Re: FILE_FLAG_WRITE_THROUGH

["Steve Graegert" <graegerts@gmail.com>]

On 4/5/06, Glynn Clements <glynn@gclements.plus.com> wrote:
>
> Rebelde wrote:
>
> > On Windows XP I can write directly to disk whitout cache write using the
> > CreateFile function with FILE_FLAG_WRITE_THROUGH. Could someone tell me
> > how to make this on Linux?
>
> Pass the O_SYNC flag to open(). This will cause write() calls to block
> until the data has been sent to the drive.
>
> The O_DIRECT flag suggested by Steve is probably overkill. It requires
> that the buffer start address, buffer size and file offset are all
> multiples of the filesystem's block size, and only works on some
> filesystems.

Well, the O_SYNC flag causes, i.e. write(2), to block until the data
has been physically written to disk, but it does not prevent the C
library/kernel from caching the bytes.  O_DIRECT, on the other hand,
circumvents the caching mechanisms and allows for raw access, although
it may degrade performance.  While sematically difficult, I think this
technique is nearest equivalent to the FILE_FLAG_WRITE_THROUGH flag
known from Windows.

	\Steve