Messages in this thread |  | | Date | Tue, 25 Sep 2001 18:41:32 -0400 | Subject | Re: [PATCH] 2.4.10 improved reiserfs a lot, but could still be better | From | (bill davidsen) |
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In article <Pine.LNX.4.20.0109250820001.28393-100000@otter.mbay.net> jalvo@mbay.net wrote:
| There used to be stupid hard disk formatting tricks where the sector | numbers were interleaved | | 1001 1008 1002 1009 1003 1010 1004 1011 1005 1012 1006 1013 1007 1014 | | just to gain that enhancement. I also remember an ancient IBM Dasd trick | where the sectors were offset just slightly so that a track to track | switch could pick up the next sector in time. | | Ancient history...
The spacing between consecutively numbered sectors is called interleave, and the offset between cylinders is called skew.
Take a look at superformat, it's still done. The object is to take any media without hardware cache and match the timing of the media under the head with the need for data. This could easily double the transfer rate on any media which runs in an unbuffered reader.
And I would be surprised if modern hard drives don't allow using an offset between tracks so that when a track is read the next track "starts" under the read heads after the delay to step one track. This can save you the rotational latency of the drive, which on a 7200rpm drive is ~8.5ms, or as great as the step time. If actual track to track is matched to the step time you save that one rotation on big block reads, like swaps.
Guess it's not quite so ancient history after all, just done by the vendor typically. If I were writing a low level formatting routine, and I haven't in over a decade, I would run some tests on how much skew is needed to have the first sector under the head. The penelty for making that number a little too large is a sector or two, the penalty for making it too small is one rotation less a sector or two.
-- bill davidsen <davidsen@tmr.com> "If I were a diplomat, in the best case I'd go hungry. In the worst case, people would die." -- Robert Lipe - 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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