Transfer Rate Disk Post: 302505635

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Top Forums UNIX for Dummies Questions & Answers Transfer Rate Disk Post 302505635 by Corona688 on Thursday 17th of March 2011 01:16:04 PM

03-17-2011

Registered User

dd is a general purpose data moving command. It's sometimes used for moving data from one raw disk to another but it's a flexible command and doesn't care what kind of files or devices it's talking to.

dd by itself just reads stdin and writes back to stdout, so dd < filename > newfile is effectively the same thing as cp.

It does it in blocks of 512 bytes by default, which is a bit inefficient, so dd bs=1048576 < filename > newfile may be much faster. Beyond a certain point making the buffer bigger doesn't help.

So all I'm doing is having dd read from one file and write to another, and while it's running, sending it the signal SIGUSR1 which casues it to print statistics. Or you could wait for it to end, or kill it with ctrl-C, and it'll print statistics as it ends.

cache effects are when the OS doesn't immediately store what you've written to file on disk, just in memory. This is great for programs since they don't have to wait for the disk to catch up but troublesome for benchmarks. dd in linux supports the oflag=sync option to tell it not to do that.

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LEARN ABOUT V7

hp

HP(4)							     Kernel Interfaces Manual							     HP(4)

NAME

       hp - RH-11/RP04, RP05, RP06 moving-head disk

DESCRIPTION

       The octal representation of the minor device number is encoded idp, where i is an interleave flag, d is a physical drive number, and p is a
       pseudodrive (subsection) within a physical unit.  If i is 0, the origins and sizes of the pseudodisks on each drive, counted  in  cylinders
       of 418 512-byte blocks, are:

	    disk start	   length
	    0	 0    23
	    1	 23   21
	    2	 0    0
	    3	 0    0
	    4	 44   386
	    5	 430  385
	    6	 44   367
	    7	 44   771

       If  i  is  1, the minor device consists of the specified pseudodisk on drives numbered 0 through the designated drive number.  Successively
       numbered blocks are distributed across the drives in rotation.

       Systems distributed for these devices use disk 0 for the root, disk 1 for swapping, and disk 4 (RP04/5) or disk 7 (RP06) for a mounted user
       file system.

       The  block  files  access  the disk via the system's normal buffering mechanism and may be read and written without regard to physical disk
       records.

       A `raw' interface provides for direct transmission between the disk and the user's read or write buffer.   A  single  read  or  write  call
       results	in  exactly  one I/O operation and therefore raw I/O is considerably more efficient when many words are transmitted.  The names of
       the raw files conventionally begin with an extra `r.'  In raw I/O the buffer must begin on a word boundary, and raw I/O to  an  interleaved
       device is likely to have disappointing results.

FILES

       /dev/rp?, /dev/rrp?

SEE ALSO

       rp(4)

BUGS

       In  raw	I/O  read  and	write(2) truncate file offsets to 512-byte block boundaries, and write scribbles on the tail of incomplete blocks.
       Thus, in programs that are likely to access raw devices, read, write and lseek(2) should always deal in 512-byte multiples.

       Raw device drivers don't work on interleaved devices.

																	     HP(4)