SET READ AHEAD comments and questions

Henry Gabryjelski Henry.Gabryjelski at microsoft.com
Tue Jul 18 15:03:39 PDT 2006


* From the T10 Reflector (t10 at t10.org), posted by:
* "Henry Gabryjelski" <Henry.Gabryjelski at microsoft.com>
*
Microsoft has some questions regarding the current attempt to obsolete
the SET READ AHEAD command.  This command is currently exposed in
Windows XP (and later) via IOCTL_STORAGE_SET_READ_AHEAD and
IOCTL_DVD_SET_READ_AHEAD.
Our understanding is that this command was created for the first (1x)
DVD-ROM drives so that PC player software would be able to optimize the
drive's read-ahead cache for DVD playback.  In particular, most
read-ahead cache on drives at that time would read ahead linearly.
Unfortunately, if a VOB file transition skipped some space, the 1x drive
would be unable to continue to offer seamless playback without a method
for the PC player software to indicate this transition point.
Clearly this optimization is not required any longer for the current
DVD-ROM drive market, as speeds for the drives are now at least 8x read
speed for all drives and can maintain (or "catch up") seamless playback
without this optimization.  However, it seems that this optimization may
once again be required for any new format (i.e. the new blue laser disc
formats) which has non-CE hardware produced at 1x speed ("1x drive A/V
stream" issue).
Please help us to understand the reasons why this command has been
requested to become obsolete.  In addition, if the above "1x drives A/V
stream" issue is already mitigated by other factors, please include this
in the reply.
Thank you,
Henry Gabryjelski
Microsoft Corporation
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