[mythtv-users] OT: best way to connect IDE harddrives and dvd

Brian Wood beww at beww.org
Mon Sep 18 15:29:21 UTC 2006

On Sep 18, 2006, at 12:53 AM, chris at cpr.homelinux.net wrote:

> On Sun, Sep 17, 2006 at 09:18:13AM -0400, Matt Mossholder wrote:
>> 	I mostly agree, but one point to mention is that it is typically  
>> a good
>> idea to make your swap partition be located at the outer edge of the
>> disk, where the rotational speed of the disk give the most  
>> advantage to
>> throughput.
> There are a lot of arguments to be made for and against each
> partition layout strategy.  In the days of ST-506 drives it made a
> lot of sense to benchmark and shuffle until you got good results,
> but these days you can do more harm than good.
> It's true that on modern drives the outer cylinders have more
> sectors per track than the inner cylinders.  That doesn't mean that
> a specific sector will be read any sooner, however, as the disk
> doesn't spin any faster out there.  At 10,000 RPM you will need, on
> average, 3 milliseconds to read any random sector *anywhere on the
> disk*, assuming the head is already on the right track.  In most
> cases, head seek and settle times will swamp rotation latency as it
> will take substantially longer than 3 milliseconds to find the
> right track (typically 3ms to move to an adjacent track, 8-12ms to
> move to a random track, and 20+ms to move from one edge of the
> drive to the other).  For example, if the drive was reading from an
> inner track and suddenly needs to get data from swap on the outer
> track, the fact that it can read data faster out there is of no
> benefit if the drive has to seek the full width of the platter
> first.  There's one school of thought that says swap should be as
> close to the center cylinder as possible so that seek times are
> minimized.  That same line of thought says that the filesystems
> containing /bin and /usr/bin should be next to swap near the middle
> of the drive, with /var and /home distributed on either side.
> It really depends on whether you are expecting long accesses or
> short accesses.  The long accesses (sequential reads from a
> contiguous file) will get the most benefit from being on the outer
> cylinder.
> In the long run, however, there are three points that make all that
> "optimizing" a moot point:
> (1) Most drives and/or drivers do command buffering and optimize
> based on real-time requirements.  For example, if the drive heads
> are on an inner track and there are 10 requests pending (including
> a read from the swap file on the outer track), the swap access may
> be the last task executed even if it was the second task in the
> queue because *on average* the best performance occurs when the
> heads don't change direction.
> (2) Unlike the ST-506 days, the CHS values reported by IDE drives
> are totally fake.  Telling fdisk that you want a partition to be in
> the middle of the drive doesn't actually mean it will be on the
> middle cylinder.
> (3) Drives do sector-substitution without your knowledge.  Three
> sequential LBA blocks are not always going to be in sequential
> sectors on the drive.

All true, and (4) Linux will do its own smart caching of oft-accessed  
info in any case.

These days even assembly code is so far removed from the physical  
platters that trying to optimize based on drive geometry etc. is  
pretty much wasted effort, and may actually work against you.

BTW - wasn't that ST-502 not 506? or is my early-onset Alzhiemer's  
kicking in ? I assume you meant the interface with the two separate  
cables for control and data, sometimes mistakenly called "MFM", which  
is an encoding method not an interface.

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