弱龄寄事外,委怀在琴书。这篇文章主要讲述NoSQL数据库之Memcached的认识及安装使用相关的知识,希望能为你提供帮助。
NoSQL数据库之Memcached的认识及安装使用,本章节从基本概念、工作原理、yum安装、编译安装等角度介绍 Memcached,让我们对
NoSQL 类数据库有基本的了解。
1.概述
1.1NoSQL介绍【NoSQL数据库之Memcached的认识及安装使用】NoSQL是对 Not Only SQL、非传统关系型数据库的统称。NoSQL一词诞生于1998年,2009年这个词汇被再次提出指非关系型、分布式、不提供ACID的数据库设计模式。随着互联网时代的到来,数据爆发式增长,数据库技术发展日新月异,要适应新的业务需求。而随着移动互联网、物联网的到来,大数据的技术中NoSQL也同样重要。
数据库排名:https://db-engines.com/en/ranking
NoSQL 分类
- Key-value Store k/v数据库
- 性能好 O(1) , 如: redis、memcached
- Document Store 文档数据库
- mongodb、CouchDB
- Column Store 列存数据库,Column-Oriented DB
- HBase、Cassandra,大数据领域应用广泛
- Graph DB 图数据库
- Neo4j
- Time Series 时序数据库
- InfluxDB、Prometheus
memcached 虽然没有像redis所具备的数据持久化功能,比如RDB和AOF都没有,但是可以通过做集群同步的方式,让各memcached服务器的数据进行同步,从而实现数据的一致性,即保证各memcached 的数据是一样的,即使有任何一台 memcached 发生故障,只要集群中有一台 memcached 可用就不会出现数据丢失,当其他memcached 重新加入到集群的时候,可以自动从有数据的memcached 当中自动获取数据并提供服务。
Memcached 借助了操作系统的 libevent 工具做高效的读写。libevent是个程序库,它将Linux的epoll、BSD类操作系统的kqueue等事件处理功能封装成统一的接口。即使对服务器的连接数增加,也能发挥高性能。memcached使用这个libevent库,因此能在Linux、BSD、Solaris等操作系统上发挥其高性能。
Memcached 支持最大的内存存储对象为1M,超过1M的数据可以使用客户端压缩或拆分报包放到多个key中,比较大的数据在进行读取的时候需要消耗的时间比较长,memcached 最适合保存用户的session实现session共享Memcached存储数据时, Memcached会去申请1MB的内存, 把该块内存称为一个slab, 也称为一个page。
Memcached 支持多种开发语言,包括:java,C,python,php,C#,Ruby,Perl等。
Memcached 官网:http://memcached.org/
1.3Memcached 和 Redis 比较
比较 项 | Redis | memcached |
支持的数据结构 | 哈希、列表、集合、有序集 | 纯kev-value |
持久化 | 有 | 无 |
高可用 | redis支持集群功能,可以实现主动复制,读写分离。官方也提供了sentinel集群管理工具,能够实现主从服务监控,故障自动转移,这一切,对于客户端都是透明的,无需程序改动,也无需人工介入 | 需要二次开发 |
存储value容量 | 最大512M | 最大1M |
内存分配 | 临时申请空间,可能导致碎片 | 预分配内存池的方式管理内存,能够省去内存分配时间 |
虚拟内存使用 | 有自己的VM机制,理论上能够存储比物理内存更多的数据,当数据超量时,会引发swap,把冷数据刷到磁盘上 | 所有的数据存储在物理内存里 |
网络模型 | 非阻塞IO复用模型,提供一些非KV存储之外的排序,聚合功能,在执行这些功能时,复杂的CPU计算,会阻塞整个IO调度 | 非阻塞IO复用模型 |
水平扩展 | redis cluster 可以横向扩展 | 暂无 |
多线程 | Redis6.0之前是只支持单线程 | Memcached支持多线程,CPU利用方面Memcache优于Redis |
过期策略 | 有专门线程,清除缓存数据 | 懒淘汰机制:每次往缓存放入数据的时候,都会存一个时间,在读取的时候要和设置的时间做TTL比较来判断是否过期 |
单机QPS | 约10W | 约60W |
源代码可读性 | 代码清爽简洁 | 代码清爽简洁可能是考虑了太多的扩展性,多系统的兼容性,代码不清爽 |
使用场景 | 复杂数据结构、有持久化、高可用需求、value存储内容较大 | 纯KV,数据量非常大,并发量非常大的业务 |
2.1内存分配机制应用程序运行需要使用内存存储数据,但对于一个缓存系统来说,申请内存、释放内存将十分频繁,非常容易导致大量内存碎片,最后导致无连续可用内存可用。
Memcached采用了Slab Allocator机制来分配、管理内存。
- Page:分配给Slab的内存空间,默认为1MB,分配后就得到一个Slab。Slab分配之后内存按照固定字节大小等分成chunk。
- Chunk:用于缓存记录k/v值的内存空间。Memcached会根据数据大小选择存到哪一个chunk中,假设chunk有128bytes、64bytes等多种,数据只有100bytes存储在128bytes中,存在少许浪费。Chunk最大就是Page的大小,即一个Page中就一个Chunk
- Slab Class:Slab按照Chunk的大小分组,就组成不同的Slab Class, 第一个Chunk大小为 96B的Slab为Class1,Chunk 120B为Class 2,如果有100bytes要存,那么Memcached会选择下图中SlabClass 2 存储,因为它是120bytes的Chunk。Slab之间的差异可以使用Growth Factor 控制,默认1.25。
2.2懒过期 Lazy Expirationmemcached不会监视数据是否过期,而是在取数据时才看是否过期,如果过期,把数据有效期限标识为0,并不清除该数据。以后可以覆盖该位置存储其它数据。
