用DD命令制作硬盘镜像

用DD命令制作硬盘镜像

本文参考http://serverfault.com/questions/4906/using-dd-for-disk-cloning写出，转载时请说明出处。

以下的说明都是针对备份整个硬盘，而不是备份某一个分区。

一、用DD命令制作硬盘镜像的特点

(1)在制作镜像时，不能对需要进行备份的硬盘经常写操作，可以只读挂载或者不挂载。

(2)在制作镜像时，MBR、硬盘的分区表、bootloader也会被备份。

(3)生成的镜像文件用于恢复时，目标硬盘的容量必须等于或大于源硬盘的容量。

(4)使用硬盘镜像完成恢复后，由于目标硬盘的分区表跟源硬盘的分区表是一样的，所以会造成目标硬盘的空间浪费。这个问题可以通过使用硬盘分区大小调整工具解决。

(5)在dd生成或恢复镜像时，默认没有显示进度，但这个问题可以解决。解决方法请看下文。

二、备份和还原操作

(1)制作硬盘sdb的镜像文件sdb_backup.img：

$dd if=/dev/sdb of=~/sdb_backup.imgbs=32M

注：bs即blocksize，bs根根系统的内存大小和硬盘读写速度而设定

(2)将硬盘sdb的内容直接克隆到硬盘sdc中（要保证sdc的容量等于或大于sdb的容量）：

$dd if=/dev/sda of=/dev/sdbbs=32M

(3)需要备份的硬盘可能存在大量的空白区域（未用于存储数据的区域），如果用压缩工具压缩生成的镜像，可大大减小镜像的大小。

在制作硬盘sdb的镜像文件时就进行压缩：

$dd if=/dev/sdb| gzip -c > ~/sdb_backup.img.gz

将备份的镜像恢复到硬盘sdc中
$gunzip -c ~/sdb_backup.img.gz| dd of=/dev/sdc

(5)只备份硬盘的MBR

$dd if=/dev/sdb of=~/MBR_backup bs=512 count=1

(6)当使用dd进行镜像备份时，如dd发现某个sector（扇区）错误，默认会停止备份操作。这时可以 "conv=noerror,sync" to ensure that it doesn't stop whenit encounters an error, and fills in the missing sector(s) with null bytes.This is usually the first step I take if trying
to recover from a failed orfailing disk -- get a copy before doing any recovery attempts, and then dorecovery on the good (cloned) disk. I leave it to the recovery tool to copewith any blank sectors that couldn't be copied.

gunzip -c ~/sdb_backup.img.gz| dd of=/dev/sdc conv=noerror,sync
注意：

If you have a disk with bad sectors,you really should be using 'ddrescue' instead of dd. It's much more efficient,and has a much better chance of recovering more data. (Don't get it confusedwith dd_rescue, which is not as good)

If the source drive is damaged at all,you'll have more luck using

dd_rhelp

with

dd_rescue

(my personal preference) or GNU

ddrescue

.

The reason behind this is that, on readerrors,

dd

keeps trying and trying and trying - potentially waiting for along time for timeouts to occur.

dd_rescue

doessmart things like reading up to an error, then picking a spot further on on thedisk and reading backwards to the last error, and

dd_rhelp

isbasically a

dd_rescue

session manager - cleverly starting and resuming

dd_rescue

runsto make it quicker again.

The end result of

dd_rhelp

ismaximum data recovered in minimum time. If you leave

dd_rhelp

running, in the end it does the exact same job as

dd

inthe same time. However, if

dd

encountered
read errors at byte 100 of your 100Gb disk, you'd haveto wait a long time to recover the other 9,999,900 bytes*, whereas

dd_rhelp

+

dd_rescue

wouldrecover the bulk of the data much faster.

(5)显示制作操作或恢复操作的进度
参考A
Youcan follow the progression of the operation with :
$ddif=/dev/sda of=/dev/sdb & pid=$!

$kill-USR1 $pid; sleep 1; kill $pid

参考B
Youcan get a dd process running in the background to report status by sending it asignal with the kill command, e.g.:
$ddif=/dev/hdb of=/image.img &

$kill -SIGUSR11234

＃这里假设1234为备份进程号

参考C
The man page says: Sending a USR1 signal to a running ‘dd’process makes it print I/O statistics to standard error and then resumecopying.

I use this feature regularly.

Thisis kind of a cheap hack, but it's a quick and dirty way to monitor your DDprocess.
Runyour dd command. Open a new shell and do a ps awx to find your dd process' PID.Now in the new shell type watch -n 10 kill -USR1 {pid of your DD process}
Thiswill do nothing in the watch output window, but back in the original DD shell,DD will start outputting status reports every 10 seconds. You can change the -n10 in the watch command to any other time frame of
course.

OS X doesn't have watch available and -USR1 kills dd. The following command works though: while [ true ]; do killall -INFO dd; sleep 30; done