dd命令详解！

dd命令功能很强大的，对于一些比较底层的问题，使用dd命令往往可以得到出人意料的效果。用的比较多的还是用dd来备份裸设备。但是不推荐，如果需要备份oracle裸设备，可以使用rman备份，或使用第三方软件备份，使用dd的话，管理起来不太方便。
建议 在有需要的时候使用dd 对物理磁盘操作，如果是文件系统的话还是使用tar backup cpio等其他命令更加方便。
另外，使用dd对磁盘操作时，最好使用块设备文件。

Description


The dd command reads the InFile parameter or standard input, does the

specified conversions, then copies the converted data to the OutFile

parameter or standard output. The input and output block size can be

specified to take advantage of raw physical I/O. Note: The term Block

refers to the quantity of data read or written by the dd command in one

operation and is not necessarily the same size as a disk block.


if=file 输入文件名　标准输入确省
of=file 输出文件名，标准输出确省
ibs=n 输入块大小，n字节（默认512）
obs=n 输出块大小,n字节（默认512）
bs=n 同时设置输入输出块大小
cbs=n 转换缓冲区大小
files=n 在中断之前拷贝和转换n个输入文件
skip=n 拷贝之前越过n个输入块
oseek=n 拷贝之前从输出文件开始查找n个块
iseek 拷贝之前从输入文件开始查找n个块
seek=n 等同于oseek
cono=ascic 将EBCDIC码转换为ASCII

ebcdic ASCII->;EBCDIC
ibm ASCII转换为EBCDIC码时轻微不同的映象

blolk 将新栈中断的ASCII码记录转换为固定长度

unblock 将固定长度的ASCII码记录转换为新行中断记录

lcase 变换字将至低等情况

ucase 变换字将至高等情况

swab 交换每对字节

noerrir 出错时不停止处理

sync 将每个块填充到ibs


输入设备：infile参数指定或标准输入
实施指定的转换，然后复制转换数据到 outfile 参数指定 或标准输出。
输入块和输出块大小可以自己指定，但必须是磁盘物理块大小的倍数。



1
Usually, you need only write access to the output file. However,

when the output file is not on a direct-access device and you use

the seek flag, you also need read access to the file.

2
The dd command inserts new-line characters only when converting


with the conv=ascii or conv=unblock flags set; it pads only when

converting with the conv=ebcdic, conv=ibm, or conv=block flags

set.

3
Use the backup, tar, or cpio command instead of the dd command

whenever possible to copy files to tape. These commands are

designed for use with tape devices. For more information on using
tape devices, see the rmt special file.

4
The block size values specified with the bs, ibs and obs flags

must always be a multiple of the physical block size for the media

being used.

参数说明：
bs=BlockSize

Specifies both the input and output block size, superseding the

ibs and obs flags. The block size values specified with the bs

flag must always be a multiple of the physical block size for the

media being used.
同时指定输入输出块大小，代替ibs和obs参数，块大小必须是 物理块的倍数。


cbs=BlockSize

Specifies the conversion block size for variable-length to fixed-

length and fixed-length to variable-length conversions, such as

conv=block.
用于转换缓冲区大小

count=InputBlocks

Copies only the number of input blocks specified by the

InputBlocks variable.
只复制由count参数指定数量的输入块

conv= Conversion,....

Specifies one or more conversion options. Multiple conversions

should be separated by commas. The following list describes the

possible options:

ascii

Converts EBCDIC to ASCII. This option is incompatible with

the ebcdic, ibm, block, and unblock options.

block


Converts variable-length records to fixed-length. The

length is determined by the conversion block size (cbs).

