dd命令功能很強大的，對於一些比較底層的問題，使用dd命令往往可以得到出人意料的效果。用的比較多的還是用dd來備份裸設備。但是不推薦，如果需要備份oracle裸設備，可以使用rman備份，或使用第三方軟件備份，使用dd的話，管理起來不太方便。
建議在有需要的時候使用dd 對物理磁盤操作，如果是文件系統的話還是使用tar backup cpio等其他命令更加方便。
另外，使用dd對磁盤操作時，最好使用塊設備文件。
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 輸入文件名 標准輸入確省
of=file 輸出文件名，標准輸出確省
ibs=n 輸入塊大小，n字節（默認512）
obs=n 輸出塊大小,n字節（默認512）
bs=n 同時設置輸入輸出塊大小
cbs=n 轉換緩沖區大小
files=n 在中斷之前拷貝和轉換n個輸入文件
skip=n 拷貝之前越過n個輸入塊
oseek=n 拷貝之前從輸出文件開始查找n個塊
iseek 拷貝之前從輸入文件開始查找n個塊
seek=n 等同於oseek
cono=ascic 將EBCDIC碼轉換為ASCII
ebcdic ASCII->;EBCDIC
ibm ASCII轉換為EBCDIC碼時輕微不同的映象
blolk 將新棧中斷的ASCII碼記錄轉換為固定長度
unblock 將固定長度的ASCII碼記錄轉換為新行中斷記錄
lcase 變換字將至低等情況
ucase 變換字將至高等情況
swab 交換每對字節
noerrir 出錯時不停止處理
sync 將每個塊填充到ibs
輸入設備：infile參數指定或標准輸入
實施指定的轉換，然後復制轉換數據到 outfile 參數指定或標准輸出。
輸入塊和輸出塊大小可以自己指定，但必須是磁盤物理塊大小的倍數。
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.
參數說明：
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.
同時指定輸入輸出塊大小，代替ibs和obs參數，塊大小必須是物理塊的倍數。
cbs=BlockSize
Specifies the conversion block size for variable-length to fixed-
length and fixed-length to variable-length conversions, such as
conv=block.
用於轉換緩沖區大小
count=InputBlocks
Copies only the number of input blocks specified by the
InputBlocks variable.
只復制由count參數指定數量的輸入塊
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).
復制由inputfile指定的文件，在中斷之前，對磁帶設備有意義
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.
跳過磁帶上的多個記錄
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參數，但如果bs參數指定，就不需要再使用ibs參數
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.
指定輸出塊大小
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.
在復制到of文件之前先查找由recordnumber指定的記錄，一個記錄的大小由bs參數指定，
注意是在of 上查找
skip=SkipInputBlocks
Skips the specified SkipInputBlocks value of input blocks before
starting to copy.
在復制之前跳過skipinputblocks指定的塊，塊大小由bs參數指定。
注意是在if上跳過
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..
以下列幾個例子：
標准輸入到標准輸出
F50:[/]#echo "123" | dd
123
0+1 records in.
0+1 records out.
先創建幾個裸設備
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
查看邏輯卷lvcb塊信息，標准輸入為邏輯卷raw，輸出到標准輸出
以下塊大小為512字節，只復制1個塊
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
跳過80個字節且復制8個字節
0000000
0000
0000002
0000 0000 0000
0000010
8+0 records in.
8+0 records out.
使用dd在裸設備之間進行數據復制
F50:[/]#dd if=/dev/raw count=1
邏輯卷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
邏輯卷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個塊沒有信息
1+0 records in.
1+0 records out.
跳過raw1的第1個塊，將raw的第lvcb信息復制到raw1的第2個塊
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破壞lvcb信息，以此驗證dd 順序讀取磁盤
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=16nobase=8n"toupper(substr($0,f,2))
}
}'|
bc 2>/dev/null`
/usr/bin/echo ""${a[0]}""${a[1]}""${a[2]}""${a[3]}""
${a[4]}""${a[5]}""${a[6]}""${a[7]}"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
由於該文件只有341個字節，可以考慮大點的文件操作
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=16nobase=8n"toupper(substr($0,f,2))
}
}'|
bc 2>/dev/null`
/usr/bin/echo ""${a[0]}""${a[1]}""${a[2]}""${a[3]}""
${a[4]}""${a[5]}""${a[6]}""${a[7]}"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
由上看出裸設備raw的lvcb信息已經被破壞，在下次importvg的時候將出現報警信息。
F50:[/]#importvg -y datavg hdisk1
0516-622 synclvodm: Warning, cannot write lv control block data.
Datavg
這在許多數據庫中經常出現這種情況，數據庫會直接去讀寫裸設備的前512個字節，所以就容易導致出現以上的錯誤，但是並不影響數據庫的運行。
可以再次使用dd 將其從raw1上的第2塊上復制回來，
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
沒有出現報警信息
dd備份裸設備到磁帶
需要注意的幾個方面：
1，
磁帶的塊大小
2，
恢復的時候注意不要把lvcb覆蓋
偷點懶，直接復制ibm的··
1.備份裸的邏輯卷的步驟：
(1)設定磁帶備份設備的合適的塊大小：
使用下述命令查看當前的塊大小：
# tctl -f /dev/rmt0 status
使用下述命令設置塊大小：
# chdev -l rmt0 -a block_size=<塊的字節數>
(2)不經過軟件壓縮進行備份，使用下述命令：
# dd if=<邏輯卷名> of=/dev/rmt0 ibs=512 obs=<塊的字節數> conv=sync
(3)備份並進行軟件壓縮，使用下述命令：
# dd if=<邏輯卷名> bs=512 | compress | dd of=/dev/rmt0 ibs=512 obs=<塊的字節數> conv=sync
2.恢復備份的裸邏輯卷數據的步驟：
(1)在恢復備份的邏輯卷數據之前需要確定是否保留當前的邏輯卷控制塊信息
(2)若保留當前的邏輯卷控制塊信息，使用下面的命令恢復數據：
a.恢復未經過軟件壓縮的數據：
# dd if=/dev/rmt0 ibs=<塊的字節數> obs=512 | dd of=/dev/<邏輯卷名> bs=512 skip=1 seek=1
b.恢復經過軟件壓縮的數據：
# dd if=/dev/rmt0 ibs=<塊的字節數> obs=512 | uncompress | dd of=/dev/<邏輯卷名> bs=512 skip=1 seek=1
(3)若覆蓋當前的邏輯卷控制塊信息，使用下面的命令恢復數據：
a.恢復未經過軟件壓縮的數據：
# dd if=/dev/rmt0 of=/dev/<邏輯卷名> ibs=<塊的字節數> obs=512
b.恢復經過軟件壓縮的數據：
# dd if=/dev/rmt0 ibs=<塊的字節數> obs=512 | uncompress | dd of=/dev/<邏輯卷名> bs=512
張宇加：
在對文件系統或物理存儲設備進行dd時，bs的大小不可太小，通常建議4MB或1MB，可以實際測試後確定。