ClickHouse/dbms/include/DB/Storages/MergeTree/DiskSpaceMonitor.h

#pragma once

#include <mutex>
#include <sys/statvfs.h>
#include <memory>
#include <boost/noncopyable.hpp>
#include <common/logger_useful.h>
#include <DB/Common/Exception.h>
#include <DB/IO/WriteHelpers.h>
#include <DB/Common/formatReadable.h>
#include <DB/Common/CurrentMetrics.h>


namespace CurrentMetrics
{
	extern const Metric DiskSpaceReservedForMerge;
}

namespace DB
{

namespace ErrorCodes
{
	extern const int CANNOT_STATVFS;
	extern const int NOT_ENOUGH_SPACE;
}


/** Determines amount of free space in filesystem.
  * Could "reserve" space, for different operations to plan disk space usage.
  * Reservations are not separated for different filesystems,
  *  instead it is assumed, that all reservations are done within same filesystem.
  */
class DiskSpaceMonitor
{
public:
	class Reservation : private boost::noncopyable
	{
	public:
		~Reservation()
		{
			try
			{
				std::lock_guard<std::mutex> lock(DiskSpaceMonitor::mutex);
				if (DiskSpaceMonitor::reserved_bytes < size)
				{
					DiskSpaceMonitor::reserved_bytes = 0;
					LOG_ERROR(&Logger::get("DiskSpaceMonitor"), "Unbalanced reservations size; it's a bug");
				}
				else
				{
					DiskSpaceMonitor::reserved_bytes -= size;
				}

				if (DiskSpaceMonitor::reservation_count == 0)
				{
					LOG_ERROR(&Logger::get("DiskSpaceMonitor"), "Unbalanced reservation count; it's a bug");
				}
				else
				{
					--DiskSpaceMonitor::reservation_count;
				}
			}
			catch (...)
			{
				tryLogCurrentException("~DiskSpaceMonitor");
			}
		}

		/// Change amount of reserved space. When new_size is greater than before, availability of free space is not checked.
		void update(size_t new_size)
		{
			std::lock_guard<std::mutex> lock(DiskSpaceMonitor::mutex);
			DiskSpaceMonitor::reserved_bytes -= size;
			size = new_size;
			DiskSpaceMonitor::reserved_bytes += size;
		}

		size_t getSize() const
		{
			return size;
		}

		Reservation(size_t size_)
			: size(size_), metric_increment(CurrentMetrics::DiskSpaceReservedForMerge, size)
		{
			std::lock_guard<std::mutex> lock(DiskSpaceMonitor::mutex);
			DiskSpaceMonitor::reserved_bytes += size;
			++DiskSpaceMonitor::reservation_count;
		}

	private:
		size_t size;
		CurrentMetrics::Increment metric_increment;
	};

	using ReservationPtr = std::unique_ptr<Reservation>;

	static size_t getUnreservedFreeSpace(const std::string & path)
	{
		struct statvfs fs;

		if (statvfs(path.c_str(), &fs) != 0)
			throwFromErrno("Could not calculate available disk space (statvfs)", ErrorCodes::CANNOT_STATVFS);

		size_t res = fs.f_bfree * fs.f_bsize;

		/// Heuristic by Michael Kolupaev: reserve 30 MB more, because statvfs shows few megabytes more space than df.
		res -= std::min(res, 30 * (1ul << 20));

		std::lock_guard<std::mutex> lock(mutex);

		if (reserved_bytes > res)
			res = 0;
		else
			res -= reserved_bytes;

		return res;
	}

	static size_t getReservedSpace()
	{
		std::lock_guard<std::mutex> lock(mutex);
		return reserved_bytes;
	}

	static size_t getReservationCount()
	{
		std::lock_guard<std::mutex> lock(mutex);
		return reservation_count;
	}

	/// If not enough (approximately) space, throw an exception.
	static ReservationPtr reserve(const std::string & path, size_t size)
	{
		size_t free_bytes = getUnreservedFreeSpace(path);
		if (free_bytes < size)
			throw Exception("Not enough free disk space to reserve: " + formatReadableSizeWithBinarySuffix(free_bytes) + " available, "
				+ formatReadableSizeWithBinarySuffix(size) + " requested", ErrorCodes::NOT_ENOUGH_SPACE);
		return std::make_unique<Reservation>(size);
	}

private:
	static size_t reserved_bytes;
	static size_t reservation_count;
	static std::mutex mutex;
};

}