

Can Apple Predict How Long a Transfer Takes? - kldavis4
http://www.wired.com/2014/10/can-apple-predict-long-transfer-takes/

======
eps
When copying files there are two costs - fixed per-file cost (of
opening/closing a file) and variable cost of bulk copying. If you track both,
you can pass the measurements through basic linear regression and get a simple
formula -

    
    
      ETA = A * number-of-file + B * number-of-bytes
    

This can be further refined to accomodate the cost of file extension for
larger file, the overhead of directory creation, etc, but in practice basic
two-term model works really well.

There was an earlier post on HN on this, let me see if I can find it ...
_edit_ ... Nope, not HN. I was thinking of this -
[http://www.bvckup.com/support/forums/topic_show.pl?tid=101](http://www.bvckup.com/support/forums/topic_show.pl?tid=101)

------
a2tech
I believe the prediction is made based on the estimated size of the copy and
it doesn't take into account the number of items to be copied. As any seasoned
computer user will tell you, a large number of small files takes longer to
copy then a similar sized large file. When the migration wizard is copying a
huge number of small files the transfer goes far slower than when it gets into
your documents and other large items.

~~~
bradleyland
All of that might be true, but it doesn't answer the question: Why isn't the
Migration Assistant time prediction algorithm more accurate?

Part of this might be the fact that the author is (or at least appears to be)
using a traditional magnetic HDD, rather than an SSD. I say this because I've
transferred many a MacBook Air over Thunderbolt in 40 minutes for smallish
amounts of data. The longest time I've seen MA take was 4 hours for a MacBook
Air with a 256 GB SSD (don't recall the utilization). A 10 hour transfer time
smells of magnetic platters.

The reason HDD vs SSD would impact prediction is that the throughput of an HDD
is impacted more severely by a greater number of variables when transferring
at the filesystem level than a SSD. Man, that was a mouthful. Stated simply,
HDD performance is less predictable than SSD performance.

Given the direction Apple is headed with primary storage, I could see them not
investing much in tailoring their estimation algorithm toward a technology
that is on the wane.

------
shalmanese
There are a couple of things Apple seems bizarrely bad at and a lot of them
seem to involve networked file manipulation (Migration Assistant, Time Machine
etc.) There seem to be network operations that have latencies orders of
magnitude slower than they should be which brings down the performance of the
entire application.

------
matthewmcg
A bit frustrating, perhaps, but how often is migration assistant used by an
individual user?

I would think it would be more important to get the estimate right for routine
batch file copies. Obligatory reference to XKCD #612:
[http://xkcd.com/612/](http://xkcd.com/612/)

------
dudus
[http://xkcd.com/612/](http://xkcd.com/612/)

