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Change the semantics of blob_garbage_collection_force_threshold to pr…
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…ovide better control over space amp (facebook#13022)

Summary:
Pull Request resolved: facebook#13022

Currently, `blob_garbage_collection_force_threshold` applies to the oldest batch of blob files, which is typically only a small subset of the blob files currently eligible for garbage collection. This can result in a form of head-of-line blocking: no GC-triggered compactions will be scheduled if the oldest batch does not currently exceed the threshold, even if a lot of higher-numbered blob files do. This can in turn lead to high space amplification that exceeds the soft bound implicit in the force threshold (e.g. 50% would suggest a space amp of <2 and 75% would imply a space amp of <4). The patch changes the semantics of this configuration threshold to apply to the entire set of blob files that are eligible for garbage collection based on `blob_garbage_collection_age_cutoff`. This provides more intuitive semantics for the option and can provide a better write amp/space amp trade-off. (Note that GC-triggered compactions still pick the same SST files as before, so triggered GC still targets the oldest the blob files.)

Reviewed By: jowlyzhang

Differential Revision: D62977860

fbshipit-source-id: a999f31fe9cdda313de513f0e7a6fc707424d4a3
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@ltamasi @facebook-github-bot
ltamasi authored and facebook-github-bot committed Sep 19, 2024
1 parent 98c33cb commit 54ace7f
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Showing 8 changed files with 32 additions and 121 deletions.
39 changes: 11 additions & 28 deletions db/version_set.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -3772,14 +3772,17 @@ void VersionStorageInfo::ComputeFilesMarkedForForcedBlobGC(
return;
}

// Compute the sum of total and garbage bytes over the oldest batch of blob
// files. The oldest batch is defined as the set of blob files which are
// kept alive by the same SSTs as the very oldest one. Here is a toy example.
// Let's assume we have three SSTs 1, 2, and 3, and four blob files 10, 11,
// 12, and 13. Also, let's say SSTs 1 and 2 both rely on blob file 10 and
// potentially some higher-numbered ones, while SST 3 relies on blob file 12
// and potentially some higher-numbered ones. Then, the SST to oldest blob
// file mapping is as follows:
// Compute the sum of total and garbage bytes over the batch of blob files
// currently eligible for garbage collection based on
// blob_garbage_collection_age_cutoff, and if the garbage ratio exceeds
// blob_garbage_collection_force_threshold, schedule compaction for the
// SST files that reference the oldest batch of blob files. Here is a toy
// example. Let's assume we have three SSTs 1, 2, and 3, and four blob files
// 10, 11, 12, and 13, which correspond to the range that is eligible for GC
// and satisfy the garbage ratio threshold. Also, let's say SSTs 1 and 2 both
// rely on blob file 10 and potentially some higher-numbered ones, while SST 3
// relies on blob file 12 and potentially some higher-numbered ones. Then, the
// SST to oldest blob file mapping is as follows:
//
// SST file number Oldest blob file number
// 1 10
Expand All @@ -3797,11 +3800,6 @@ void VersionStorageInfo::ComputeFilesMarkedForForcedBlobGC(
//
// Then, the oldest batch of blob files consists of blob files 10 and 11,
// and we can get rid of them by forcing the compaction of SSTs 1 and 2.
//
// Note that the overall ratio of garbage computed for the batch has to exceed
// blob_garbage_collection_force_threshold and the entire batch has to be
// eligible for GC according to blob_garbage_collection_age_cutoff in order
// for us to schedule any compactions.
const auto& oldest_meta = blob_files_.front();
assert(oldest_meta);

Expand All @@ -3818,25 +3816,10 @@ void VersionStorageInfo::ComputeFilesMarkedForForcedBlobGC(
const auto& meta = blob_files_[count];
assert(meta);

if (!meta->GetLinkedSsts().empty()) {
// Found the beginning of the next batch of blob files
break;
}

sum_total_blob_bytes += meta->GetTotalBlobBytes();
sum_garbage_blob_bytes += meta->GetGarbageBlobBytes();
}

if (count < blob_files_.size()) {
const auto& meta = blob_files_[count];
assert(meta);

if (meta->GetLinkedSsts().empty()) {
// Some files in the oldest batch are not eligible for GC
return;
}
}

if (sum_garbage_blob_bytes <
blob_garbage_collection_force_threshold * sum_total_blob_bytes) {
return;
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78 changes: 4 additions & 74 deletions db/version_set_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -727,20 +727,7 @@ TEST_F(VersionStorageInfoTest, ForcedBlobGCSingleBatch) {
ASSERT_TRUE(vstorage_.FilesMarkedForForcedBlobGC().empty());
}