This option is incompatible with the ascii, ebcdic, ibm,

and unblock options.

ebcdic

Converts ASCII to standard EBCDIC. This option is

incompatible with the ascii, ibm, block, and unblock

options.

ibm

Converts ASCII to an IBM version of EBCDIC. This option is

incompatible with the ascii, ebcdic, block, and unblock


options.

iblock, oblock

Minimize data loss resulting from a read or write error on


direct access devices. If you specify the iblock variable

and an error occurs during a block read (where the block

size is 512 or the size specified by the ibs=InputBlockSize

variable), the dd command attempts to reread the data block

in smaller size units. If the dd command can determine the

sector size of the input device, it reads the damaged block

one sector at a time. Otherwise, it reads it 512 bytes at a

time. The input block size ( ibs) must be a multiple of

this retry size. This option contains data loss associated

with a read error to a single sector. The oblock conversion


works similarly on output.

lcase

Makes all alphabetic characters lowercase.

noerror

Does not stop processing on an error.

notrunc

Does not truncate the output file. Instead, blocks not

explicitly written to output are preserved.

ucase

Makes all alphabetic characters uppercase.

swab

Swaps every pair of bytes.

sync


Pads every input block to the ibs value.

unblock

Converts fixed-length blocks to variable-length. The length

is determined by the conversion block size (cbs). This

option is incompatible with the ascii, ebcdic, ibm, and

block options.
files=InputFiles

Copies the number of files specified by the InputFiles variable

value of input files before ending (makes sense only where input


is a magnetic tape or similar device).
复制由inputfile指定的文件，在中断之前，对磁带设备有意义


fskip=SkipEOFs

Skips past the number of end-of-file characters specified by the

SkipEOFs variable before starting to copy; this SkipEOFs variable

is useful for positioning on multifile magnetic tapes.
跳过磁带上的多个记录

ibs=InputBlockSize

Specifies the input-block size; the default is 512 bytes or one

block. The block-size values specified with the ibs flag must

always be a multiple of the physical block size for the media

being used.
同bs参数，但如果bs参数指定，就不需要再使用ibs参数


if=InFile

Specifies the input file name; standard input is the default.


obs=OutputBlockSize

Specifies the output-block size; the default is 512 bytes or one

block. The block size values specified with the obs flag must

always be a multiple of the physical block size for the media

being used.
指定输出块大小


of=OutFile


Specifies the output file name; standard output is the default.


seek=RecordNumber

Seeks the record specified by the RecordNumber variable from the

beginning of output file before copying.
在复制到of文件之前先查找由recordnumber指定的记录，一个记录的大小由bs参数指定，
注意是在of 上查找



skip=SkipInputBlocks

Skips the specified SkipInputBlocks value of input blocks before

starting to copy.
在复制之前跳过skipinputblocks指定的块，块大小由bs参数指定。
注意是在if上跳过

span=yes|no

Allows spanning across devices if specified yes and works as

default if specified as no. See Spanning Across Devices, for more

information..

以下列几个例子：

标准输入到标准输出
F50:[/]#echo "123" | dd
123
0+1 records in.
0+1 records out.

先创建几个裸设备
F50:[/]#mkvg -y datavg hdisk1
Datavg
F50:[/]#mklv -y raw -t raw datavg 5
raw
F50:[/]#lsvg -l datavg
datavg:
LV NAME
TYPE
LPs
PPs
PVs
LV STATE
MOUNT POINT
raw
raw
5
5
1
closed/syncd
N/A
F50:[/]#mklv -y raw1 -t raw datavg 5
raw1
查看逻辑卷lvcb块信息，标准输入为逻辑卷raw，输出到标准输出
以下块大小为512字节，只复制1个块
F50:[/]#dd if=/dev/rraw count=1 | od -x
1+0 records in.
1+0 records out.
0000000
4149 5820 4c56 4342 0000 7261 7700 0000
0000020
0000 0000 0000 0000 0000 0000 0000 0000
0000040
0000 0000 0000 0000 0000 3030 3061 6637
0000060
3064 3030 3030 3463 3030 3030 3030 3031
0000100
3061 0072 6177 0000 0000 0000 0000 0000
0000120
0000 0000 0000 0000 0000 0000 0000 0000
*
0000200
0000 0054 6875 2041 7072 2032 3720 3134
0000220
3a35 343a 3539 2032 3030 360a 0000 0000
0000240
0054 6875 2041 7072 2032 3720 3134 3a35
0000260
343a 3539 2032 3030 360a 0000 0000 0041
0000300
4637 3044 3443 3030 0079 6d6d 0079 0020
0000320
0005 0001 4e6f 6e65 0000 0000 0000 0000
0000340
0000 0000 0000 0000 0000 0000 0000 0000
*
0000720
0000 0000 0000 0000 0000 6461 3230 3066
0000740
3233 2e31 0000 0000 0000 0000 0000 0000
0000760
0000 0000 0000 0000 0000 0000 0000 0000
0001000
F50:[/]#dd if=/dev/raw bs=1 skip=80 count=8 |od –x
跳过80个字节且复制8个字节
0000000
0000
0000002
0000 0000 0000
0000010
8+0 records in.
8+0 records out.

