1、下载voltdb后直接解压：

$ tar -zxvf LINUX-voltdb-ent-3.6.tar.gz

2、两点注意事项，切记切记：

1.1.

Disable Swapping

Swapping is an operating system feature that optimizes memory usage when running multiple processes. However, memory is a critical component of the VoltDB server process. Any contention for memory, including swapping, will have a very negative impact on performance and functionality.

We recommend using dedicated servers and disabling swapping when running the VoltDB database server process. Use the swapoff command to disable swapping on Linux systems. If swapping cannot be disabled for any reason, you can reduce the likelihood of VoltDB being swapped out by setting the kernel parameter vm.swappiness to zero.

voltdb是内存数据库，常驻内存非常重要，因此需要将交换分区(虚拟内存)完全关闭。可以使用swapoff命令关闭，如果无法使用swapoff命令，则将内核参数vm.swappiness修改为0。

1.2.

Turn off TCP segmentation offload and generic receive offload if cluster stability is a problem.

There is an issue where, under certain conditions, the use of TCP segmentation offload (TSO) and generic receive offload (GRO) can cause nodes to randomly drop out of a cluster. The symptoms of this problem are that nodes timeout — that is, the rest of the cluster thinks they have failed — although the node is still running and no other network issues (such as a network partition) are the cause.

Disabling TSO and GRO is recommended for any VoltDB clusters that experience such instability. The commands to disable offloading are the following, where N is replaced by the number of the ethernet card:

ethtool -K ethN tso off
ethtool -K ethN gro off

Note that these commands disable offloading temporarily. You must issue these commands every time the node reboots.

这个是使用集群时需要注意的事项。

3、如果是旧版本升级需要按以下顺序执行：

The process for upgrading VoltDB for a running database is as follows:

    Place the database in admin mode using the @Pause system procedure (or VoltDB Enterprise Manager).

    Perform a manual snapshot of the database (using @SnapShotSave).

    Shutdown the database (using @Shutdown).

    Upgrade VoltDB.

    Recompile the application catalog using the new version of VoltDB.

    Restart the database using the create option, the new catalog, and starting in admin mode (specified in the deployment file).

    Restore the snapshot created in Step #2 (using voltadmin restore).

    Return the database to normal operations (using voltadmin resume).

4、将voltdb添加至环境变量中.

JAVA_HOME=/opt/jdk1.7.0_40
VOLTDB_HOME=/opt/voltdb-3.5.0.1
PATH=$PATH:$HOME/bin:$JAVA_HOME/bin:$VOLTDB_HOME/bin

export JAVA_HOME VOLTDB_HOME PATH

5、以下开始学习使用简单的voltdb命令。

先创建一个指定目录：helloworld

定义表结构:

CREATE TABLE HELLOWORLD (
   HELLO VARCHAR(15),
   WORLD VARCHAR(15),
   DIALECT VARCHAR(15) NOT NULL,
   PRIMARY KEY (DIALECT)
);

decoder中需要使用byte[]接收数据并转为protobuf对象。

本来考虑的是使用netty自带的缓存功能或直接在每个decoder中分配一个byte[]。这时想起来了fastjson的作者提过使用的线程绑定的byte[]功能。

查找到源代码，摘出来用。也感谢fastjson的作者开源代码。

package cn.joylab.game.util;

import java.lang.ref.SoftReference;

/**
 * 提供线程绑定数组缓存功能。
 * 
 * 可以避免频繁创建byte数组，最大长度限制为128k，超过128k的数据不进行缓存。
 * 
 * 用例1：decoder.
 * 
 * @author ZhuKunqian
 * 
 */
public class ThreadLocalCache {
	private final static int BYTE_CACHE_INIT_SIZE = 1024;
	private final static int BYTE_CACHE_MAX_SIZE = 1024 * 128;
	private static final ThreadLocal<SoftReference<byte[]>> byteBufLocal = new ThreadLocal<SoftReference<byte[]>>();

	public static void clearCache() {
		byteBufLocal.set(null);
	}

	public static byte[] getBytes(int length) {
		SoftReference<byte[]> ref = byteBufLocal.get();
		if (ref == null) {
			return allocateBytes(length);
		}

		byte[] bytes = ref.get();
		if (bytes == null) {
			return allocateBytes(length);
		}
		if (bytes.length < length) {
			bytes = allocateBytes(length);
		}
		return bytes;
	}

	private static int getAllocateLength(int init, int max, int length) {
		int value = init;
		while (true) {
			if (value >= length) {
				return value;
			}
			value *= 2;
			if (value > max) {
				break;
			}
		}
		return length;
	}

	private static byte[] allocateBytes(int length) {
		int allocateLength = getAllocateLength(BYTE_CACHE_INIT_SIZE,
				BYTE_CACHE_MAX_SIZE, length);
		if (allocateLength <= BYTE_CACHE_MAX_SIZE) {
			byte[] bytes = new byte[allocateLength];
			byteBufLocal.set(new SoftReference<byte[]>(bytes));
			return bytes;
		}
		return new byte[length];
	}
}

今天仔细看了下voltdb，全内存数据库。

在新项目选择上，主要考查了3个数据库mysql,mongodb,voltdb。

mysql就不需要多说了，传统的关系数据库，而且用了多年，也比较熟悉。

mongodb，nosql数据库，使用二进制json格式保存，支持sql语句。

voltdb，关系数据库，也可当做nosql数据库来用，全内存，支持sql语句。因为是全内存操作，所以速度是非常快的。

mysql,mangodb需要读写磁盘，所以相对要慢一些。但voltdb官方推荐是4G内存启动。虽然也可以1G内存启动起来，会有会有其它后果还不清楚。而且还有一个很恶心的问题，voltdb不像mysql那样或以多个database启动，一个voltdb实例，只是一个database。这还不算问题，如果要在一台机器上开启20个database，则无法像官方所说的那样，按4G内存分配。估计每个database只能分配1G。

但voltdb可以当作一个jar嵌入到工程中，这样可以避免线程中的数据传输，又是一个好处。

voltdb也提供了jdbc driver，因此新项目中可以尝试使用voltdb，如果发现有解决不了的问题，可以直接切换至mysql.