Making things run faster Post: 302248460

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Top Forums UNIX for Advanced & Expert Users Making things run faster Post 302248460 by shamrock on Friday 17th of October 2008 05:33:21 PM

10-17-2008

Registered User

Quote:

Originally Posted by Legend986

Added to that, I have a small question (not so sure if its silly though but can't seem to understand it completely)...

If I have four datasets like in the problem above and all I have to do is grep some text out of it, does it really make a difference doing the jobs parallely on all the datasets or doing them in a sequential order? In fact, to be more precise, the argument goes something like this:

Four datasets are stored on the disk. The CPU has to fetch some data everytime for the four processes to process them and write back to the disk. Now, if it has to provide data to all the four processes, then shouldn't the head keep moving around to provide the data as opposed to just one process where it just keeps reading the data (provided there is no fragmentation). As I said, I'm sorry if my question seems silly but just want to clear some basic concepts.

And that is the reason for caching data and striping it over multiple disks in order to reduce disk arm contention. This way reads/writes are done in parallel and with caching in play most reads/write are logical instead of physical. As you have terabytes of data I am assuming that all of it isn't on a single drive like a JBOD of some sort and that it is on a high end storage array with significant intelligence and caching built into it while being striped for performace and mirrored for availability.

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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)