Date: Wed, 15 Feb 2006 08:48:37 -0600 From: "Jack Stone" <antennex@hotmail.com> To: kdk@daleco.biz, freebsd-questions@freebsd.org Cc: keramida@ceid.upatras.gr Subject: Re: Using dd to Make a Clone of a Drive Message-ID: <BAY106-F20DA823330BD0DD3AD5C8DCCFA0@phx.gbl> In-Reply-To: <43F1DF7A.7000504@daleco.biz>
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I have used dump/restore and dd as well. For a block size, I chose 102400
which was the fastest -- but still slow compared to dump/restore.
Dump/restore is not limited to making a whole image, blanks and all like dd.
Once upon a time, I used this as the best:
dd if=/dev/ad0 of=/dev/ad1 bs=102400
The obvious intention is to minimize the number of transfers, so
theoretically the larger the transfer, the better. The maximum I/O transfer
size is limited to the value of MAXPHYS, which is defined in
sys/param.h:
#ifndef MAXPHYS
#define MAXPHYS (128 * 1024) /* max raw I/O transfer size */
#endif
The ATA subsystem uses this value. SCSI drives were limited to 60 kB
transfers, though this could have changed. I don't currently have any
machine with SCSI disks connected, so I can't confirm that. A way to find
is to run a command like will show the I/O:
#dd if=/dev/ad0c of=/dev/null bs=128k &
and in the background do an 'iostat ad0 1'. Here's an example with an
IDE drive:
#iostat ad0 1
tty ad0 cpu
tin tout KB/t tps MB/s us ni sy in id
0 3 5.19 7 0.03 11 0 4 1 84
0 126 127.36 183 22.74 0 0 6 2 92
0 44 128.00 190 23.76 0 0 2 0 98
0 44 128.00 191 23.89 0 0 5 0 95
0 44 128.00 191 23.88 0 0 7 1 92
As you can see, it's really doing 128 kB transfers, for an average transfer
rate of almost 24 MB/s.
=============================================================
I do all of my backups every night by scripts by using dump/restore to a
dedicated separate HD. It is bootable and ready to go in case of a problem
with my master HD. Just shutdown, switch HDs and reboot -- voila! Back on
line within a minute and the data is always only a few hours old at the
most.
Once you master dump/restore, it is indispensable for handling file systems.
Best regards,
Jack
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