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edited Mar 9, 2015 at 18:48

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There's a bit of a cargo cult around dd. Originally, there were two bugs in cp that caused problems: It would misdetect files as sparse when reported with a block size other than 512 (Linux used a block size of 1024), and it did not clear empty blocks from the destination when copying from a sparse file to a block device.

You can find some references to this in the early Linux mailing list archives.

So people got used to dd being the correct way to deal with disk images, and cp fell by the wayside. And since dd uses a default block size of 512, it's slow (slower than cp on modern systems). But it's not obvious what block size you should use. Probably in your case someone has read that 2048 is the "natural" block size for a CD-ROM (it is, CD-ROMs are divided into 2,352 byte sectors containing 2,048 bytes of data along with error correcting information) and has decided that this is the "right" size to use with dd, when in fact you would probably get faster results if you used a (moderately) larger block size. In fact, GNU cp uses a default block size of 64k for this reason.

tl;dr: cp /dev/dvd foobar.iso should work fine. The default block size for dd is 512. The only effect leaving it alone is likely to have in most modern circumstances is to make the copying process slower.

There's a bit of a cargo cult around dd. Originally, there were two bugs in cp that caused problems: It would misdetect files as sparse when reported with a block size other than 512 (Linux used a block size of 1024), and it did not clear empty blocks from the destination when copying from a sparse file to a block device.

You can find some references to this in the early Linux mailing list archives.

So people got used to dd being the correct way to deal with disk images, and cp fell by the wayside. And since dd uses a default block size of 512, it's slow (slower than cp on modern systems). But it's not obvious what block size you should use. Probably in your case someone has read that 2048 is the "natural" block size for a CD-ROM (it is, CD-ROMs are divided into 2,352 byte sectors containing 2,048 bytes of data along with error correcting information) and has decided that this is the "right" size to use with dd, when in fact you would probably get faster results if you used a (moderately) larger block size. In fact, cp uses a default block size 64k for this reason.

tl;dr: cp /dev/dvd foobar.iso should work fine. The default block size is 512. The only effect leaving it alone is likely to have in most modern circumstances is to make the copying process slower.

There's a bit of a cargo cult around dd. Originally, there were two bugs in cp that caused problems: It would misdetect files as sparse when reported with a block size other than 512 (Linux used a block size of 1024), and it did not clear empty blocks from the destination when copying from a sparse file to a block device.

You can find some references to this in the early Linux mailing list archives.

So people got used to dd being the correct way to deal with disk images, and cp fell by the wayside. And since dd uses a default block size of 512, it's slow (slower than cp on modern systems). But it's not obvious what block size you should use. Probably in your case someone has read that 2048 is the "natural" block size for a CD-ROM (it is, CD-ROMs are divided into 2,352 byte sectors containing 2,048 bytes of data along with error correcting information) and has decided that this is the "right" size to use with dd, when in fact you would probably get faster results if you used a (moderately) larger block size. In fact, GNU cp uses a default block size of 64k for this reason.

tl;dr: cp /dev/dvd foobar.iso should work fine. The default block size for dd is 512. The only effect leaving it alone is likely to have in most modern circumstances is to make the copying process slower.

answered Mar 9, 2015 at 18:38

11.3k
2
39
42

There's a bit of a cargo cult around dd. Originally, there were two bugs in cp that caused problems: It would misdetect files as sparse when reported with a block size other than 512 (Linux used a block size of 1024), and it did not clear empty blocks from the destination when copying from a sparse file to a block device.

You can find some references to this in the early Linux mailing list archives.

So people got used to dd being the correct way to deal with disk images, and cp fell by the wayside. And since dd uses a default block size of 512, it's slow (slower than cp on modern systems). But it's not obvious what block size you should use. Probably in your case someone has read that 2048 is the "natural" block size for a CD-ROM (it is, CD-ROMs are divided into 2,352 byte sectors containing 2,048 bytes of data along with error correcting information) and has decided that this is the "right" size to use with dd, when in fact you would probably get faster results if you used a (moderately) larger block size. In fact, cp uses a default block size 64k for this reason.

tl;dr: cp /dev/dvd foobar.iso should work fine. The default block size is 512. The only effect leaving it alone is likely to have in most modern circumstances is to make the copying process slower.