[lug] Converting 95%ile into GB/month.

Sean Reifschneider jafo at tummy.com
Fri Jan 9 17:02:22 MST 2009

In response to the question at the meeting last night about how you
convert between gigabytes per month and 95th percentile pricing for
bandwidth...  As I said, it really depends on your exact traffic profile,
but if you have a typical sinusoidal traffic profile where days are higher
than nights, one number I've found works for one of our clients is to use
half of the megabit number, multiplied out.

So, for example, a megabit can transfer 309MiB in a 30 day month.  Without
the leap second.  So, 3mbps at 95th %ile may translate to 3*309/2 MiB, or
around 464MiB.

Of course, this also depends on whether you are being charged for the 95th
%ile of the greater of in or out, or in+out, and what exactly your traffic
profile is...

For example, mirrors.tummy.com over the last week is:

   95th %ile in: 0.8MiB  (6.4mbps)
   95th %ile out: 2.5MiB (20mbps)
   Average in: 178.7KiB  (1.4mbps)
   Average out: 1.094MiB (8.8mbps)

So that's an average of 10.2mbps and a 95th %ile of the greater of in or
out of 20.0mbps.  So very close to half.

However, if you had a fairly flat traffic profile over a day, meaning
constant use, your average and 95th %ile may be similar.  It would even be
possible for the average to be over the 95th %ile.  Say, you ran at 1mbps
steady rate most of the day, but for around an hour a night you saturated a
gigabit for backups.  Your average would be WAY over the 95th percentile...

Hope this helps.

Sean Reifschneider, Member of Technical Staff <jafo at tummy.com>
tummy.com, ltd. - Linux Consulting since 1995: Ask me about High Availability

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