Seminar on Linux-based embedded systemsRoot File system for Raspberry Pi

Aalto University13th Dec 2013

Presenter: Bastian Arjun Shajit (338303)

Agenda

1. Introduction

2. Types of File system

3. Selecting a Root file system

4. Performance Comparison

5. Summary

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What is a File system ?

• Gargantuan amount of data stored as 1s and 0s on storage media (conceivable by machine)

• Impossible to understand files/folders from the sea of binary data.

• File system was designed as a novel way to organize such data easily understandable by all users.

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Definition

• It can refer to – The abstract structure in which files are named and logically

stored in a storage media (RFS)

eg. In any *NIX distribution,

(or)

– The software which provides the core support for implementing the abstraction defined above in the operating system/kernel.

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/

bin dev home etc lib . . . . . . . . . . . . . .

Root/Top levelDirectory

Agenda

1. Introduction

2. Types of File system

3. Selecting a Root file system

4. Performance Comparison

5. Summary

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Block-device File Systems

• Considers storage device as a simple linear collection of blocks or sectors.– Size of blocks vary (512, 1024 …)

• Have two main operation : Read & Write• Type of Devices:

– LBA devices (HDDs)– FTL devices (USB flash drive, SD, SSDs etc).

• In case of bad blocks, the FTL or hardware (for LBA devices) takes care of hiding from user.

• File system drivers : ext2, ext3, ext4, FAT, NTFS, ReiserFS etc.

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Flash File Systems

• Mainly used on Raw Flash Devices• Linux provides MTD as a primitive translation Layer.

• How are they Different?– They are not block devices– Mostly raw NAND / NOR storage media.– They consist of ERASEBLOCKS– Have 3 main operation : Read, Write & Erase Eraseblock– Each block has limited lifespan (1000s of write cycles)

• Needs write to be spread.

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Flash File Systems II

• FTL devices Vs Raw Flash device– FTL devices has a micro-controller which runs FTL firmware.

• Helps to emulate a block device.

• Why FFS is unique?– Needs to spread write– Perform Garbage collection– Bad blocks handling– Better reliability compared to FTL (Since FTL is vendor specific)

• E.g. UBIFS, JFFS2 etc.

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In-Memory File system

• Uses primary memory e.g. RAM• Features:

– Has higher I/O performance– Volatile Memory

• E.g, ramfs, tmpfs, ramdisk.• Why not ramdisk?

– It emulates block device in RAM.– Block device store recently accessed files in caches. Hence

using ramdisk creates a large memory foot print– Hard limit on max size of ram disk.

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User Space File system

• Mainly intended for block devices.• Resides in the user space.• Overhead of communicating to and from kernel and

User space.• Distinct Advantage:

– A bad user space file-system can never cause a system crash.

• E.g. ntfs-3g

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Compressed Read-only File systems

• Read-only filesystem• Data Stored in compressed format.• Especially useful in low memory systems• E.g.

– Cramfs:• Metadata not stored (No time stamps or user permissions)

• Possible security issues.

• Max file system size = 256 MB

– Squashfs• stores metadata information.

• Max filesystem size = 2^64.

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Linux Virtual File system

• Role of VFS:– Register/de-register– Associate devices and file

system drivers.– Generic File operations

• Caching

• Look ups

– Forward File-system specific

operation:• Read

• Write

• etc

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Agenda

1. Introduction

2. Types of File system

3. Selecting a Root file system

4. Performance Comparison

5. Summary

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Selecting a root file-system

• Based on the type of device– Is it Raw flash, RAM or block device

• Based on available system resource.• Based on the type of I/O on the device

– If Read-only, then handle partitions required by kernel for write.

• Based on the data volatility– To persist data or not.

• Based on data usage and demand.

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Bare minimal Root Filesystem

• Created a bare minimal root filesystem using buildroot for RPi. Refer buildroot manual for instructions, http://buildroot.uclibc.org/downloads/manual/manual.html

• Includes basic utilities.• Saves disc space.• Following figure represents the size of rootfs obtained

from buildroot.

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TypeRaw Root

Filesystem (MB)

With Kernel Libraries

(MB)

Bare Minimal 4.97 5.32

With Essential Utils 20.65 32.33

Raspbian 1624.89 1641.1

Agenda

1. Introduction

2. Types of File system

3. Selecting a Root file system

4. Performance Comparison

5. Summary

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Experiment

• Aims at providing a new perspective on how to choose file system based on performance.

• Comparison between ramfs vs ntfs-3g vs ext2• Moved root file-system across different storage media to

analyze performance.• Performance aspects considered:

– I/O speed– CPU usage.

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Performance Benchmark I

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Performance Benchmark II

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Agenda

1. Introduction

2. Types of File system

3. Selecting a Root file system

4. Performance Comparison

5. Summary

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Summary

• Successfully created a bare minimal and fully loaded root file system.

• There isn’t “One file system to rule them all”. • Varying requirement give rise to numerous possibilities

and a mix & match of different file systems can perhaps solve the need of the hour.

• Many file systems were not analyzed– Raw flash device unavailability to test flash file systems.– Failed experiments with squashfs.– Out of scope file-systems like msdos, nfs, smbfs, cifs.

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Thank you !!!

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