Why is the performance of "cp" and "cat" so poor compared with "dd"?  -Mike


System: 1.5T Thunderbird 1.1GHz/0.5 GB memory

wd0 at pciide0 channel 0 drive 0: <IBM-DTLA-307030>
wd0: drive supports 16-sector PIO transfers, LBA addressing
wd0: 29314 MB, 16383 cyl, 16 head, 63 sec, 512 bytes/sect x 60036480 sectors
wd0: 32-bit data port
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 5 (Ultra/100)
wd0(pciide0:0:0): using PIO mode 4, Ultra-DMA mode 5 (Ultra/100) (using DMA data  transfers)

# time dd if=/dev/rwd0e of=1g bs=4096k count=256

256+0 records in
256+0 records out
1073741824 bytes transferred in 80 secs (13421772 bytes/sec)

real    1m19.987s
user    0m0.000s
sys     0m9.586s

# time cp 1g 1g.cp 

real    3m32.457s
user    0m0.010s
sys     0m10.826s

# rm 1g.cp

# time cat 1g>1g.cat

real    3m31.184s
user    0m0.079s
sys     0m11.248s

# rm 1g.cat

# time dd if=1g of=1g.dd bs=4096k

256+0 records in
256+0 records out
1073741824 bytes transferred in 79 secs (13591668 bytes/sec)

real    1m18.696s
user    0m0.000s
sys     0m15.441s

# rm 1g.dd