ECE344-Lecture24-FileSystems

ECE344-Lecture24-Fil - Lecture 24 File Systems David Lie ECE344 University of Toronto 1 Outline File Systems Overview of file system Disk Basics

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1 Lecture 24: File Systems David Lie ECE344 University of Toronto
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2 ECE344: Operating Systems Outline File Systems – Overview of file system – Disk Basics File system design Consistency and crash recovery Sharing files – Unix file system Disks – Disk scheduling algorithms – Redundancy in storage systems
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3 ECE344: Operating Systems Buffer Cache Management File operations often access the same disk block – E.g., block containing contents of root (/) directory Caching disk blocks in memory can reduce disk I/O Traditionally block cache is called a buffer cache Cache operations – Block lookup • If block in memory, returns data from buffer – Block miss • Read disk block into buffer, update buffer cache – Block flush • If buffer is modified, write it back to disk block
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4 ECE344: Operating Systems Buffer Cache Organization Many blocks can be cached in memory (e.g., 1000s) Need to lookup block in memory efficiently how? Use a hash table Device Block # key Disk blocks in memory
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ECE344: Operating Systems Buffer Cache Write Policy When a block needs to be written, the corresponding buffer in buffer cache is updated When is the disk block updated? Immediately (synchronously) • Write-through cache • Correct, but very slow Later (asynchronously) • Write-back cache • Fast, but what if system crashes? • File system can become inconsistent
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This document was uploaded on 02/20/2012.

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ECE344-Lecture24-Fil - Lecture 24 File Systems David Lie ECE344 University of Toronto 1 Outline File Systems Overview of file system Disk Basics

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