Messages in this thread | | | Date | Wed, 7 Apr 2004 09:42:22 -0700 | From | Andrew Morton <> | Subject | Re: dd PATCH: add conv=direct |
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Bruce Allen <ballen@gravity.phys.uwm.edu> wrote: > > > > 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.
A conv=fsync option is an irrelevant wishlist item. If you were to provide conv=fsync then dd should still fsync the file after performing the direct-io read or write.
You're correct that an fsync after a direct-io read or write should not need to perform any file data I/O. But it will often need to perform file metadata I/O - file allocation tables, inode, etc. direct-io only syncs the file data, not the file metadata. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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