dd命令是Linux下的一个非常有用的命令,可以指定大小的块拷贝文件并在拷贝的同时进行各种指定的转换。
使用举例
测试写
dd if=/dev/zero of=/infokist/test/data1 bs=1M count=50000
/dev/zero 是一个输入设备,可以无穷尽的提供0,用来向设备或者文件中写入字符串0
dd if=/dev/zero of=/infokist/test/data2 bs=1M count=100000 oflag=direct
oflag=direct表明直接调用I/O接口进行磁盘读写,不通过系统缓存,相对上面的方式,可以更准确的测出磁盘的写入速度。
测试读
测试磁盘读能力
time dd if=/dev/sdb of=/dev/null bs=4k
/dev/null 是空设备,也成为位桶,外号无底洞,任何写入它的输出都会被抛弃,如果不想让消息以标准输出显示或者写入文件,就可以将消息重定向到/dev/null
同样的,可以通过直接打开direct开关,不通过系统缓存,更准确的测出磁盘的读取速度
time dd if=/dev/sdb of=/dev/null bs=4k oflag=direct
注:bs是一次IO读写的规模,count是读多少个bs,也可以写成count=16G
命令说明
名称: dd
使用权限: 所有使用者
使用方法:dd [操作] ...或者 dd 选项
功能说明:复制文件,根据指定操作进行转换和格式化。
bs=BYTES read and write up to BYTES bytes at a time (default: 512);
overrides ibs and obs
cbs=BYTES convert BYTES bytes at a time
conv=CONVS convert the file as per the comma separated symbol list
count=N copy only N input blocks
ibs=BYTES read up to BYTES bytes at a time (default: 512)
if=FILE read from FILE instead of stdin
iflag=FLAGS read as per the comma separated symbol list
obs=BYTES write BYTES bytes at a time (default: 512)
of=FILE write to FILE instead of stdout
oflag=FLAGS write as per the comma separated symbol list
seek=N skip N obs-sized blocks at start of output
skip=N skip N ibs-sized blocks at start of input
status=LEVEL The LEVEL of information to print to stderr;
'none' suppresses everything but error messages,
'noxfer' suppresses the final transfer statistics,
'progress' shows periodic transfer statistics
N and BYTES may be followed by the following multiplicative suffixes:
c =1, w =2, b =512, kB =1000, K =1024, MB =1000*1000, M =1024*1024, xM =M,
GB =1000*1000*1000, G =1024*1024*1024, and so on for T, P, E, Z, Y.
CONV参数如下:
ascii from EBCDIC to ASCII
ebcdic from ASCII to EBCDIC
ibm from ASCII to alternate EBCDIC
block pad newline-terminated records with spaces to cbs-size
unblock replace trailing spaces in cbs-size records with newline
lcase change upper case to lower case
ucase change lower case to upper case
sparse try to seek rather than write the output for NUL input blocks
swab swap every pair of input bytes
sync pad every input block with NULs to ibs-size; when used
with block or unblock, pad with spaces rather than NULs
excl fail if the output file already exists
nocreat do not create the output file
notrunc 不截断输出文件
noerror 读取数据发生错误后仍然继续
fdatasync 结束前将输出文件数据写入磁盘
fsync 类似上面,但是元数据也一同写入
FLAG参数如下:
FLAG 符号可以是:
append 追加模式(仅对输出有意义;隐含了conv=notrunc)
direct 使用直接I/O 存取模式
directory 除非是目录,否则 directory 失败
dsync 使用同步I/O 存取模式
sync 与上者类似,但同时也对元数据生效
fullblock 为输入积累完整块(仅iflag)
nonblock 使用无阻塞I/O 存取模式
noatime 不更新存取时间
nocache Request to drop cache. See also oflag=sync
noctty 不根据文件指派控制终端
nofollow 不跟随链接文件
count_bytes treat 'count=N' as a byte count (iflag only)
skip_bytes treat 'skip=N' as a byte count (iflag only)
seek_bytes treat 'seek=N' as a byte count (oflag only)
dd命令执行过程中可以通过两种方式获取执行进度,一种是前面已经说过的,status参数,这里就不细说了,在dd命令的说明里还有一个方法,使用如下:
Sending a USR1 signal to a running 'dd' process makes it
print I/O statistics to standard error and then resume copying.
即周期性的发送USR1信号获取dd进度,自行处理即可。
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