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What is dd tool?
dd is a command-line utility for Unix and Unix-like operating systems, the primary purpose of which is to convert and copy files. As a result, dd can be used for tasks such as backing up the boot sector of a hard drive, and obtaining a fixed amount of random data.
How do you monitor dd?
How to show progress during a dd copy
- Determine the process-id (pid) for the running dd command: $ pgrep -l ‘^dd$’
- Send the USR1 signal to the pid:
- Switch to the terminal running dd and view the output:
- Use the ‘watch’ command to execute the USR1 kill every minute:
- Kill the watch when the copy has completed with CTRL-C.
When to use iflag and Oflag in DD?
This flag can be used only with ‘iflag’. ‘seek_bytes’ Interpret the ‘seek=’ operand as a byte count, rather than a block count, which allows specifying an offset that is not a multiple of the I/O block size. This flag can be used only with ‘oflag’.
Why is DD with the O _ DIRECT flag so?
There is a nice explanation about how copying a buffer to the kernel isn’t instantaneous and how page pressure can slow things down in an answer to the Linux async (io_submit) write v/s normal (buffered) write question.
What happens if you do not use Oflag direct?
If you do not use oflag=direct, then large amounts of writes can build up in the system page cache. This build-up will affect the progress you see [2]. But also, Linux sometimes responds badly to the build-up, and degrades performance for all devices [3].
What does sync and nocache do in DD?
Metadata (e.g., last-access and last-modified time) is not necessarily synchronized. ‘sync’ Use synchronized I/O for both data and metadata. ‘nocache’ Request to discard the system data cache for a file. When count=0 all cached data for the file is specified, otherwise the cache is dropped for the processed portion of the file.