Re: External Backup Drives
- From: dempson@xxxxxxxxxxxxx (David Empson)
- Date: Wed, 15 Aug 2007 04:15:28 +1200
Jim Higgins <gordian238@xxxxxxxxxxx> wrote:
So far I have tried two external USB drives, a 120GB WD and a 160GB
Seagate, and neither has had anything anywhere remotely close to 480MBPS
data transfer for backup.
USB 2.0's theoretical maximum raw speed is 480 Mbps.
This speed really only refers to the maximum rate at which raw bits can
be sent over the USB cable. There is a fair amount of protocol overhead,
which adds additional data and delays.
The net result is that the achievable data rate for a USB hard drive is
much lower.
In my previous tests on a PowerBook G4, I never saw data transfers
faster than about 17 megabytes per second over USB 2.0. (That's about
17*8 = 136 Mbps). It is possible that Intel Mac models are somewhat
faster, but I wouldn't expect much better than 25 MB per second even on
the best computer in the world.
The same hard drive and computer were able to achieve at least 40 MB per
second over Firewire 400, and I've heard of some drives and computers
achieving almost 50 MB per second on Firewire 400 (which is its
theoretical limit).
The numbers suggest that Firewire 400 will be slower than USB 2.0 (480
on the same scale), but Firewire is much more efficient than USB, so it
works a lot faster in reality.
Firewire 800 is faster again, and is typically faster than any single
hard drive.
A second factor is the speed of the hard drive. A 2.5" (laptop-sized)
hard drive running at 5400 RPM or slower could be limiting the transfer
rate even further on USB 2.0, and definitely will with Firewire. Modern
higher capacity drives will be faster than older and lower capacity
ones, even at the same rotation speed.
A 3.5" 7200 RPM drive will be fast enough to saturate USB 2.0 but might
not be fast enough to saturate Firewire 400 (depending on capacity and
which part of the drive is being accessed). It will definitely be the
limiting factor with Firewire 800.
Also consider the speed of your internal drive, if you are copying files
between it and an external one - the slowest drive will be the limiting
factor.
A third factor is the mechanical processes involved in a hard drive:
seeking will introduce significant delays (relative to the data transfer
rate), resulting in reduced performance.
A fourth factor is the size of the files involved. If you are
transferring a lot of small files, there is a lot more overhead for the
CPU to access and deal with all of the information about the files, and
to do lots of small I/O operations to read a small amount of data at a
time (plus time required to seek). If you are transferring large files
which are stored in consecutive sectors on the disk, a single large I/O
request can be done, and this has a lot less overhead, resulting in
better average performance.
The notebook is a MacBook Pro purchased in July '07. What am I missing
here? Is FW400 the only way to go or have I overlooked something obvious?
You need Firewire if you want your backups to go faster.
Firewire 400 is probably "fast enough". Firewire 800 is really only
useful for high end applications or speed junkies, and probably won't
help much for internal to external drive backups on a laptop, as the
internal drive will probably not be fast enough to tax Firewire 400.
--
David Empson
dempson@xxxxxxxxxxxxx
.
- References:
- External Backup Drives
- From: Jim Higgins
- External Backup Drives
- Prev by Date: External Backup Drives
- Next by Date: Re: External Backup Drives
- Previous by thread: External Backup Drives
- Next by thread: Re: External Backup Drives
- Index(es):
Relevant Pages
|
Loading
