is relatively checksum intensive (small blocks) and salt generation intensive (small files). ``` (for I in `seq 1 100`; do TEST_TMPDIR=/dev/shm/dbbench2 ./db_bench -benchmarks=fillseq -memtablerep=vector -disable_wal=1 -allow_concurrent_memtable_write=false -num=3000000 -compaction_style=2 -fifo_compaction_max_table_files_size_mb=10000 -fifo_compaction_allow_compaction=0 -write_buffer_size=100000 -compression_type=none -block_size=1000; done) 2>&1 \| grep micros/op \| tee out awk '{ tot += $5; n += 1; } END { print int(1.0 * tot / n) }' < out ``` Each value below is ops/s averaged over 100 runs, run simultaneously with competing configuration for load fairness Before -> after (both fv=5): 483530 -> 483673 (negligible) Re-run 1: 480733 -> 485427 (1.0% faster) Re-run 2: 483821 -> 484541 (0.1% faster) Before (fv=5) -> after (fv=6): 482006 -> 485100 (0.6% faster) Re-run 1: 482212 -> 485075 (0.6% faster) Re-run 2: 483590 -> 484073 (0.1% faster) After fv=5 -> after fv=6: 483878 -> 485542 (0.3% faster) Re-run 1: 485331 -> 483385 (0.4% slower) Re-run 2: 485283 -> 483435 (0.4% slower) Re-run 3: 483647 -> 486109 (0.5% faster) Pull Request resolved: https://github.com/facebook/rocksdb/pull/9058 Test Plan: unit tests included (table_test, db_properties_test, salt in env_test). General DB tests and crash test updated to test new format_version. Also temporarily updated the default format version to 6 and saw some test failures. Almost all were due to an inadvertent additional read in VerifyChecksum to verify the index block checksum, though it's arguably a bug that VerifyChecksum does not appear to (re-)verify the index block checksum, just assuming it was verified in opening the index reader (probably usually true but probably not always true). Some other concerns about VerifyChecksum are left in FIXME comments. The only remaining test failure on change of default (in block_fetcher_test) now has a comment about how to upgrade the test. The format compatibility test does not need updating because we have not updated the default format_version. Reviewed By: ajkr, mrambacher Differential Revision: D33100915 Pulled By: pdillinger fbshipit-source-id: 8679e3e572fa580181a737fd6d113ed53c5422ee	2 years ago

Author

SHA1

Message

Date

Peter Dillinger

7a1b0207e6

format_version=6 and context-aware block checksums (#9058 )

Summary:
## Context checksum
All RocksDB checksums currently use 32 bits of checking
power, which should be 1 in 4 billion false negative (FN) probability (failing to
detect corruption). This is true for random corruptions, and in some cases
small corruptions are guaranteed to be detected. But some possible
corruptions, such as in storage metadata rather than storage payload data,
would have a much higher FN rate. For example:
* Data larger than one SST block is replaced by data from elsewhere in
the same or another SST file. Especially with block_align=true, the
probability of exact block size match is probably around 1 in 100, making
the FN probability around that same. Without `block_align=true` the
probability of same block start location is probably around 1 in 10,000,
for FN probability around 1 in a million.

To solve this problem in new format_version=6, we add "context awareness"
to block checksum checks. The stored and expected checksum value is
modified based on the block's position in the file and which file it is in. The
modifications are cleverly chosen so that, for example
* blocks within about 4GB of each other are guaranteed to use different context
* blocks that are offset by exactly some multiple of 4GiB are guaranteed to use
different context
* files generated by the same process are guaranteed to use different context
for the same offsets, until wrap-around after 2^32 - 1 files

Thus, with format_version=6, if a valid SST block and checksum is misplaced,
its checksum FN probability should be essentially ideal, 1 in 4B.

## Footer checksum
This change also adds checksum protection to the SST footer (with
format_version=6), for the first time without relying on whole file checksum.
To prevent a corruption of the format_version in the footer (e.g. 6 -> 5) to
defeat the footer checksum, we change much of the footer data format
including an "extended magic number" in format_version 6 that would be
interpreted as empty index and metaindex block handles in older footer
versions. We also change the encoding of handles to free up space for
other new data in footer.

## More detail: making space in footer
In order to keep footer the same size in format_version=6 (avoid change to IO
patterns), we have to free up some space for new data. We do this two ways:
* Metaindex block handle is encoded down to 4 bytes (from 10) by assuming
it immediately precedes the footer, and by assuming it is < 4GB.
* Index block handle is moved into metaindex. (I don't know why it was
in footer to begin with.)

## Performance
In case of small performance penalty, I've made a "pay as you go" optimization
to compensate: replace `MutableCFOptions` in BlockBasedTableBuilder::Rep
with the only field used in that structure after construction: `prefix_extractor`.
This makes the PR an overall performance improvement (results below).

Nevertheless I'm seeing essentially no difference going from fv=5 to fv=6,
even including that improvement for both. That's based on extreme case table
write performance testing, many files with many blocks. This is relatively
checksum intensive (small blocks) and salt generation intensive (small files).

```
(for I in `seq 1 100`; do TEST_TMPDIR=/dev/shm/dbbench2 ./db_bench -benchmarks=fillseq -memtablerep=vector -disable_wal=1 -allow_concurrent_memtable_write=false -num=3000000 -compaction_style=2 -fifo_compaction_max_table_files_size_mb=10000 -fifo_compaction_allow_compaction=0 -write_buffer_size=100000 -compression_type=none -block_size=1000; done) 2>&1 | grep micros/op | tee out
awk '{ tot += $5; n += 1; } END { print int(1.0 * tot / n) }' < out
```

Each value below is ops/s averaged over 100 runs, run simultaneously with competing
configuration for load fairness

Before -> after (both fv=5): 483530 -> 483673 (negligible)
Re-run 1: 480733 -> 485427 (1.0% faster)
Re-run 2: 483821 -> 484541 (0.1% faster)
Before (fv=5) -> after (fv=6): 482006 -> 485100 (0.6% faster)
Re-run 1: 482212 -> 485075 (0.6% faster)
Re-run 2: 483590 -> 484073 (0.1% faster)
After fv=5 -> after fv=6: 483878 -> 485542 (0.3% faster)
Re-run 1: 485331 -> 483385 (0.4% slower)
Re-run 2: 485283 -> 483435 (0.4% slower)
Re-run 3: 483647 -> 486109 (0.5% faster)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9058

Test Plan:
unit tests included (table_test, db_properties_test, salt in env_test). General DB tests
and crash test updated to test new format_version.

Also temporarily updated the default format version to 6 and saw some test failures. Almost all
were due to an inadvertent additional read in VerifyChecksum to verify the index block checksum,
though it's arguably a bug that VerifyChecksum does not appear to (re-)verify the index block
checksum, just assuming it was verified in opening the index reader (probably *usually* true but
probably not always true). Some other concerns about VerifyChecksum are left in FIXME
comments. The only remaining test failure on change of default (in block_fetcher_test) now
has a comment about how to upgrade the test.

The format compatibility test does not need updating because we have not updated the default
format_version.

Reviewed By: ajkr, mrambacher

Differential Revision: D33100915

Pulled By: pdillinger

fbshipit-source-id: 8679e3e572fa580181a737fd6d113ed53c5422ee

2 years ago

1 Commits (cf95821fb6b18b30edb56098da3afccdf2c88916)