Affordable and Manageable Storage, A Server Guide

Today i found an interresting article at anandtech, which talk about storage on a server side. This article can be a guide and more information to figure about storage systems, i copy paste a bit here, happy reading.


Disk performance?

Before we start discussing the different topologies and technologies in the storage world, it is good to get back to basics. The basic component of 99.9% of the storage technology out there is still the hard disk.

To understand the basic performance of a disk, take a look at what happens when a request is sent to the disk:

  1. The Disk controller translates a logical address into a physical address (cylinder, track, and sector). The request is a matter of a few tens of nanoseconds, the command decoding and translating can take up to 1 ms.
  2. The head is moved by the actuator to the correct track. This is called seek time, the average seek time is somewhere between 3.5 and 10 ms
  3. The rotational motor makes sure that the correct sector is located under the head. This is called rotational latency and it takes from 5.6 ms (5400 rpm) to 2 ms (15000 rpm). Rotational latency is thus determined by how fast the rotational motor spins.
  4. The data is then read or written. The time it takes is dependent on how many sectors the disk has to write or read. The rate at which data is accessed is called the media transfer rate (MTR).
  5. If data is read, the data goes into disk buffer, and is transferred by the disk interface to the system.

Media transfer rate (MTR) depends on the rotation speed and on the density with which data is stored. The higher the density, the more data moves under the head in the same amount of time.

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