FLASH MEMORY AS A HARD DRIVE REPLACEMENT
Seminar by
Hisham Pasha.N.K.
Roll No:8S5 MCA
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OVERVIEW
• FLASH BASICS
• FLASH DEGRADATION
• FLASH MEMORY V/S HARD DISK DRIVES
• FLASH AS A HARD DRIVE REPLACEMENT
• PROLONGING FLASH LIFE
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HARD DISK STRUCTURE
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HARD DISK STRUCTURE
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STRUCTURE OF A USB FLASH DRIVE
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Applications of flash memory
• DIGITAL CAMERAS
• PERSONAL DIGITAL ASSISTANTS (PDAs)
• GAME CONSOLES
• USB DRIVES
• GENERAL PURPOSE SERVERS
• BLADE SYSTEMS
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FLASH MEMORY USED IN DEVICES
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FLASH DEGRADATION
Storage units perform two basic functions:
- reading
- writing The lifetime of a flash device is measured in
“write/erase” or “program/clear” cycles. Write operations are performed on “pages”. Erase operations are performed on “blocks”. The lifespan of a flash device is long, but
limited
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FLASH MEMORY V/S HDD
Key factors:-
Pricing trends
Technology trends
Energy consumption
Robustness
File size
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Cost is a Key Factor
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Advantages of Flash over HDD
• In portable applications• In limited capacity applications• In applications where file size or the total no: of files to be stored is more important than total system cost
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FLASH AS A HARD DRIVE REPLACEMENT
• Wear leveling– Technique to rotate the write/erase cycles.– Goal
Get the entire device to wear at approximately the same rate.
– Generally implemented in h/w on a microcontroller on the same PCB as the flash memory
– Wear leveling failing formula:-
C*L*O/W*60*60*24*365 C*L*O/W*60*60*24*365
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PROLONGING FLASH LIFE
1. Reducing Writes
2. Move /tmp to tmpfs
3. Manage swap space
4. Migrate frequently written files off
flash
5. Tune file system to minimize
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1. PROLONGING FLASH LIFE BY REDUCING WRITES
• Redirect Log Output
Mounting Log Files Via NFS
Redirecting Log Output Using Syslog
Redirecting Application Logs to Syslog
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Mounting Log Files Via NFS
• To mount /var as an NFS share– Entry required in /etc/fstab to avoid traffic
problem in NFS`network
<server>:</path/to/dir> /var nfs sync 0 0<server>:</path/to/dir> /var nfs sync 0 0
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• Kernel 2.6.23– RHEL5
– Increase Performance– Large no: of writes fsc no fscTo disable fsc; following /etc/fstab entry used
In log server side; following entry in /etc/exports to authorize the client to mount
and write to the share
<server>:</path/to/dir> /var nfs sync,nofsc 0 0<server>:</path/to/dir> /var nfs sync,nofsc 0 0
/path/to/logs/<client host name><client IP>(rw,no_root_squash,no_subtree_check)/path/to/logs/<client host name><client IP>(rw,no_root_squash,no_subtree_check)
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1. PROLONGING FLASH LIFE BY REDUCING WRITES
• Redirect Log Output
Mounting Log Files Via NFS
Redirecting Log Output Using Syslog
Redirecting Application Logs to Syslog
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Redirecting Log Output Using Syslog
• Log some or all messages to a remote log server
• UDP is used to send log messages• Security Implications:-
Eg:- Logging over an untrusted n/w– Replacing Syslog by Syslogng
• Logging using TCP
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1. PROLONGING FLASH LIFE BY REDUCING WRITES
• Redirect Log Output
Mounting Log Files Via NFS
Redirecting Log Output Using Syslog
Redirecting Application Logs to Syslog
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Redirecting Application Logs to Syslog
• Hosted applications write to log filesEg:- BIND & some types of Apache
error loggingTo enable Apache access logs:
CustomLog “|/path/to/logging/script.pl”common
CustomLog “|/path/to/logging/script.pl”common
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2.PROLONGING FLASH LIFE BY MOVING /tmp TO tmpfs
The /tmp directory is a large source of writes.The common parameters are size, nr_inodes and mode.
/tmp can be mounted as a ramdisk.Ramdisks:- Areas of RAM reserved by OS Mounted as disks Provide very High Speed Storage
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Advantages of tmpfs over Ramdisks
tmpfs dynamically grows the file system up to a user-specified limit
tmpfs is able to swap out seldom-used pages
tmpfs can be resized on the fly by simply remounting it
To mount 100MB tmpfs partition, run the command
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%mount –t tmpfs –o size=100M,nr_inodes=1k,mode=777 tmpfs /tmp%mount –t tmpfs –o size=100M,nr_inodes=1k,mode=777 tmpfs /tmp
3. PROLONGING FLASH LIFE BY MANAGING SWAP SPACE
• Analyzing swap space• Running without swap space• Swapping to external storage
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Analyzing swap space
• To determine current memory and swap usage of a Linux system
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%free -mt%free -mt
Running without swap space1. Determine which device is the
swap device
2. Turn off swapping temporarily
3. Configure system to run with out swap on a more permanent mode
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% grep swap /etc/fstab
/dev/sda2 swap swap 0 0
% grep swap /etc/fstab
/dev/sda2 swap swap 0 0
% swapoff /dev/sda2% swapoff /dev/sda2
# /dev/sda2 swap swap defaults 0 0# /dev/sda2 swap swap defaults 0 0
4. PROLONGING FLASH LIFE BY MIGRATING FREQUENTLY WRITTEN FILES
OFF FLASHMigration of frequently written files can be in
any of the following ways:-
Move a file into a Ramdisk and sacrifices
permanence
Move a file onto an NFS share and impact
speed
Move a file into externally attached storage
and add cost to the system
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5. PROLONGING FLASH LIFE BY TUNING FILE SYSTEMS TO MINIMIZE WRITES
Areas to be considered:- Make sure reads don’t cause writes Check that the file system is not
using some form of secure deletion Utilize on-the-fly file system
compression Evaluate if any partitions can be
mounted read-only
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Standard Linux File System
The Ext2 File System Highly configurable Supports secure deletion Access time logging The Ext3 File System Journaling file system Not recommended for flash drives The JFFS file system Used on non-wear leveled flash devices Log file system Intelligent garbage collection and built-in
compression Targeted at embedded device market
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Queries?
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