slow speed on copying files between slices

V

varnie

greetings.

i've discovered that in the most cases the speed of copying files from slice_A to slice_B on my FreeBSD 7.2-RELEASE-p4 is about 15/20 MB/sec. it is too slow if i'm not mistaken. are there any causes for that?

i have two slices: the first for system and the second as file-storage, with UFS on both of them.

as for dd tool, it gives me the following output:
Code: 
varnie# /bin/dd if=/dev/ad16 of=/dev/null bs=1m count=2000
2000+0 records in
2000+0 records out
2097152000 bytes transferred in 21.986160 secs (95385097 bytes/sec)
it is definitely *not* 20MB/sec!
as for my hard-drive, i have:
Code: 
ad16: 476940MB <WDC WD5000AAKS-00A7B0 01.03B01> at ata8-master SATA300
could you clarify why there's such slow speed during copying files between those two slices. thanks.
 
You'll never get more than 50% of the max speed when copying from/to the same disk. The disk has to read a chunk of data, move the heads, write a chunk of data, move the heads, read a chunk of date, etc.

Copying from one disk to another disk will get you the max speed, as one disk will do nothing but reads and the other disk will do nothing but writes, and the data just streams continuously from one to the other.
 
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