Linux-kernel: is this patch horribly wrong? 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.
Alas, I don't do autoconf, so I hope someone else can add the appropriate stanza to autoconfiscate HAS_O_DIRECT. -andy diff -ur coreutils-5.0.91/doc/coreutils.texi coreutils-5.0.91-adi/doc/coreutils.texi --- coreutils-5.0.91/doc/coreutils.texi 2003-09-04 16:26:51.000000000 -0500 +++ coreutils-5.0.91-adi/doc/coreutils.texi 2004-04-06 13:48:54.000000000 -0500 @@ -6373,6 +6373,11 @@ Pad every input block to size of @samp{ibs} with trailing zero bytes. When used with @samp{block} or @samp{unblock}, pad with spaces instead of zero bytes. + [EMAIL PROTECTED] direct [EMAIL PROTECTED] direct +Open the output file with O_DIRECT, avoiding (on Linux) using the buffer +cache. @end table @end table diff -ur coreutils-5.0.91/src/dd.c coreutils-5.0.91-adi/src/dd.c --- coreutils-5.0.91/src/dd.c 2003-07-25 02:43:09.000000000 -0500 +++ coreutils-5.0.91-adi/src/dd.c 2004-04-06 13:44:09.000000000 -0500 @@ -66,6 +66,9 @@ /* Default input and output blocksize. */ #define DEFAULT_BLOCKSIZE 512 +/* XXX hack */ +#define HAVE_O_DIRECT 1 + /* Conversions bit masks. */ #define C_ASCII 01 #define C_EBCDIC 02 @@ -78,6 +81,7 @@ #define C_NOERROR 0400 #define C_NOTRUNC 01000 #define C_SYNC 02000 +#define C_DIRECT 010000 /* Use separate input and output buffers, and combine partial input blocks. */ #define C_TWOBUFS 04000 @@ -162,6 +166,9 @@ {"noerror", C_NOERROR}, /* Ignore i/o errors. */ {"notrunc", C_NOTRUNC}, /* Do not truncate output file. */ {"sync", C_SYNC}, /* Pad input records to ibs with NULs. */ +#ifdef HAVE_O_DIRECT + {"direct", C_DIRECT}, /* open files with O_DIRECT */ +#endif {NULL, 0} }; @@ -1190,6 +1197,11 @@ = (O_CREAT | (seek_records || (conversions_mask & C_NOTRUNC) ? 0 : O_TRUNC)); +#if HAVE_O_DIRECT + if (conversions_mask & C_DIRECT) + opts |= O_DIRECT; +#endif + /* Open the output file with *read* access only if we might need to read to satisfy a `seek=' request. If we can't read the file, go ahead with write-only access; it might work. */ _______________________________________________ Bug-coreutils mailing list [EMAIL PROTECTED] http://mail.gnu.org/mailman/listinfo/bug-coreutils