使用dd在裸设备之间进行数据复制
F50:[/]#dd if=/dev/raw count=1
逻辑卷raw的 lvcb信息
AIX LVCBraw000af70d00004c000000010arawThu Apr 27 14:54:59 2006
Thu Apr 27 14:54:59 2006
AF70D4C00ymmy Noneda200f23.1
1+0 records in.
1+0 records out.

F50:[/]#dd if=/dev/raw1 count=1
逻辑卷raw1的lvcb信息
AIX LVCBraw000af70d00004c000000010araw1Thu Apr 27 14:55:22 2006
Thu Apr 27 14:55:22 2006
AF70D4C00ymmy Noneda200f23.2
1+0 records in.
1+0 records out.

F50:[/]#dd if=/dev/raw1 skip=1 count=1
第2个块没有信息
1+0 records in.
1+0 records out.

跳过raw1的第1个块，将raw的第lvcb信息复制到raw1的第2个块
F50:[/]#dd if=/dev/raw count=1 of=/dev/raw1 seek=1

1+0 records in.
1+0 records out.
F50:[/]#dd if=/dev/raw1 skip=1 count=1
AIX LVCBraw000af70d00004c000000010arawThu Apr 27 14:54:59 2006
Thu Apr 27 14:54:59 2006
AF70D4C00ymmy Noneda200f23.1
1+0 records in.
1+0 records out.
F50:[/]#dd if=/dev/raw1 count=1
AIX LVCBraw000af70d00004c000000010araw1Thu Apr 27 14:55:22 2006
Thu Apr 27 14:55:22 2006
AF70D4C00ymmy Noneda200f23.2
1+0 records in.
1+0 records out.