2.3LRU当内存不足时,memcached会使用LRU(Least Recently Used)机制来查找可用空间,分配给新记录使用。
2.4集群
- Memcached集群,称为基于客户端的分布式集群,即由客户端实现集群功能,即Memcached本身不支持集群。
- Memcached集群内部并不互相通信,一切都需要客户端连接到Memcached服务器后自行组织这些节点,并决定数据存储的节点。
3.1yum 安装
[root@CentOS84-IP08 ]#dnf info memcached
[root@CentOS84-IP08 ]#dnf -y install memcached
[root@CentOS84-IP08 ]#rpm -ql memcached
/etc/sysconfig/memcached
/usr/bin/memcached
/usr/bin/memcached-tool
/usr/lib/.build-id
/usr/lib/.build-id/25
/usr/lib/.build-id/25/9ffe16926831a542c5c1a8fbdfdf063429439e
/usr/lib/systemd/system/memcached.service
/usr/share/doc/memcached
/usr/share/doc/memcached/AUTHORS
/usr/share/doc/memcached/CONTRIBUTORS
/usr/share/doc/memcached/COPYING
/usr/share/doc/memcached/ChangeLog
/usr/share/doc/memcached/NEWS
/usr/share/doc/memcached/README.md
/usr/share/doc/memcached/new_lru.txt
/usr/share/doc/memcached/protocol-binary-range.txt
/usr/share/doc/memcached/protocol-binary.txt
/usr/share/doc/memcached/protocol.txt
/usr/share/doc/memcached/readme.txt
/usr/share/doc/memcached/storage.txt
/usr/share/doc/memcached/threads.txt
/usr/share/doc/memcached/tls.txt
/usr/share/man/man1/memcached-tool.1.gz
/usr/share/man/man1/memcached.1.gz
[root@CentOS84-IP08 ]#rpm -qa memcached
memcached-1.5.22-2.el8.x86_64
[root@CentOS84-IP08 ]#rpm -qi memcached
Name: memcached
Epoch: 0
Version: 1.5.22
Release: 2.el8
Architecture: x86_64
Install Date: Thu 14 Apr 2022 07:30:40 AM CST
Group: System Environment/Daemons
Size: 414767
License: BSD
Signature: RSA/SHA256, Thu 18 Jun 2020 01:57:20 AM CST, Key ID 05b555b38483c65d
Source RPM: memcached-1.5.22-2.el8.src.rpm
Build Date: Thu 18 Jun 2020 01:19:21 AM CST
Build Host: x86-02.mbox.centos.org
Relocations : (not relocatable)
Packager: CentOS Buildsys < bugs@centos.org>
Vendor: CentOS
URL: https://www.memcached.org/
Summary: High Performance, Distributed Memory Object Cache
Description :
memcached is a high-performance, distributed memory object caching
system, generic in nature, but intended for use in speeding up dynamic
web applications by alleviating database load.
[root@CentOS84-IP08 ]#cat /etc/sysconfig/memcached
PORT="11211"
USER="memcached"
MAXCONN="1024"
CACHESIZE="64"
OPTIONS="-l 127.0.0.1,::1"
# 查看yum安装的默认服务启动文件去除注释行后实际生效的信息
[root@CentOS84-IP08 ]#grep -Ev "^#|^$" /usr/lib/systemd/system/memcached.service
[Unit]
Description=memcached daemon
Before=httpd.service
After=network.target
[Service]
EnvironmentFile=/etc/sysconfig/memcached
ExecStart=/usr/bin/memcached -p $PORT -u $USER -m $CACHESIZE -c $MAXCONN $OPTIONS
PrivateTmp=true
ProtectSystem=full
NoNewPrivileges=true
PrivateDevices=true
CapabilityBoundingSet=CAP_SETGID CAP_SETUID CAP_SYS_RESOURCE
RestrictAddressFamilies=AF_INET AF_INET6 AF_UNIX
[Install]
