\begin{slide}{violet}
\violet
\begin{center}
Workload
\end{center}
System:
\begin{itemize}
\itemsep=12pt
\item Aviion AV 4000 running DG/UX 4.3
\item test disk: 179Mbyte SCSI 
\item Adaptec AIC-6250 Protocol Chip as bus interface
\end{itemize}
Immediate problems:
\begin{itemize}
\itemsep=12pt
\item SCSI's try to hide disk operation
\item finally revealed: variable number of sectors per track (31 to 53)
$\rightarrow$ 155 to 265 sectors per cylinder (5 surfaces)
\end{itemize}
Conclude: work with {\it virtual cylinders}\\

\pagebreak[4]

virtual cylinder size:
\begin{itemize}
\itemsep=12pt
\item small enough to capture real cylinder request dependencies
\item large enough to make model manageable (\#states = \#vc's) 
\item multiple of file system transfer size (16 sectors = 8K bytes)
\end{itemize}
choice: 160 sectors $\Rightarrow$ 2,181 virtual cylinders\\
\quad\\
procedure:\\
\quad\\
monitor (NFS attached) AViion AV 200\\
\quad\\
3 days $\rightarrow$ 163,681 requests\\
\quad\\
cast (WSCAN) queue departure stream in terms of 2,181 virtual cylinders
\end{slide}