使用dd破坏lvcb信息，以此 验证dd 顺序读取磁盘
F50:[/]#more pvid.txt
#!/usr/bin/ksh
pvid=$1
disk=$2
set -A a `echo $pvid|\
awk ' {
for (f=1; f <= length($0); f=f+2) {
print "ibase=16\nobase=8\n"toupper(substr($0,f,2))
}
}'|\
bc 2>/dev/null`
/usr/bin/echo "\0"${a[0]}"\0"${a[1]}"\0"${a[2]}"\0"${a[3]}"\0"\
${a[4]}"\0"${a[5]}"\0"${a[6]}"\0"${a[7]}"\0\0\0\0\0\0\0\0\c"|\
dd bs=1 seek=128 of=/dev/$disk
F50:[/]#ls -al pvid.txt
-rw-r-----
1 root
system
341 Apr 21 18:07 pvid.txt
由于该文件只有341个字节，可以考虑大点的文件操作
F50:[/]#dd if=/pvid.txt bs=1024 of=/dev/raw
0+1 records in.
0+1 records out.
F50:[/]#dd if=/dev/raw count=1
#!/usr/bin/ksh
pvid=$1
disk=$2
set -A a `echo $pvid|\
awk ' {
for (f=1; f <= length($0); f=f+2) {
print "ibase=16\nobase=8\n"toupper(substr($0,f,2))
}
}'|\
bc 2>/dev/null`
/usr/bin/echo "\0"${a[0]}"\0"${a[1]}"\0"${a[2]}"\0"${a[3]}"\0"\
${a[4]}"\0"${a[5]}"\0"${a[6]}"\0"${a[7]}"\0\0\0\0\0\0\0\0\c"|\
dd bs=1 seek=128 of=/dev/$diskda200f23.1
1+0 records in.
1+0 records out.
F50:[/]#lsvg -l datavg
datavg:
LV NAME
TYPE
LPs
PPs
PVs
LV STATE
MOUNT POINT
raw
5
5
1
closed/syncd
N/A
raw1
raw
5
5
1
closed/syncd
N/A
由上看出 裸设备raw的lvcb信息已经被破坏，在下次importvg的时候将出现报警信息。
F50:[/]#importvg -y datavg hdisk1
0516-622 synclvodm: Warning, cannot write lv control block data.
Datavg
这在许多数据库中 经常出现这种情况，数据库会直接去读写裸设备的前512个字节，所以就容易导致出现以上的错误，但是并不影响数据库的运行。
可以再次使用dd 将其从raw1上的第2块上复制回来，
F50:[/]#dd if=/dev/raw1 skip=1 count=1
AIX LVCBraw000af70d00004c000000010arawThu Apr 27 14:54:59 2006
Thu Apr 27 14:54:59 2006
AF70D4C00ymmy Noneda200f23.1
1+0 records in.
1+0 records out.
F50:[/]#dd if=/dev/raw1 skip=1 count=1 of=/dev/raw
1+0 records in.
1+0 records out.
F50:[/]#varyoffvg datavg
F50:[/]#exportvg datavg
F50:[/]#importvg -y datavg hdisk1
Datavg
没有出现报警信息


dd备份裸设备到磁带
需要注意的几个方面：
1，
磁带的块大小
2，
恢复的时候注意不要把lvcb覆盖
偷点懒，直接复制ibm的··
1.备份裸的逻辑卷的步骤：
(1)设定磁带备份设备的合适的块大小：
使用下述命令查看当前的块大小：
# tctl -f /dev/rmt0 status
使用下述命令设置块大小：
# chdev -l rmt0 -a block_size=<块的字节数>
(2)不经过软件压缩进行备份，使用下述命令：
# dd if=<逻辑卷名> of=/dev/rmt0 ibs=512 obs=<块的字节数> conv=sync
(3)备份并进行软件压缩，使用下述命令：
# dd if=<逻辑卷名> bs=512 | compress | dd of=/dev/rmt0 ibs=512 obs=<块的字节数> conv=sync
2.恢复备份的裸逻辑卷数据的步骤：
(1)在恢复备份的逻辑卷数据之前需要确定是否保留当前的逻辑卷控制块信息
(2)若保留当前的逻辑卷控制块信息，使用下面的命令恢复数据：
a.恢复未经过软件压缩的数据：
# dd if=/dev/rmt0 ibs=<块的字节数> obs=512 | dd of=/dev/<逻辑卷名> bs=512 skip=1 seek=1
b.恢复经过软件压缩的数据：
# dd if=/dev/rmt0 ibs=<块的字节数> obs=512 | uncompress | dd of=/dev/<逻辑卷名> bs=512 skip=1 seek=1
(3)若覆盖当前的逻辑卷控制块信息，使用下面的命令恢复数据：
a.恢复未经过软件压缩的数据：
# dd if=/dev/rmt0 of=/dev/<逻辑卷名> ibs=<块的字节数> obs=512
b.恢复经过软件压缩的数据：
# dd if=/dev/rmt0 ibs=<块的字节数> obs=512 | uncompress | dd of=/dev/<逻辑卷名> bs=512

转自 http://blog.chinaunix.net/u/16252/showart.php?id=94788