On 9/27/05, m ike <[EMAIL PROTECTED]> wrote:
> for extracting a portion of a file, the dd command can be hastened
> dramatically (by a factor of 10,000) by changing the to bs=1024
> (for example) and increasing count to be inclusive, and then piping
> the result to head -c to trim it down to exact byte-size.
>
> 10,000 may be an exaggeration. okay it is an exaggeration. but does
> not seem to be far off.
There are two reasons for using large block sizes in dd. One is to
eliminate the overhead of e.g. issuing a million system calls each to
read one byte, vs. one system call to read a million bytes. The other
is to reduce the effect of missing the "next block" in a disk read.
If you have to wait for a whole disk revolution to read a block, your
data transfer slows down proportional to the number of blocks per
cylinder. Nowadays this can range from 600 at the inner radius to
1200 at the outer. (these are real physical blocks, not the fictional
blocks that LBA software uses).
carl
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carl lowenstein marine physical lab u.c. san diego
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