On Wed, 14 Jan 2004, Kevin Oberman wrote: > Thanks to both John and Dan. I clearly did not correctly understand how > dd operated. Guess it's time to read the sources a bit. I have no problems using dd to copy partitions, at least in versions of FreeBSD without GEOM. If the target contains metadata (e.g., disk label(s)), then it may need to be skipped using something like: dd if =/dev/ad0s1a of=/dev/ad1s1a count=1 # copy up to label # Adjust target metadata later (or before) if necessary dd if =/dev/ad0s1a of=/dev/ad1s1a iseek=2 oseek=2 # copy a bit after label dd if =/dev/ad0s1a of=/dev/ad1s1a bs=64k iseek=1 oseek=1 # copy rest If the source contains metadata (e.g., disk label(s)), then the metadata might not be copied correctly or might spring to life on the target inconveniently as soon as it is copied. If so, it can be skipped or adjusted as necessary. Versions of FreeBSD without GEOM have weaker restrictions on clobbering metadata. Reads and writes to who disk devices are always permitted and no translation of metadata is done. So any disk region can be copied using: dd if =/dev/ad0 of=/dev/ad1 iseek=whatever1 oseek=whatever2 count=whatever3 This is rather dangerous, so I rarely use it except with a count of 1 to zap metadata blocks after carefully checking that the seek offsets are correct. A safer version of this is to put the seek offsets and count in slice or partition entries and then copy between the slices or partitions. FreeBSD partitions have strong enough restrictions on overlapping and on clobbering of active (mounted) partitions to prevent clobbering via simple most mistakes like typos in the the offsets (not to mention the drive number). > I understand why dump|restore is a better choice than dd in may ways, > but, if a partition is large and full, dd is MUCH faster. That's why I > use it to backup my system disk. I can copy 40 GB in about 40 minutes on > my laptop and dumping takes just a bit longer. I normally use tar, since dump just doesn't work for non-ffs partitions. Is dump really only a bit slower than dd? dd is 10-20 times faster than tar for me, since I have lots of small files in src and cvs trees and these trees were very fragmented until I rebuilt them recently (tar can be no faster than ffs itself). BruceReceived on Wed Jan 14 2004 - 10:40:20 UTC
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