Synchronous hard drive writes from shell script

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From - Thu Aug  5 12:24:25 1999
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From: "Radek Tomis" <rts@mediumsoft.cz>
Subject: Re: Disk write shell script
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> From: Fabio Giannotti <fabiog@venmar.com>
> Sent: Thursday, August 05, 1999 12:51 AM

> Is there a way using bourne shell to write directly to the hard disk?
>
> That is to say, if you write a "regular" shell script that writes 100
> lines to a file, it may only hit the disk 1 or 2 times.  I assuming it's
> buffering or caching a bunch, then writing a bunch all at once.
>
> What I want is for each line written to "go right to disk" and not be
> buffered/cached.
>
> Any ideas?

If you want transparent synchronous output, then download this 35 KB binary:

  http://members.xoom.com/rts101/tmp/fildes

and use it inside (or outside) of your script in one or more of the
following ways:

1. Inside your script for individual output commands:

     fildes 1=~,A+S : echo "line" >> /tmp/file

   This will append "line" to "/tmp/file" synchronously (i.e. the line will
   be physically written to the disk drive before the command 'echo'
   returns).

   Consider the following example using different output command:

     fildes 1=~,S : tail -300 /usr/adm/messages > /tmp/file

   This will synchronously append last 300 lines of "/usr/adm/messages"
   to "/tmp/file. However, 'tail' (and almost all other commands/utilities)
   uses user-level standard buffered I/O (from standard C library), and if
   the output is directed to something other than a terminal device, then
   the output is buffered by constant block size, usually 1 KB (depends on
   implementation). So, the 'tail' above actually writes (calls kernel
   routine write(S)) to "/tmp/file" only once per 1 KB (no matter whether
   this happens to be in the middle of the read line or not). In this case,
   each such 1K block gets written synchronously (the 1K block physically
   appears on the disk drive before the write(S) call returns).


2. Inside your script for several output commands at once:

     fildes 1=/tmp/file,A+S : sh -c '(
       set -- comp unix sco misc
       echo "`date`: arguments:"
       for arg
       do
         echo "  arg: ${arg}"
         sleep 1    # some processing
       done
     )'

   This will append several lines to "/tmp/file". Each line will be written
   synchronously (the line will be physically written to the disk drive
   before the command 'echo' returns).


3. Outside of your script for the entire script:

     fildes 1=/tmp/file,S : <your_script>

   This will run your script with standard output redirected to "/tmp/file".
   Every standard output from your script (and all its children, i.e. all
   external commands used within your script) will be synchronous, i.e. all
   underlying write(S) calls completes after all data in the given write(S)
   call have been physically written to the disk drive.


You can also use standard system command 'sync' after each line output,
however, it will write out not only this last line to the disk, but also all
other dirty file system blocks from system memory buffers for all file
systems. Depending on the recent write activity in the system, it may of
course take much longer than actually needed for your one-line write.

--
Radek Tomis
rts@mediumsoft.cz

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