If you're doing performance analysis of a RAID-1 setup, one of theinteresting questions is 'which drive gets read from when?'

(Measuring how balanced the current IO load is is useful, but it doesn'ttell you how a changed load will affect the balance.)

Since seeks are the expensive thing on modern drives, you want yourRAID-1 system to deal in requests, not in individual blocks, and to senda sequence of sequential reads off to the same disk. If

Today I got to run into an interesting performance impact of having aRAID-1 mirror resync running on a big partition of a live system.

An important system was having performance problems today, so we werepoking around it. When we watched the disk statistics, we noticed thatonly the first disk was seeing read traffic; the second disk was loafingalong with just occasional bursts of writes. Looking more closely wenoticed that a RAID-1 resync of a big partition was in progress; becausethe system was loaded

A while back, I wrote an entry about a bad web spider that at the time appeared to belong toUptilt Inc. About a week after I published the entry, some of thesystem administration folks from Uptilt stumbled across it and got intouch with me to look into the whole situation.

In fact they were pretty puzzled about the incident, because (asthey put it) Uptilt didn't even do outgoing HTTP, much less have aweb crawler; their business is based on email. After I provided

I'm not sure where I ran across this, but until recently I wasn't awarethat you could pre-explode lists and tuples when you declared whatparameters a function takes. That sounds stilted, so I'll give anexample:

def foo(a, (b, c, d)):  print a, b, c, d

This is slightly more compact than the equivalent:

def bar(a, tmp):  b, c, d = tmp  print a, b,