// Part of the oldest batch of blob files (specifically, #12 and #13) is
// ineligible for GC due to the age cutoff

{
constexpr double age_cutoff = 0.5;
constexpr double force_threshold = 0.0;
vstorage_.ComputeFilesMarkedForForcedBlobGC(
age_cutoff, force_threshold, /*enable_blob_garbage_collection=*/true);

ASSERT_TRUE(vstorage_.FilesMarkedForForcedBlobGC().empty());
}

// Oldest batch is eligible based on age cutoff but its overall garbage ratio
// is below threshold
// Overall garbage ratio of eligible files is below threshold

{
constexpr double age_cutoff = 1.0;
Expand All @@ -751,8 +738,7 @@ TEST_F(VersionStorageInfoTest, ForcedBlobGCSingleBatch) {
ASSERT_TRUE(vstorage_.FilesMarkedForForcedBlobGC().empty());
}

// Oldest batch is eligible based on age cutoff and its overall garbage ratio
// meets threshold
// Overall garbage ratio of eligible files meets threshold

{
constexpr double age_cutoff = 1.0;
Expand Down Expand Up @@ -878,20 +864,7 @@ TEST_F(VersionStorageInfoTest, ForcedBlobGCMultipleBatches) {
ASSERT_TRUE(vstorage_.FilesMarkedForForcedBlobGC().empty());
}

// Part of the oldest batch of blob files (specifically, the second file) is
// ineligible for GC due to the age cutoff

{
constexpr double age_cutoff = 0.25;
constexpr double force_threshold = 0.0;
vstorage_.ComputeFilesMarkedForForcedBlobGC(
age_cutoff, force_threshold, /*enable_blob_garbage_collection=*/true);

ASSERT_TRUE(vstorage_.FilesMarkedForForcedBlobGC().empty());
}

// Oldest batch is eligible based on age cutoff but its overall garbage ratio
// is below threshold
// Overall garbage ratio of eligible files is below threshold

{
constexpr double age_cutoff = 0.5;
Expand All @@ -902,8 +875,7 @@ TEST_F(VersionStorageInfoTest, ForcedBlobGCMultipleBatches) {
ASSERT_TRUE(vstorage_.FilesMarkedForForcedBlobGC().empty());
}

// Oldest batch is eligible based on age cutoff and its overall garbage ratio
// meets threshold
// Overall garbage ratio of eligible files meets threshold

{
constexpr double age_cutoff = 0.5;
Expand All @@ -929,48 +901,6 @@ TEST_F(VersionStorageInfoTest, ForcedBlobGCMultipleBatches) {
ASSERT_EQ(ssts_to_be_compacted[0], expected_ssts_to_be_compacted[0]);
ASSERT_EQ(ssts_to_be_compacted[1], expected_ssts_to_be_compacted[1]);
}

// Now try the last two cases again with a greater than necessary age cutoff

// Oldest batch is eligible based on age cutoff but its overall garbage ratio
// is below threshold

{
constexpr double age_cutoff = 0.75;
constexpr double force_threshold = 0.6;
vstorage_.ComputeFilesMarkedForForcedBlobGC(
age_cutoff, force_threshold, /*enable_blob_garbage_collection=*/true);

ASSERT_TRUE(vstorage_.FilesMarkedForForcedBlobGC().empty());
}

// Oldest batch is eligible based on age cutoff and its overall garbage ratio
// meets threshold

{
constexpr double age_cutoff = 0.75;
constexpr double force_threshold = 0.5;
vstorage_.ComputeFilesMarkedForForcedBlobGC(
age_cutoff, force_threshold, /*enable_blob_garbage_collection=*/true);

auto ssts_to_be_compacted = vstorage_.FilesMarkedForForcedBlobGC();
ASSERT_EQ(ssts_to_be_compacted.size(), 2);

std::sort(ssts_to_be_compacted.begin(), ssts_to_be_compacted.end(),
[](const std::pair<int, FileMetaData*>& lhs,
const std::pair<int, FileMetaData*>& rhs) {
assert(lhs.second);
assert(rhs.second);
return lhs.second->fd.GetNumber() < rhs.second->fd.GetNumber();
});

const autovector<std::pair<int, FileMetaData*>>
expected_ssts_to_be_compacted{{level, level_files[0]},
{level, level_files[1]}};

