External Value Store

Summary:
Developing a capability for storing values on external backing file(s).

This is just a highly unoptimized first pass - supports:
1) Allocating some portion of external file to be used to store value
2) Freeing the range, enabling it to be reused by other values

As next steps, I plan to:
1) Create some kind of stress testing. Once I can measure stuff, I can focus on optimizing.
2) Optimize locking.
3) Optimize freelist data structure. Currently we have O(n) for both freeing and allocation.
4) Figure out how to do recovery.

Test Plan: Created a unit test.

Reviewers: dhruba, haobo, kailiu

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13389

Makefile

@@ -48,6 +48,7 @@ TESTS = \
 	db_test \
 	dbformat_test \
 	env_test \
+	blob_store_test \
 	filelock_test \
 	filename_test \
 	filter_block_test \
@@ -204,6 +205,9 @@ cache_test: util/cache_test.o $(LIBOBJECTS) $(TESTHARNESS)
 coding_test: util/coding_test.o $(LIBOBJECTS) $(TESTHARNESS)
 	$(CXX) util/coding_test.o $(LIBOBJECTS) $(TESTHARNESS) $(EXEC_LDFLAGS) -o $@ $(LDFLAGS) $(COVERAGEFLAGS)
 
+blob_store_test: util/blob_store_test.o $(LIBOBJECTS) $(TESTHARNESS) $(TESTUTIL)
+	$(CXX) util/blob_store_test.o $(LIBOBJECTS) $(TESTHARNESS) $(EXEC_LDFLAGS) -o$@ $(LDFLAGS) $(COVERAGEFLAGS)
+
 stringappend_test: utilities/merge_operators/string_append/stringappend_test.o $(LIBOBJECTS) $(TESTHARNESS)
 	$(CXX) utilities/merge_operators/string_append/stringappend_test.o $(LIBOBJECTS) $(TESTHARNESS) $(EXEC_LDFLAGS) -o $@ $(LDFLAGS) $(COVERAGEFLAGS)
 

