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rocksdb/utilities/persistent_cache/persistent_cache_tier.h

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// Copyright (c) 2013, Facebook, Inc. All rights reserved.
// This source code is licensed under the BSD-style license found in the
// LICENSE file in the root directory of this source tree. An additional grant
// of patent rights can be found in the PATENTS file in the same directory.
//
#pragma once
#ifndef ROCKSDB_LITE
#include <limits>
#include <list>
#include <map>
#include <string>
#include <vector>
#include "rocksdb/env.h"
#include "rocksdb/persistent_cache.h"
#include "rocksdb/status.h"
#include "util/histogram.h"
// Persistent Cache
//
// Persistent cache is tiered key-value cache that can use persistent medium. It
// is a generic design and can leverage any storage medium -- disk/SSD/NVM/RAM.
// The code has been kept generic but significant benchmark/design/development
// time has been spent to make sure the cache performs appropriately for
// respective storage medium.
// The file defines
// PersistentCacheOptions : Options for persistent cache
// PersistentCacheTier : Implementation that handles individual cache tier
// PersistentTieresCache : Implementation that handles all tiers as a logical
// unit
//
// PersistentTieredCache architecture:
// +--------------------------+ PersistentCacheTier that handles multiple tiers
// | +----------------+ |
// | | RAM | PersistentCacheTier that handles RAM (VolatileCacheImpl)
// | +----------------+ |
// | | next |
// | v |
// | +----------------+ |
// | | NVM | PersistentCacheTier implementation that handles NVM
// | +----------------+ (BlockCacheImpl)
// | | next |
// | V |
// | +----------------+ |
// | | LE-SSD | PersistentCacheTier implementation that handles LE-SSD
// | +----------------+ (BlockCacheImpl)
// | | |
// | V |
// | null |
// +--------------------------+
// |
// V
// null
namespace rocksdb {
// Persistent Cache Options
//
// This struct captures all the options that are used to configure persistent
// cache. Some of the terminologies used in naming the options are
//
// dispatch size :
// This is the size in which IO is dispatched to the device
//
// write buffer size :
// This is the size of an individual write buffer size. Write buffers are
// grouped to form buffered file.
//
// cache size :
// This is the logical maximum for the cache size
//
// qdepth :
// This is the max number of IOs that can issues to the device in parallel
//
// pepeling :
// The writer code path follows pipelined architecture, which means the
// operations are handed off from one stage to another
//
// pipelining backlog size :
// With the pipelined architecture, there can always be backlogging of ops in
// pipeline queues. This is the maximum backlog size after which ops are dropped
// from queue
struct PersistentCacheOptions {
explicit PersistentCacheOptions(Env* const _env, const std::string& _path,
const uint64_t _cache_size,
const std::shared_ptr<Logger>& _log,
const uint32_t _write_buffer_size = 1 * 1024 *
1024) {
env = _env;
path = _path;
log = _log;
cache_size = _cache_size;
writer_dispatch_size = write_buffer_size = _write_buffer_size;
}
//
// Validate the settings. Our intentions are to catch erroneous settings ahead
// of time instead going violating invariants or causing dead locks.
//
Status ValidateSettings() const {
// (1) check pre-conditions for variables
if (!env || path.empty()) {
return Status::InvalidArgument("empty or null args");
}
// (2) assert size related invariants
// - cache size cannot be less than cache file size
// - individual write buffer size cannot be greater than cache file size
// - total write buffer size cannot be less than 2X cache file size
if (cache_size < cache_file_size || write_buffer_size >= cache_file_size ||
write_buffer_size * write_buffer_count() < 2 * cache_file_size) {
return Status::InvalidArgument("invalid cache size");
}
// (2) check writer settings
// - Queue depth cannot be 0
// - writer_dispatch_size cannot be greater than writer_buffer_size
// - dispatch size and buffer size need to be aligned
if (!writer_qdepth || writer_dispatch_size > write_buffer_size ||
write_buffer_size % writer_dispatch_size) {
return Status::InvalidArgument("invalid writer settings");
}
return Status::OK();
}
//
// Env abstraction to use for systmer level operations
//
Env* env;
//
// Path for the block cache where blocks are persisted
//
std::string path;
//
// Log handle for logging messages
//
std::shared_ptr<Logger> log;
//
// Logical cache size
//
uint64_t cache_size = std::numeric_limits<uint64_t>::max();
// cache-file-size
//
// Cache consists of multiples of small files. This parameter defines the
// size of an individual cache file
//
// default: 1M
uint32_t cache_file_size = 100ULL * 1024 * 1024;
// writer-qdepth
//
// The writers can issues IO to the devices in parallel. This parameter
// controls the max number if IOs that can issues in parallel to the block
// device
//
// default :1
uint32_t writer_qdepth = 1;
// pipeline-writes
//
// The write optionally follow pipelined architecture. This helps
// avoid regression in the eviction code path of the primary tier. This
// parameter defines if pipelining is enabled or disabled
//
// default: true
bool pipeline_writes_ = true;
// max-write-pipeline-backlog-size
//
// Max pipeline buffer size. This is the maximum backlog we can accumulate
// while waiting for writes. After the limit, new ops will be dropped.