ASSERT_EQ(ssts_to_be_compacted[0], expected_ssts_to_be_compacted[0]);
ASSERT_EQ(ssts_to_be_compacted[1], expected_ssts_to_be_compacted[1]);
}
}

class VersionStorageInfoTimestampTest : public VersionStorageInfoTestBase {
Expand Down
2 changes: 1 addition & 1 deletion db_stress_tool/db_stress_gflags.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -497,7 +497,7 @@ DEFINE_double(blob_garbage_collection_force_threshold,
ROCKSDB_NAMESPACE::AdvancedColumnFamilyOptions()
.blob_garbage_collection_force_threshold,
"[Integrated BlobDB] The threshold for the ratio of garbage in "
"the oldest blob files for forcing garbage collection.");
"the eligible blob files for forcing garbage collection.");

DEFINE_uint64(blob_compaction_readahead_size,
ROCKSDB_NAMESPACE::AdvancedColumnFamilyOptions()
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13 changes: 6 additions & 7 deletions include/rocksdb/advanced_options.h
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Original file line number Diff line number Diff line change
Expand Up @@ -929,13 +929,12 @@ struct AdvancedColumnFamilyOptions {
// Dynamically changeable through the SetOptions() API
double blob_garbage_collection_age_cutoff = 0.25;

// If the ratio of garbage in the oldest blob files exceeds this threshold,
// targeted compactions are scheduled in order to force garbage collecting
// the blob files in question, assuming they are all eligible based on the
// value of blob_garbage_collection_age_cutoff above. This option is
// currently only supported with leveled compactions.
// Note that enable_blob_garbage_collection has to be set in order for this
// option to have any effect.
// If the ratio of garbage in the blob files currently eligible for garbage
// collection exceeds this threshold, targeted compactions are scheduled in
// order to force garbage collecting the oldest blob files. This option is
// currently only supported with leveled compactions. Note that
// enable_blob_garbage_collection has to be set in order for this option to
// have any effect.
//
// Default: 1.0
//
Expand Down
9 changes: 4 additions & 5 deletions java/src/main/java/org/rocksdb/AdvancedMutableColumnFamilyOptionsInterface.java
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Original file line number Diff line number Diff line change
Expand Up @@ -733,11 +733,10 @@ T setReportBgIoStats(
double blobGarbageCollectionAgeCutoff();

/**
* If the ratio of garbage in the oldest blob files exceeds this threshold,
* targeted compactions are scheduled in order to force garbage collecting
* the blob files in question, assuming they are all eligible based on the
* value of {@link #blobGarbageCollectionAgeCutoff} above. This option is
* currently only supported with leveled compactions.
* If the ratio of garbage in the blob files currently eligible for garbage
* collection exceeds this threshold, targeted compactions are scheduled in
* order to force garbage collecting the oldest blob files. This option is
* currently only supported with leveled compactions.
* <p>
* Note that {@link #enableBlobGarbageCollection} has to be set in order for this
* option to have any effect.
Expand Down
9 changes: 4 additions & 5 deletions java/src/main/java/org/rocksdb/ColumnFamilyOptions.java
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Original file line number Diff line number Diff line change
Expand Up @@ -1204,11 +1204,10 @@ public double blobGarbageCollectionAgeCutoff() {
}

/**
* If the ratio of garbage in the oldest blob files exceeds this threshold,
* targeted compactions are scheduled in order to force garbage collecting
* the blob files in question, assuming they are all eligible based on the
* value of {@link #blobGarbageCollectionAgeCutoff} above. This option is
* currently only supported with leveled compactions.
* If the ratio of garbage in the blob files currently eligible for garbage
* collection exceeds this threshold, targeted compactions are scheduled in
* order to force garbage collecting the oldest blob files. This option is
* currently only supported with leveled compactions.
* <p>
* Note that {@link #enableBlobGarbageCollection} has to be set in order for this
* option to have any effect.
Expand Down
2 changes: 1 addition & 1 deletion tools/db_bench_tool.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1104,7 +1104,7 @@ DEFINE_double(blob_garbage_collection_force_threshold,
ROCKSDB_NAMESPACE::AdvancedColumnFamilyOptions()
.blob_garbage_collection_force_threshold,
"[Integrated BlobDB] The threshold for the ratio of garbage in "
"the oldest blob files for forcing garbage collection.");
"the eligible blob files for forcing garbage collection.");

DEFINE_uint64(blob_compaction_readahead_size,
ROCKSDB_NAMESPACE::AdvancedColumnFamilyOptions()
Expand Down
1 change: 1 addition & 0 deletions unreleased_history/behavior_changes/blob_garbage_collection_force_threshold.md
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@@ -0,0 +1 @@
Changed the semantics of the BlobDB configuration option `blob_garbage_collection_force_threshold` to define a threshold for the overall garbage ratio of all blob files currently eligible for garbage collection (according to `blob_garbage_collection_age_cutoff`). This can provide better control over space amplification at the cost of slightly higher write amplification.

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