🚀 go-pugleaf

Steve Firth <filter@firthcom.demon.co.uk> wrote:

> Rebecca and Rowland <real-address-in-sig@flurbletigibbet.rhubarb.foobar>
> wrote:
> 
[snip good explanation]

Ta.

> > 
> > >If
> > > you try using Wipe Infor to overwrite all the data on the disk, it takes
> > > even longer. HDT has an option to to a secure erase, performing multiple
> > > overwrites with random data, it takes hours.
> > 
> > Hmm.  Isn't that more a reflection of the inefficiency of the software?
> > Given that the SCSI-2 interface on my Performa 475 can shove data into
> > the 1GB ext HDD at 3.8MB/s (measured sustained data rate), you could
> > expect efficient software to write once over the entire disc in
> > something of the order of 5 minutes.
> 
> Not really inefficiency as such, you quote the peak SDTR, try doing the
> same trick with a sequence of random reads and writes which makes
> allowance for real-world events like moving the heads. You will find
> that the rate is a lot slower than 3.8Mb/s. 


Yes, but formatting a disc doesn't use random disc access - it's a
sequential process, creating the blocks at one end of the disc and
working your way to the other end.  One would assume that you'd get
close to maximum efficiency.

>I checked the bench test
> results forthe drive on this Mac, a 6100/DOS and it gives 4.9Mb/s as the
> peak SDTR, but with 2K blocks the rate falls to under 450Kb/s.
> Formatting involves writing blocks which are 512 bytes, at which point
> it is hard to read the scale on the graph, but it would appear to be
> around 50-120 Kb/s.
> 
> HD Toolkit used to give the option of improving transfer rate by
> increasing block size, however this also vastly reduced the storage of
> the drive since the smallest amount of space used by a file is the block
> size. Hence with a 2Kb block size, even a file using 64 bytes will
> consume 2Kb of storage.

Given that Macs are limited to 2^16 blocks (unless you've got hfs+,
which very few people have - certainly not me), you use a block size
larger than 512 bytes normally anyway.  My smallest disc partition uses
2K blocks - at that block size, data transfer to my HD is still very
efficient.

Rowland.

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