On Tue, Nov 1, 2022 at 3:14 PM Roger Heflin <rogerheflin@xxxxxxxxx> wrote:
I usually add a bs=4k or bs=16k or a bs=256k when using dd so the
blocksize is larger. That usually makes it quite a bit faster.
fdisk shows block size
And if you background the dd you can use "vmstat 1" and the bi/bo
columns tell you the actual disk io rates, and you can use that to
reliably estimate.
Elderly rescue CD’s have busybox, which provides nmeter. Fdisk gives
values for logical and physical block size, but those may be fictional.
% doas fdisk -l /dev/sdb1
Disk /dev/sdb1: 1.91 TiB, 2100425654272 bytes, 4102393856 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
On the elderly iMac, logical and physical were both 512.
Default is 512 and does not align with most (all?) underlying physical
devices block sizes and may cause various layers extra work and take
much longer.
I have done experiments preparatory to using dd to clone a box of disks.
Modern storage devices are more complicated and network speed means
sneakernet is no longer needed.
George N. White III
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