Originally posted by: TonyPearson

On his blog post on preparation, Seth Godin mentioned an appropriate Swedish saying:

There is no bad weather, just bad clothing.

Appropriate because it snowed here in Tucson, Arizona on Sunday evening, leaving many of us here figuring out how to drive through the stuff on Monday. In my entire lifetime, I have only witness snow down in the Tucson valley a handful of times. It got me thinking about coats, and the wonderful schemes for coat check rooms, as an analogy for data access. A lot of people ask me to compare and contrast one technology from another, say block-level virtualization from content-addressable storage, and so on, and I always try to find a good analogy to help explain things.

Let's start with the setting. It is snowing outside and people are wearing coats. When they come inside, they check their coats at a coat check room, a large room with rows and rows of racks with hangers. A coat check attendant takes your coat and puts it on a hanger, and gives you a ticket or other identifier that will allow you to retrieve your coat later. The ticket must have sufficient information to retrieve the coat quickly, rather than searching rows and rows of hangers for it.

Block-based disk storage

You walk to the coat-check desk, tell the attendant to hang your coat on a specific hanger, say hanger number 387. When you come back, you ask for the coat on hanger 387. The coat-check attendant knows exactly where hanger 387 is, and is able to retrieve it quickly. Most disk systems use this approach, including IBM SAN Volume Controller and DS family of disk systems.

Name-based disk storage

You walk to the coat-check desk, tell the person the name that you want to call your coat. An empty hanger is located, and a list of coat names, with their associated hanger number, is then kept. Upon return, you ask for your coat by name, and the coat-check attendant looks up the hanger number to match, and retrieves your coat. This is the scheme used by the IBM System Storage DR550, N series for NAS storage, and the IBM Healthcare and Life Sciences Grid Medical Archive Solution (GMAS).

Content-addressable storage (CAS)

You walk to the coat-check desk and hand them your coat. The attendant weighs your coat, checks the brand, the size, the number of buttons and zippers, types it all in, and the computer spits out a "hash code" from 1 to 99999. An empty hanger is found, and the hash code is associated to the hanger number. Upon return, you provide the hash code you were given, and the coat-check attendant looks up the hanger number to match, and retrieves your coat.This is the scheme used for some non-erasable, non-rewriteable storage, such as the EMC Centera.

IBM invented hash codes in 1953 as a way to speed up searches. For example, if you want to look up a word in the dictionary, knowing the first letter of the word makes it much quicker, because you can thumb directly to that section. A hash code was intended to give a more even distribution, so that if a million words are stored in a "hash code dictionary" then you would calculate the hash code, then look up only that section of words associated with that specific hash code number.

A problem arises when you generate "hash codes" for storage. It is possible for two different pieces of data to resolve to the same hash code. When an application tries to write a piece of data, and it resolves to a hash code that already exists, that is called a collision. One response is to either compare the incoming data to the data that is already stored, confirm they are identical, but that can be time consuming. The other response is to just assume they are identical, and reject the secondary copy, a process often referred to as "de-duplication".

What's the chance of getting a collision for data that is really different? Let's take for example the famousBirthday paradox. Suppose the coat check room assigned the hanger based on your birthday (month and day). How may coats before you run the risk of having two people turn in coats with the same birthday? After only 23 people, the likelihood is 50%. At 60 people, it goes up to 99%.

For this reason, IBM does not offer content-addressable storage. For non-erasable, non-rewriteable storage, the IBM System Storage DR550 requires the application to give each object a name, and that name is then used to storage the data, eliminating the possibility that data might accidently be thrown away.

It's safer that way.

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