util/blob_store.cc

@@ -0,0 +1,297 @@
+// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+
+#include "util/blob_store.h"
+
+namespace rocksdb {
+
+using namespace std;
+
+// BlobChunk
+bool BlobChunk::ImmediatelyBefore(const BlobChunk& chunk) const {
+  // overlapping!?
+  assert(!Overlap(chunk));
+  // size == 0 is a marker, not a block
+  return size != 0 &&
+    bucket_id == chunk.bucket_id &&
+    offset + size == chunk.offset;
+}
+
+bool BlobChunk::Overlap(const BlobChunk &chunk) const {
+  return size != 0 && chunk.size != 0 && bucket_id == chunk.bucket_id &&
+    ((offset >= chunk.offset && offset < chunk.offset + chunk.size) ||
+     (chunk.offset >= offset && chunk.offset < offset + size));
+}
+
+// Blob
+string Blob::ToString() const {
+  string ret;
+  for (auto chunk : chunks) {
+    PutFixed32(&ret, chunk.bucket_id);
+    PutFixed32(&ret, chunk.offset);
+    PutFixed32(&ret, chunk.size);
+  }
+  return ret;
+}
+
+Blob::Blob(const std::string& blob) {
+  for (uint32_t i = 0; i < blob.size(); ) {
+    uint32_t t[3] = {0};
+    for (int j = 0; j < 3 && i + sizeof(uint32_t) - 1 < blob.size();
+                    ++j, i += sizeof(uint32_t)) {
+      t[j] = DecodeFixed32(blob.data() + i);
+    }
+    chunks.push_back(BlobChunk(t[0], t[1], t[2]));
+  }
+}
+
+// FreeList
+FreeList::FreeList() {
+  // We add (0, 0, 0) blob because it makes our life easier and
+  // code cleaner. (0, 0, 0) is always in the list so we can
+  // guarantee that free_chunks_list_ != nullptr, which avoids
+  // lots of unnecessary ifs
+  free_chunks_list_ = (FreeChunk *)malloc(sizeof(FreeChunk));
+  free_chunks_list_->chunk = BlobChunk(0, 0, 0);
+  free_chunks_list_->next = nullptr;
+}
+
+FreeList::~FreeList() {
+  while (free_chunks_list_ != nullptr) {
+    FreeChunk* t = free_chunks_list_;
+    free_chunks_list_ = free_chunks_list_->next;
+    free(t);
+  }
+}
+
+Status FreeList::Free(const Blob& blob) {
+  MutexLock l(&mutex_);
+
+  // add it back to the free list
+  for (auto chunk : blob.chunks) {
+    FreeChunk* itr = free_chunks_list_;
+
+    // find a node AFTER which we'll add the block
+    for ( ; itr->next != nullptr && itr->next->chunk <= chunk;
+            itr = itr->next) {
+    }
+
+    // try to merge with previous block
+    if (itr->chunk.ImmediatelyBefore(chunk)) {
+      // merge
+      itr->chunk.size += chunk.size;
+    } else {
+      // Insert the block after itr
+      FreeChunk* t = (FreeChunk*)malloc(sizeof(FreeChunk));
+      if (t == nullptr) {
+        throw runtime_error("Malloc failed");
+      }
+      t->chunk = chunk;
+      t->next = itr->next;
+      itr->next = t;
+      itr = t;
+    }
+
+    // try to merge with the next block
+    if (itr->next != nullptr &&
+        itr->chunk.ImmediatelyBefore(itr->next->chunk)) {
+      FreeChunk *tobedeleted = itr->next;
+      itr->chunk.size += itr->next->chunk.size;
+      itr->next = itr->next->next;
+      free(tobedeleted);
+    }
+  }
+
+  return Status::OK();
+}
+
+Status FreeList::Allocate(uint32_t blocks, Blob* blob) {
+  MutexLock l(&mutex_);
+  FreeChunk** best_fit_node = nullptr;
+
+  // Find the smallest free chunk whose size is greater or equal to blocks
+  for (FreeChunk** itr = &free_chunks_list_; (*itr) != nullptr;
+       itr = &((*itr)->next)) {
+    if ((*itr)->chunk.size >= blocks &&
+        (best_fit_node == nullptr ||
+         (*best_fit_node)->chunk.size > (*itr)->chunk.size)) {
+      best_fit_node = itr;
+    }
+  }
+
+  if (best_fit_node == nullptr || *best_fit_node == nullptr) {
+    // Not enough memory
+    return Status::Incomplete("");
+  }
+
+  blob->SetOneChunk((*best_fit_node)->chunk.bucket_id,
+                    (*best_fit_node)->chunk.offset,
+                    blocks);
+
+  if ((*best_fit_node)->chunk.size > blocks) {
+    // just shorten best_fit_node
+    (*best_fit_node)->chunk.offset += blocks;
+    (*best_fit_node)->chunk.size -= blocks;
+  } else {
+    assert(blocks == (*best_fit_node)->chunk.size);
+    // delete best_fit_node
+    FreeChunk* t = *best_fit_node;
+    (*best_fit_node) = (*best_fit_node)->next;
+    free(t);
+  }
+
+  return Status::OK();
+}
+
+bool FreeList::Overlap(const Blob &blob) const {
+  MutexLock l(&mutex_);
+  for (auto chunk : blob.chunks) {
+    for (auto itr = free_chunks_list_; itr != nullptr; itr = itr->next) {
+      if (itr->chunk.Overlap(chunk)) {