WantedBy=multi-user.target
[root@CentOS84-IP08 ]#
# 查看yum安装的默认服务启动文件全部信息
[root@CentOS84-IP08 ]#cat /usr/lib/systemd/system/memcached.service
# Its not recommended to modify this file in-place, because it will be
# overwritten during upgrades.If you want to customize, the best
# way is to use the "systemctl edit" command to create an override unit.
#
# For example, to pass additional options, create an override unit
# (as is done by systemctl edit) and enter the following:
#
#[Service]
#Environment=OPTIONS="-l 127.0.0.1,::1"
[Unit]
Description=memcached daemon
Before=httpd.service
After=network.target
[Service]
EnvironmentFile=/etc/sysconfig/memcached
ExecStart=/usr/bin/memcached -p $PORT -u $USER -m $CACHESIZE -c $MAXCONN $OPTIONS
# Set up a new file system namespace and mounts private /tmp and /var/tmp
# directories so this service cannot access the global directories and
# other processes cannot access this services directories.
PrivateTmp=true
# Mounts the /usr, /boot, and /etc directories read-only for processes
# invoked by this unit.
ProtectSystem=full
# Ensures that the service process and all its children can never gain new
# privileges
NoNewPrivileges=true
# Sets up a new /dev namespace for the executed processes and only adds API
# pseudo devices such as /dev/null, /dev/zero or /dev/random (as well as
# the pseudo TTY subsystem) to it, but no physical devices such as /dev/sda.
PrivateDevices=true
# Required for dropping privileges and running as a different user
CapabilityBoundingSet=CAP_SETGID CAP_SETUID CAP_SYS_RESOURCE
# Restricts the set of socket address families accessible to the processes
# of this unit. Protects against vulnerabilities such as CVE-2016-8655
RestrictAddressFamilies=AF_INET AF_INET6 AF_UNIX
# Some security features are not in the older versions of systemd used by
# e.g. RHEL7/CentOS 7. The below settings are automatically edited at package
# build time to uncomment them if the target platform supports them.
# Attempts to create memory mappings that are writable and executable at
# the same time, or to change existing memory mappings to become executable
# are prohibited.
##safer##MemoryDenyWriteExecute=true
# Explicit module loading will be denied. This allows to turn off module
# load and unload operations on modular kernels. It is recommended to turn
# this on for most services that do not need special file systems or extra
# kernel modules to work.
##safer##ProtectKernelModules=true
# Kernel variables accessible through /proc/sys, /sys, /proc/sysrq-trigger,
# /proc/latency_stats, /proc/acpi, /proc/timer_stats, /proc/fs and /proc/irq
# will be made read-only to all processes of the unit. Usually, tunable
# kernel variables should only be written at boot-time, with the sysctl.d(5)
# mechanism. Almost no services need to write to these at runtime; it is hence
# recommended to turn this on for most services.
##safer##ProtectKernelTunables=true
# The Linux Control Groups (cgroups(7)) hierarchies accessible through
# /sys/fs/cgroup will be made read-only to all processes of the unit.
# Except for container managers no services should require write access
# to the control groups hierarchies; it is hence recommended to turn this
# on for most services
##safer##ProtectControlGroups=true
# Any attempts to enable realtime scheduling in a process of the unit are
# refused.
##safer##RestrictRealtime=true
# Takes away the ability to create or manage any kind of namespace
##safer##RestrictNamespaces=true
[Install]
WantedBy=multi-user.target
# 认识memcached 的一些常规信息
[root@CentOS84-IP08 ]#getent passwd memcached
memcached:x:975:974:Memcached daemon:/run/memcached:/sbin/nologin
[root@CentOS84-IP08 ]#systemctl enable --now memcached
Created symlink /etc/systemd/system/multi-user.target.wants/memcached.service → /usr/lib/systemd/system/memcached.service.
[root@CentOS84-IP08 ]#pstree -p |grep memcached
|-memcached(40702)-+-memcached(40703)
||-memcached(40704)
||-memcached(40705)
||-memcached(40706)
||-memcached(40707)
||-memcached(40708)
||-memcached(40709)
||-memcached(40710)
|`-memcached(40711)
[root@CentOS84-IP08 ]#ss -ntlup|grep memcached
tcpLISTEN 01024127.0.0.1:112110.0.0.0:*users:(("memcached",pid=40702,fd=27))
tcpLISTEN 01024[::1]:11211[::]:*users:(("memcached",pid=40702,fd=28))
# 修改短空监听到非127本地端口,确保其他的主机可以通过网络访问
[root@CentOS84-IP08 ]#vim /etc/sysconfig/memcached
PORT="11211"
USER="memcached"
MAXCONN="1024"
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