Re: [9fans] copying fossil filesystem to a bigger disk

[dexen deVries <dexen.devries@gmail.com>]

On Tuesday 04 of October 2011 14:33:27 Richard Miller wrote:
> > 250000*4096/20.78 = 49 mb/s.  this is less than 1/2 the available
> > bandwidth giving the drive lots of wiggle room.  and since you're
> > doing sequential i/o the drive can do write combining.
> 
> Is there any experiment I can do (not involving a crowbar and a
> microscope) to find out the real physical sector size?  Bigger
> transfers get more of the bandwidth, but then a smaller proportion
> of the transfer needs read/modify/write.  I could do random addressing
> but then I would expect seek time to dominate.

compare write bandwidth for 4096B data chunks at offset modulo 512B:
(n*512B+k), where `n' is random. compare 8 runs, each with const `k' from { 0, 
1, ...7 }. sync after every write. write much more than drive cache size at 
each run (probably 64MB on modern HDs).

if your drive is 4k, one of the runs will be at exact sector boundary, and 7 
others will read-modify-write two sectors every time. thus one run will have 
much better performance.

if your drive is 512b, all runs will have same performance.


-- 
dexen deVries

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http://xkcd.com/732/