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Re: dd PATCH: add conv=direct

From: Bruce Allen
Subject: Re: dd PATCH: add conv=direct
Date: Wed, 7 Apr 2004 11:21:40 -0500 (CDT)

> > On modern Linux, apparently the correct way to bypass the buffer cache
> > when writing to a block device is to open the block device with
> > O_DIRECT.  This enables, for example, the user to more easily force a
> > reallocation of a single sector of an IDE disk with a media error
> > (without overwriting anything but the 1k "sector pair" containing the
> > error).  dd(1) is convenient for this purpose, but is lacking a method
> > to force O_DIRECT.  The enclosed patch adds a "conv=direct" flag to
> > enable this usage.
> This would be rather nice to have.  You'll need to ensure that the data
> is page-aligned in memory.
> While you're there, please add an fsync-before-closing option.

Andrew, am I right that this is NOT needed for the proposed O_DIRECT
option, since open(2) says: 
  "The I/O is synchronous, i.e., at the completion of the read(2) or
   write(2) system call, data is guaranteed to have been transferred."
so the write will block until data is physically on the disk.


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