//
// Default: 1GiB
uint64_t max_write_pipeline_backlog_size = 1ULL * 1024 * 1024 * 1024;
// write-buffer-size
//
// This is the size in which buffer slabs are allocated.
//
// Default: 1M
uint32_t write_buffer_size = 1ULL * 1024 * 1024;
// write-buffer-count
//
// This is the total number of buffer slabs. This is calculated as a factor of
// file size in order to avoid dead lock.
size_t write_buffer_count() const {
assert(write_buffer_size);
return static_cast<size_t>((writer_qdepth + 1.2) * cache_file_size /
write_buffer_size);
}
// writer-dispatch-size
//
// The writer thread will dispatch the IO at the specified IO size
//
// default: 1M
uint64_t writer_dispatch_size = 1ULL * 1024 * 1024;
PersistentCacheOptions MakePersistentCacheOptions(
const std::string& path, const uint64_t size,
const std::shared_ptr<Logger>& log);
};
// Persistent Cache Tier
//
// This a logical abstraction that defines a tier of the persistent cache. Tiers
// can be stacked over one another. PersistentCahe provides the basic definition
// for accessing/storing in the cache. PersistentCacheTier extends the interface
// to enable management and stacking of tiers.
class PersistentCacheTier : public PersistentCache {
public:
typedef std::shared_ptr<PersistentCacheTier> Tier;
typedef std::map<std::string, double> TierStats;
virtual ~PersistentCacheTier() {}
// Open the persistent cache tier
virtual Status Open();
// Close the persistent cache tier
virtual Status Close();
// Flush the pending writes
virtual void Flush();
// Reserve space up to 'size' bytes
virtual bool Reserve(const size_t size);
// Erase a key from the cache
virtual bool Erase(const Slice& key);
// Print stats to string recursively
virtual std::string PrintStats();
// Expose stats
virtual std::vector<TierStats> Stats() = 0;
// Insert to page cache
virtual Status Insert(const Slice& page_key, const char* data,
const size_t size) = 0;
// Lookup page cache by page identifier
virtual Status Lookup(const Slice& page_key, std::unique_ptr<char[]>* data,
size_t* size) = 0;
// Return a reference to next tier
virtual Tier& next_tier() { return next_tier_; }
// Set the value for next tier
virtual void set_next_tier(const Tier& tier) {
assert(!next_tier_);
next_tier_ = tier;
}
private:
Tier next_tier_; // next tier
};
// PersistentTieredCache
//
// Abstraction that helps you construct a tiers of persistent caches as a
// unified cache. The tier(s) of cache will act a single tier for management
// ease and support PersistentCache methods for accessing data.
class PersistentTieredCache : public PersistentCacheTier {
public:
virtual ~PersistentTieredCache();
Status Open() override;
Status Close() override;
void Flush() override;
bool Erase(const Slice& key) override;
std::string PrintStats() override;
Status Insert(const Slice& page_key, const char* data,
const size_t size) override;
Status Lookup(const Slice& page_key, std::unique_ptr<char[]>* data,
size_t* size) override;
void AddTier(const Tier& tier);
Tier& next_tier() override {
auto it = tiers_.end();
return (*it)->next_tier();
}
void set_next_tier(const Tier& tier) override {
auto it = tiers_.end();
(*it)->set_next_tier(tier);
}
protected:
std::list<Tier> tiers_; // list of tiers top-down
};
} // namespace rocksdb
#endif