What Linux bind mounts are really doing
Lots of Unixes have some form of 'loopback' mounts, where you can mount a bit of an existing filesystem somewhere else; they're called loopback mounts by analogy with the loopback interface.
The general idea behind them is that they are a more efficient (and easier to use) version of doing an NFS mount from localhost
.
Linux's bind mounts (so called because they are done with mount --bind
, or by specifying bind
as the filesystem type in /etc/fstab
) look like any other sort of loopback mounting. However, they actually operate in a way quite different from the usual idea of loopback mounting, and the difference has some important consequences.
What bind mounts are really doing is more or less mounting the filesystem again with a different inode as the root inode. Thus, if you do:
mount /dev/md1 /foo
mount --bind /foo/bar /bar
what you really have is /dev/md1 mounted twice, once with the root inode of the filesystem on md1 as the root of the mount point, and once with the inode for 'bar' in the root of the filesystem on md1 as the root of the mount point.
The mount
command makes this hard to see by being misleading in its output, reporting things like'/data/home on /home type none (rw,bind)
'.
Because they use /etc/mtab
, which mount
maintains, things like df
also report like this. More of the real state of affairs is visible in /proc/mounts
, where the kernel itself reports:
/dev/md5 /data ext3 rw,data=ordered 0 0
/dev/md5 /home ext3 rw,data=ordered 0 0
Unfortunately the kernel doesn't report that what root inode /home
is mounted with, which generally makes mount
's output more useful once you know what is really going on.
One consequence of this is that once you've set up your bind mounts, you can unmount the original mount point, something which I believe is not true of things like Solaris's loopback mounts (and which definitely wouldn't be true of NFS mounts from localhost). There might be a use for this in obscure situations.
Sidebar: Deeper under the hood
Disclaimer: I am not sure I understand this correctly.
Under the hood, there are two things: actual mounts of filesystems from devices (or the network), and namespace-based views of such filesystems. Rather than create new copies of both, bind mounts create new views ('mounts' or 'vfsmounts') of the same underlying mounted filesystem.
This explains one limitation of bind mounts, which is that you can't change mount flags when you do a bind mount (so you can't have a bind mount that is a read-only version of part of a read-write filesystem). Currently, all mount flags are associated with the filesystem, not with the view, so all views have to have the same mount flags.
What Linux bind mounts are really doing的更多相关文章
- docker从零开始 存储(三)bind mounts
使用bind mounts 自Docker早期以来bind mounts 一直存在.与volumes相比,绑定挂载具有有限的功能.使用bind mounts时,主机上的文件或目录将装入容器中.文件或目 ...
- Docker 基础知识 - 使用绑定挂载(bind mounts)管理应用程序数据
绑定挂载(bind mounts)在 Docker 的早期就已经出现了.与卷相比,绑定挂载的功能有限.当您使用绑定挂载时,主机上的文件或目录将挂载到容器中.文件或目录由其在主机上的完整或相对路径引用. ...
- docker 应用数据的管理之bind mounts
创建容器使用bind mounts 挂载文件系统.宿主机文件系统会覆盖掉容器里初始数据 [root@localhost ~]# mkdir /www/htpm -pv mkdir: 已创建目录 &qu ...
- Bind Mounts and File System Mount Order
When you use the bind option of the mount command, you must be sure that the file systems are m ...
- `docker数据持久化volume和bind mounts两种方式
将数据从宿主机到容器的三种方式: ,volumes:docker管理宿主机文件系统的一部分(/var/lib/docker/volumes)保存数据的最佳方式 ,bind mounts 将宿主机上的任 ...
- Linux就这个范儿 第10章 生死与共的兄弟
Linux就这个范儿 第10章 生死与共的兄弟 就说Linux系统的开机.必须经过加载BIOS.读取MBR.Boot Loader.加载内核.启动init进程并确定运行等级.执行初始化脚本.启动内核模 ...
- linux namespace note
--------------------------------- from http://oldwiki.linux-vserver.org/Namespaces //开源不只是代码,还有思想 Na ...
- [转帖]Docker的数据管理(volume/bind mount/tmpfs)
Docker(十五)-Docker的数据管理(volume/bind mount/tmpfs) https://www.cnblogs.com/zhuochong/p/10069719.html do ...
- Linux mount 命令进阶
笔者在<Linux mount 命令>一文中介绍了 mount 命令的基本用法,本文我们接着介绍 mount 命令的一些高级用法,比如 bind mounts(绑定挂载)和 shared ...
随机推荐
- Python 解析式、生成器
标准库datetime datetime模块 对日期.时间.时间戳的处理 datetime类 类方法 today() 返回本地时区当前时间的datetime对象 now(tz=None) 返回当前时间 ...
- iOS-MPMoviePlayerController视频播放
视频MPMoviePlayerController 在iOS中播放视频可以使用MediaPlayer.framework种的MPMoviePlayerController类来完成,它支持本地 视频和网 ...
- 好工具必须SHOW出来! NGFW下一代防火墙性能评估利器:Safire !
2019-09-26 00:05:54 今天先起个头,后面陆续完善 NGFW下一代防火墙是什么? 我们要关注NGFW下一代防火墙的哪些指标? 为什么说NGFW的性能不好评估?现有的评估手段工具介绍? ...
- python全栈索引
书签 python基础 太白金星 TigerLee python基础一 pytcharm安装详细教程 python基础二 python基础数据类型 Python最详细,最深入的代码块小数据池剖析 深浅 ...
- Spring 使用单选按钮
模型层需要提供数据选项,设置错误信息 关键代码 @NotNull(message = "请选择性别") private String gender; 控制器层需要在显示视图前,通过 ...
- vue-cli webpack打包后加载资源的路径问题
vue项目,访问打包后的项目,输入路径后,页面加载空白.这时会有两类问题,都是路径问题. 1.一个是css,js,ico等文件加载不到,是目录里少了dist 打开页面时一片空白 解决办法: confi ...
- 下载GDB调试工具peda
命令: 1.git clone https://github.com/longld/peda.git ~/peda 2.echo "source ~/peda/peda.py" & ...
- 1234: 约瑟夫问题-输出最后的编号(Java)
WUSTOJ 1234: 约瑟夫问题-输出最后的编号 参考资料 约瑟夫问题--百度百科 Description n个人围成一圈,依次从1至n编号.从编号为1的人开始1至k报数,凡报数为k的人退出圈子, ...
- Fabric分支/版本切换问题
(以下示例是从 release-1.4 切换到 release-1.3) 首先将 $GOAPTH/src/github.com/hyperledger/ 下1.4版本的fabric-samples给删 ...
- Destination高级特性
一.组合队列 Composite Destinations 组合队列允许用一个虚拟的destination代表多个destinations.这样就可以通过composite destinations在 ...