## 4 Feature Extraction and Selection
### Feature Extraction  
As file requests are executed on a storage system, the file metadata information is updated, and the various patterns discussed in Section 3 can be computed and stored at the level of files, directories, and file formats. This aggregated information is then used for generating the features presented in Table 2 in an online fashion, as explained later in Section 5. All features are normalized to values between 0 and 1 but with different normalization approaches. Next, we discuss the set of extracted features and their normalization, grouped by their granularity. **Request Features:** These are collected at the level of individual file requests and include the _Request Operation_, _Request Offset_, and _Request Length_. The Request Operation is encoded with a specific number assigned per operation type: 1 for Open, 2 for Write, 3 for Read, 4 for Close, and 5 for Delete, normalized by dividing it by 5. The Request Offset and Request Length are normalized using the size of the file accessed by the request. **File Features:** These are extracted from basic file metadata on a per-file basis, and they include the _file ID_, _file size_, _time since creation_, _time since last access_, _time since last update_, \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline
**Feature** & \begin{tabular}{c} **Next File** \\ **Hotness** \\ \end{tabular} & \begin{tabular}{c} **Next Offset** \\ **Hotness** \\ \end{tabular} & \begin{tabular}{c} **Next File** \\ **Hotness** \\ \end{tabular} & \
\begin{tabular}{c} **Next Offset** \\ **Class** \\ \end{tabular} &
\begin{tabular}{c} **Next Offset** \\ **Class** \\ \end{tabular} \\ \hline \hline
**File Access Patterns** & & & & & & & \\ \hline Length Pattern & 0.99 & ✓ & 1.62 & ✓ & 1.59 & ✓ & 2.56 & ✓ \\ Length Pattern Frequency & 1.53 & ✓ & 1.56 & ✓ & 1.53 & ✓ & 2.11 & ✓ \\ Spatial Pattern & 1.10 & ✓ & 1.59 & ✓ & 1.60 & ✓ & 2.00 & ✓ \\ Spatial Pattern Frequency & 1.91 & ✓ & 1.72 & ✓ & 1.66 & ✓ & 2.19 & ✓ \\ Temporal Pattern & 1.17 & ✓ & 1.64 & ✓ & 1.47 & ✓ & 1.59 & ✓ \\ Temporal Pattern Frequency & 1.79 & ✓ & 2.17 & ✓ & 1.90 & ✓ & 2.00 & ✓ \\ Access Frequency & 1.83 & ✓ & 1.38 & ✓ & 1.84 & ✓ & 1.64 & ✓ \\ Open Frequency & 1.28 & ✓ & 1.10 & ✓ & 1.99 & ✓ & 1.47 & ✓ \\ Fully Read Frequency & 2.47 & ✓ & 2.00 & ✓ & 2.92 & ✓ & 2.51 & ✓ \\ \hline \hline
**Directory Access Patterns** & & & & & & & \\ \hline Directory ID & 1.59 & ✓ & 1.51 & ✓ & 2.15 & ✓ & 1.86 & ✓ \\ Temporal Intense Ratio & 1.88 & ✓ & 1.48 & ✓ & 1.88 & ✓ & 1.77 & ✓ \\ Temporal Mild Ratio & 2.05 & ✓ & 1.69 & ✓ & 2.08 & ✓ & 1.93 & ✓ \\ Spatial Sequential Ratio & 1.50 & ✓ & 1.11 & ✓ & 1.58 & ✓ & 1.56 & ✓ \\ Spatial Random Ratio & 1.65 & ✓ & 1.39 & ✓ & 1.81 & ✓ & 1.91 & ✓ \\ Length Uniform Ratio & 1.39 & ✓ & 0.90 & ✓ & 1.50 & ✓ & 1.42 & ✓ \\ Length Variable Ratio & 1.68 & ✓ & 1.46 & ✓ & 1.79 & ✓ & 1.61 & ✓ \\ File Count & 1.26 & ✓ & 1.18 & ✓ & 1.85 & ✓ & 1.70 & ✓ \\ Access Frequency & 1.56 & ✓ & 1.04 & ✓ & 1.36 & ✓ & 1.21 & ✓ \\ \hline \hline
**File Format Access Patterns** & & & & & & & \\ \hline File Format ID & 1.03 & ✓ & 1.13 & ✓ & 1.67 & ✓ & 1.46 & ✓ \\ Temporal Intense Ratio & 1.66 & & 1.24 & & 1.39 & & 1.13 & \\ Temporal Mild Ratio & 1.77 & & 1.36 & & 1.52 & & 1.13 & \\ Spatial Sequential Ratio & 1.32 & & 0.71 & & 1.17 & & 1.01 & \\ Spatial Random Ratio & 1.47 & & 1.08 & & 1.42 & & 1.30 & \\ Length Uniform Ratio & 1.32 & & 0.76 & & 1.27 & & 1.05 & \\ Length Variable Ratio & 1.65 & & 1.24 & & 1.57 & & 1.43 & \\ File Count & 1.10 & ✓ & 0.94 & & 1.60 & ✓ & 1.36 & ✓ \\ Access Frequency & 1.40 & & 0.86 & & 1.12 & & 0.90 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._history of open times_, and _current file hotness_. They identify the file and aim to determine how frequently this file is accessed and/or updated. The file size is normalized by the fifth root, over the fifth root of the biggest file size found in the system (defaults to 32 GB). Different approaches to normalization were also considered.