Hi Terry,
> The above link also suggests that dd is the longest method.
dd(1) is quick. It read(2)s a block of bytes and write(2)s that block.
The kernel does the transfer of bytes from the device to dd's memory and
vica versa. If you don't choose a block size then it might be quite
small,
$ dd if=/dev/zero of=/dev/null count=1
1+0 records in
1+0 records out
512 bytes copied, 0.000276964 s, 1.8 MB/s
$
and most of your time is spent in overhead of switching between dd and
the kernel. Using bs=1M would cut down that overhead as you're unlikely
to be using a device that insists on a particular block size.
> I must say that I used to get exceeding bored when copying a 4 GB SD
> Card.
The destination media is the bottleneck there.
> However, I tried it and then realised that I would have to wait for it
> to finish before I found out the duration
dd(1) says to send a USR1 signal. The arrows are the lines I typed.
The bulleted lines are the response to the signal.
$ dd bs=2
→ foo
foo
• 2+0 records in
• 2+0 records out
• 4 bytes copied, 4.40433 s, 0.0 kB/s
→ bar
bar
4+0 records in
4+0 records out
8 bytes copied, 7.53309 s, 0.0 kB/s
$
> sudo dcfldd if=/dev/urandom of=/dev/sdc
/dev/urandom can be quite slow for large amounts.
$ dd if=/dev/urandom bs=1M count=1K of=/dev/null
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 12.6809 s, 84.7 MB/s
$
$ openssl rand $((2**20)) >rnd
$ r=rnd
$ r="$r $r $r $r"
$ r="$r $r $r $r"
$ r="$r $r $r $r"
$ r="$r $r $r $r"
$ r="$r $r $r $r"
$ cat $r | wc -c
1073741824
$ echo $((2**30))
1073741824
$ while cat $r; do :; done |
> dd iflag=fullblock bs=1M count=1K of=/dev/null
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 3.04574 s, 353 MB/s
$
One can go faster still by cutting out a read(2) for every write(2) by
having a little Perl script or C program that loops, flinging the same
data read once into every write().
> It seems to be pretty quick having reached 22 GB done in around 40
> minutes.
I'm assuing that's GiB to the drive's TB.
$ units -1v 22GiB/40minutes hour/TB
reciprocal conversion
1 / (22GiB/40minutes) = 28.221896 hour/TB
$
Cheers, Ralph.
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