What's Better: USB or SCSI?

What's Better: USB or SCSI?

Abstract
Data usage and archiving is just exploding everywhere. The bus options for adding data increase often, with new bus protocols being added regularly. With systems so prevalent throughout businesses and homes, when should one choose a different bus protocol for accessing the data? This set of tests will be done with some older mid-range internal SCSI drives against a brand new massive external USB drive.

Test: Baseline
The Ultra60 test system is an SUN UltraSPARC II server, running dual 450MHz CPU's and 2 Gigabytes of RAM. Internally, there are 280 pin 180Gigabyte SCSI drives. Externally, there is one external 1.5 Terabyte Seagate Extreme drive. A straight "dd" will be done, from a 36Gig root slice, to the internal drive, and external disk.


Test #1a: Write Internal SCSI with UFS
The first copy was to an internal disk running UFS file system. The system hovered around 60% idle time with about 35% CPU time pegged in the SYS category, the entire time of the copy.

Ultra60-root$ time dd if=/dev/dsk/c0t0d0s0 of=/u001/root_slice_0
75504936+0 records in
75504936+0 records out

real 1h14m6.95s
user 12m46.79s
sys 58m54.07s

Test #1b: Read Internal SCSI with UFS
The read back of this file was used to create a baseline for other comparisons. The system hovered around 50% idle time with about 34% CPU time pegged in the SYS category, the entire time of the copy. About 34 minutes was the span of the read.

Ultra60-root$ time dd if=/u001/root_slice_0 of=/dev/null
75504936+0 records in
75504936+0 records out

real 34m13.91s
user 10m37.39s
sys 21m54.72s

Test #2a: Write Internal SCSI with ZFS
The internal disk was tested again using the ZFS file system, instead of UFS file system. The system hovered around 50% idle with about 45% being pegged in the sys category. The write time lengthened about 50%, using ZFS.

Ultra60-root$ time dd if=/dev/dsk/c0t0d0s0 of=/u002/root_slice_0
75504936+0 records in
75504936+0 records out

real 1h49m32.79s
user 12m10.12s
sys 1h34m12.79s

Test #2b: Read Internal SCSI with ZFS
The 36 Gigabyte read took ZFS took about 50% longer than UFS. The CPU capacity was not strained much more, however.

Ultra60-root$ time dd if=/u001/root_slice_0 of=/dev/null
75504936+0 records in
75504936+0 records out

real 51m15.39s
user 10m49.16s
sys 36m46.53s

Test #3a: Write External USB with ZFS
The third copy was to an external disk running ZFS file system. The system hovered around 0% idle time with about 95% CPU time pegged in the SYS category, the entire time of the copy. The copy consumed about the same amount of time as the ZFS copy to the internal disk.

Ultra60-root$ time dd if=/dev/dsk/c0t0d0s0 of=/u003/root_slice_0
75504936+0 records in
75504936+0 records out

real 1h52m13.72s
user 12m49.68s
sys 1h36m13.82s

Test #3b: Read External USB with ZFS
Read performance is slower over USB than it is over SCSI with ZFS. The time is 82% slower than the UFS SCSI read and 21% slower than the ZFS SCSI read. CPU utilization seems to be slightly more with USB (a factor of 10% less idle time with USB over SCSI.)

Ultra60-root$ time dd if=/u003/root_slice_0 of=/dev/null
75504936+0 records in
75504936+0 records out

real 1h2m50.76s
user 12m6.22s
sys 42m34.05s

Untested Conditions

Attempted was Firewire and eSATA, but these bus protocols would not reliably work on the Seagate Extreme 1.5TB drive, under any platform tested (several Macintoshes and SUN Workstations.) If you are interested in a real interface besides USB, this external drive is not the one you should be investigating - it is a serious mistake to purchase.

Conclusion

The benefits of ZFS does not come with a cost in time. Reads and writes are about 50% slower, but the cost may be worth it for the benefits of: unlimited snapshots, unlimited file system expansion, error correction, compression, 1 or 2 disk failure tolerance, future 3 disk failure tolerance, future encryption, and future clustering are features.

If you are serious about your system performance, SCSI is definitely a better choice, over USB, to provide throughput with minimum CPU utilization - regardless of file system. If you invested in CPU capacity and have CPU capacity to burn (i.e. muti-core CPU), then buying external USB storage may be reasonable, over purchasing SCSI.