+        return true;
+      }
+    }
+  }
+  return false;
+}
+
+// BlobStore
+BlobStore::BlobStore(const string& directory,
+                     uint64_t block_size,
+                     uint32_t blocks_per_bucket,
+                     Env* env) :
+    directory_(directory),
+    block_size_(block_size),
+    blocks_per_bucket_(blocks_per_bucket),
+    env_(env) {
+  env_->CreateDirIfMissing(directory_);
+
+  storage_options_.use_mmap_writes = false;
+  storage_options_.use_mmap_reads = false;
+
+  CreateNewBucket();
+}
+
+BlobStore::~BlobStore() {
+  // TODO we don't care about recovery for now
+}
+
+Status BlobStore::Put(const char* value, uint64_t size, Blob* blob) {
+  // convert size to number of blocks
+  Status s = Allocate((size + block_size_ - 1) / block_size_, blob);
+  if (!s.ok()) {
+    return s;
+  }
+
+  uint64_t offset = 0; // in bytes, not blocks
+  for (auto chunk : blob->chunks) {
+    uint64_t write_size = min(chunk.size * block_size_, size);
+    assert(chunk.bucket_id < buckets_.size());
+    s = buckets_[chunk.bucket_id].get()->Write(chunk.offset * block_size_,
+                                               Slice(value + offset,
+                                                     write_size));
+    if (!s.ok()) {
+      Delete(*blob);
+      return s;
+    }
+    offset += write_size;
+    size -= write_size;
+    if (write_size < chunk.size * block_size_) {
+      // if we have any space left in the block, fill it up with zeros
+      string zero_string(chunk.size * block_size_ - write_size, 0);
+      s = buckets_[chunk.bucket_id].get()->Write(chunk.offset * block_size_ +
+                                                    write_size,
+                                                 Slice(zero_string));
+    }
+  }
+
+  if (size > 0) {
+    Delete(*blob);
+    return Status::IOError("Tried to write more data than fits in the blob");
+  }
+
+  return Status::OK();
+}
+
+Status BlobStore::Get(const Blob& blob,
+                      string* value) const {
+  // assert that it doesn't overlap with free list
+  // it will get compiled out for release
+  assert(!free_list_.Overlap(blob));
+
+  uint32_t total_size = 0; // in blocks
+  for (auto chunk : blob.chunks) {
+    total_size += chunk.size;
+  }
+  assert(total_size > 0);
+  value->resize(total_size * block_size_);
+
+  uint64_t offset = 0; // in bytes, not blocks
+  for (auto chunk : blob.chunks) {
+    Slice result;
+    assert(chunk.bucket_id < buckets_.size());
+    Status s;
+    s = buckets_[chunk.bucket_id].get()->Read(chunk.offset * block_size_,
+                                              chunk.size * block_size_,
+                                              &result,
+                                              &value->at(offset));
+    if (!s.ok() || result.size() < chunk.size * block_size_) {
+      value->clear();
+      return Status::IOError("Could not read in from file");
+    }
+    offset += chunk.size * block_size_;
+  }
+
+  // remove the '\0's at the end of the string
+  value->erase(find(value->begin(), value->end(), '\0'), value->end());
+
+  return Status::OK();
+}
+
+Status BlobStore::Delete(const Blob& blob) {
+  return free_list_.Free(blob);
+}
+
+Status BlobStore::Allocate(uint32_t blocks, Blob* blob) {
+  // TODO we don't currently support fragmented blobs
+  MutexLock l(&allocate_mutex_);
+  assert(blocks <= blocks_per_bucket_);
+  Status s;
+
+  s = free_list_.Allocate(blocks, blob);
+  if (!s.ok()) {
+    CreateNewBucket();
+    s = free_list_.Allocate(blocks, blob);
+  }
+
+  return s;
+}
+
+Status BlobStore::CreateNewBucket() {
+  int new_bucket_id;
+  new_bucket_id = buckets_.size();
+  buckets_.push_back(unique_ptr<RandomRWFile>());
+
+  char fname[200];
+  sprintf(fname, "%s/%d.bs", directory_.c_str(), new_bucket_id);
+
+  Status s = env_->NewRandomRWFile(string(fname),
+                                   &buckets_[new_bucket_id],
+                                   storage_options_);
+  if (!s.ok()) {
+    buckets_.erase(buckets_.begin() + new_bucket_id);
+    return s;
+  }
+
+  s = buckets_[new_bucket_id].get()->Allocate(
+      0, block_size_ * blocks_per_bucket_);
+  if (!s.ok()) {
+    buckets_.erase(buckets_.begin() + new_bucket_id);
+    return s;
+  }
+
+  return free_list_.Free(Blob(new_bucket_id, 0, blocks_per_bucket_));
+}
+
+} // namespace rocksdb