Andy Millar

You may have noticed that it’s been a little quiet around here.

This would be because I am working on my patch to provide Cache Cohereny Event Profiling for Cachegrind, a Valgrind tool.

This is a university project for Imperial College London and is due in on Wednesday 18th June.

Further updates, including code and a pretty report will be available online once this is over.

PANIC will be over soon :-)

This rather strange thought came to me whilst on the loo today!

I worked out that I must spend somewhere between 30 and 45 mins per day on the loo!

Is this a long time?

How long do you spend on the loo every day?

fact

When I was first told this, my initial response was “rubbish“, there was no way that using RAID1 (mirroring) could increase the chance of disk failure, was there?

Then I actually thought about the statement again, and I was wrong!

Consider 2 hard disks A and B. The make/model/manufacturer is irrelevant for this proof.

Given that P(e) is the probability of event e occurring, we can state that:

P(disk A fails) = a

P(disk B fails) = b

Trivially, in a single disk system using either disk A or disk B, the probability of disk failure will be a or b, or for ease of demonstration we shall pick the average of the two.

Therefore, the average chance of a single disk machine suffering disk failure is (a+b)/2.

Now, consider the RAID1 system containing disks A and B.

The chance of a disk failing is the probability of disk A failing or the probability of disk B failing; so:

P(disk A fails or disk B fails) = P(disk A fails) + P(disk B fails) = a + b.

In fact, the chance of disk failure in a RAID1 system doubles. Hardly surprising if you stop and think about it.

Now, the statement I initially inferred actually was: RAID1 increases chances of data loss, which is obviously rubbish, as can be easily shown.

We know the chance of disk failure (and in this case data loss) with a single disk, (a+b)/2.

Now, using RAID1, the chance of data loss is defined as:

The probability of both disk A and disk B failing.

This is defined as:

P(disk A failing and disk B failing) = P(disk A failing) * P(disk B failing) = a * b.

It can be stated that any probability must be in the range 0 <= P(e) <= 1.

For any two numbers i,j; if both i and j satisfy 0 <= i <= 1; 0 <= j <= 1 then:

i * j < i; i * j < j

Therefore, the probability of data loss is lower when using RAID1, however the chance of disk failure doubles!

My “current location” is currently disabled.

This is because I now have a new phone and have yet to get the GPS working properly.

Apart from the GPS not being easy to get working, the Samsung i780 is a pretty nice phone!

More details on the i780 to follow later!

Since the internet sucks for finding out…

Cover with one to one and a half times its volume of hot water and leave to soak for 10 minutes.

Simple, now why can’t every site say this? :-)

Why, just why?

Do people really add these stupid facebook applications just because someone they may or may not know has added them and then sent them the request?

Sorry, but I don’t think I’ll ever want to play “Have I Ever…”.

I recently experienced a problem with an Exchange 2003 deployment where pop3svc was stuck starting.

To resolve it, try killing inetinfo.exe using Task Manager.

You should then be able to restart the pop3 and imp services via MMC.

Make sure you start the pop3/imap services via the Exchange System Manager as well as via MMC.