With a Fujitsu MAM3184 (U160, 15Krpm 18GB) disk and a Tekram
DC-390U3W controller (sym53c8xx_2 driver) on lk 2.5.15 I get:
$ time dd if=/dev/sdb of=/dev/null bs=64k count=16k
16384+0 records in
16384+0 records out
real 0m18.948s user 0m0.010s sys 0m4.090s
That is 56.67 MB/sec (MB == 10^6).
$ time sg_dd if=/dev/sg1 of=/dev/null bs=512 count=2m time=1
time to transfer data was 18.786448 secs, 57.16 MB/sec
2097152+0 records in
2097152+0 records out
real 0m18.799s user 0m0.030s sys 0m3.010s
$ time sgm_dd if=/dev/sg1 of=/dev/null bs=512 count=2m time=1
time to transfer data was 18.777035 secs, 57.18 MB/sec
2097152+0 records in
2097152+0 records out
real 0m18.781s user 0m0.020s sys 0m0.100s
The MAM3184 disk was recently reviewed
( see http://www4.tomshardware.com/storage/02q2/020415/index.html )
and those speeds are very close to the maximum in their benchmarks
(and Fujitsu's published specifications) for outer track reads.
I am impressed by dd's performance in the lk 2.5 series.
When sg_dd and sgm_dd are used they bypass the block subsystem
and issue 64KB SCSI read commands (in this case). As can be seen
above, this improves the throughput by about 1 % compared to dd.
CPU utilization (on a Athlon 1.2 GHz box with 512 MB of DDR ram)
is a little more expensive with dd (4 seconds compared with 3
seconds). The "sgm_dd" command uses mmap() to do "zero copy" reads
which is why its CPU utilization is so low.
lower than sg_dd. No doubt FreeBSD would also perform well but I
doubt it could beat linux (2.5) by the type of margin your measurements
indicate. [For sequential reads, tagged queueing will not have a
significant impact.] It is also worth noting that the new aic7xxx
and sym53c8xx_2 drivers are essentially the same on Linux and
FreeBSD (i.e. same code base, same maintainers).
Using scsi_debug (a ram disk) as a dummy scsi load yields:
# time dd if=/dev/sdc of=/dev/null bs=64k count=2k
2048+0 records in
2048+0 records out
real 0m1.082s user 0m0.000s sys 0m0.990s
That's 124 MB/sec and the CPU utilization is dominating. The
"sgm_dd" command yields 850 MB/sec for the same transfer.
Doug Gilbert
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/