File Systems in Real-Time Embedded Applications

March 7thEric Julien

Choosing the Right Storage Media

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IntroductionStorage media provides a nonvolatile backing device for one or multiple partitions.

Different media, different attributes:• Removability,• Latency,• Sequential write/read throughput,• Random write/read throughput,• Cost, density,• Reliability, wear, etc.

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Latency in embedded systems

Latency plays an often neglected role in FS performances on embedded systems.

Limited memory availability in embedded system prevent writes or reads using large buffers.

Global throughput becomes more affected by latency and less by I/O speed.

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Latency: SD Card example

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Bus speed Read length Rd time (WC) Throughput Improvement

25 MHz 512 B 1541 µs 324.5 KB/s 0 %

50 MHz 512 B 1520 µs 328.9 KB/s 1.4 %

25 MHz 1024 B 1582 µS 632.1 KB/s 94.8 %

SanDisk 2GB MicroSD Card, SD 4 bit mode•Access time: 1.5 ms (worst case).•I/O time, 512B @ 25MHz: 40.96 µs.•Total read time: 1541 µs (I/O 2.7% of total).

Storage Media Types

• RAM (volatile)• Hard Disk Drive, Solid State Drive • Unmanaged Flash Memory

• NOR Flash Memory• NAND Flash Memory

• Managed Flash Memory• Secure Digital (SD)/MultiMediaCard• CompactFlash

• USB Mass Storage Device

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RAM

Volatile storage technology, unless battery-backed that has almost ideal performance characteristics.

•Very low latency, esp. static RAM,•Fast write and read throughput.

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Hard Disk Drive

Stores data on rotating discs called platters coated with a magnetic material. Integrate a controller providing a sector based interface.•Head movement imply seek time (latency):

• Slower random access time.• Vulnerability to file system fragmentation.• Average latency around 12ms (mobile drives).• More RPM = less latency = more noise.

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Hard Disk Drive• Huge capacity up to a few TB.

• Very low cost per bit.

• Good sequential throughput– Typical modern 7200 RPM drive can sustain “disk-to-

buffer” rates up to 130 MB/s (outer tracks).

• Reliability• Controller provides ECC, bad sector management.• Mechanical parts can cause data loss (eg. head

crash): less suitable for mobile embedded systems.

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Unmanaged Flash MemoryFlash memory technology stores bits of information by modulating the charge on the floating-gate of field effect transistors.

The presence of a charge on the floating gate affects the threshold required to activate the transistor, allowing its state to be read.

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Unmanaged Flash Memory Types

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Bit line

Word line

Sourceline

Word line

Bit line

Source lineNOR

NAND

• Byte/word-addressable.• More metal contacts: bigger cell size.

• Page-addressable.• Less metal contacts: smaller cell size.

NOR Flash

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• Cells must be erased prior to programming.• Byte/Word-addressable.

• Suitable for eXecute In Place (XIP).

• Cells are almost independent:• Byte/Word program possible.• Reads and programs don’t disturb neighboring cells.

• NORs generally don’t require ECC.

• Erases do affect multiple cells (page, sector, block).

NOR Flash

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• Single-byte/word random reads are fast and low-latency. No moving mechanical parts.• Very good random and sequential read throughput.

• Slow programming and erase operations.• Poor sequential and random write throughput.

• High cost per bit, medium density (bigger cells).• Requires wear levelling: cell life 10 000 to 100 000

P/E cycles.• No bad blocks.

NOR Flash: interface

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Two types of interfaces:• Parallel memory-mapped, using standard external bus

interface.• Best throughput/latency, XIP support, but uses more pins.

• Serially-interfaced devices (often SPI and variants).

Common Flash memory Interface (CFI) standard.• Provides info on queried device including:

• Size, block configuration, voltage, command set, etc.

• Eliminate need for descriptor tables.• Allows new devices to be used without SW updates.

NAND Flash• Cells must be erased prior to programming• Page addressable.

• Not suitable for XIP (unless special controller).

• Cells aren’t independent:• Smallest programmable unit: page.• Smallest erasable unit: block (many pages).• Reads and programs cause disturbance to other

cells in a block.• Data reliability affected; use of ECC mandatory.

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NAND Flash• Page random reads are fast and low-latency.

• Very good random and sequential read throughput.

• Fast programming and erase operation.– Good sequential and random write throughput.

• Low cost per bit, high density (smaller cells).• Requires wear leveling:

– Average cell life SLC: 100 000 to 1 000 000 P/E cycles.– Average cell life MLC: 10 000 to 100 000 P/E cycles.

• Bad blocks must be dealt with (incl. factory).

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NAND Flash: interface• Most common interface: parallel on IO bus.

• Less pins than parallel NOR.

• Some serial devices (often integrate ECC).• Multiple plane, multiple die devices.• Open NAND Flash Interface (ONFI) standard.

• Standard pinout for multiple packages.• Standard command set.• Parameter page for querying device parameters.• Timing requirements, etc.

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Unmanaged Flash Comparison

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Type NOR NAND

Addressable Unit Byte/Word Page (512B, 2kB, 4kB)

Sequential Write Very Slow Fast

Single Byte Write Slow Slow

Sequential Read Fast Very fast

Single Byte Read Fast Slow

eXecute In Place (XIP) Yes No

Density Low High

Cost per Bit High Low

Bad Blocks No Yes

Avg. cell life (P/E c.) 10 000-100 000 SLC: 100 000-1 000 000MLC: 10 000-100 000

Error Correction Codes Not required SLC: 1-8 bit/512 bytesMLC: 8+ bits/512 bytes

Managed Flash MemoryNOR and NAND Flash memories, due to their erase before write nature, need to be handled differently than traditional devices.The algorithms involved are often complex and/or costly.Managed flash memory devices provide a simpler interface, taking care of logical block abstraction, wear leveling, EDC data protection, bad block management, etc.

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Secure Digital/MultiMediaCardSecure Digital (SD) and MultiMediaCard are removable devices that contain a storage device (often NAND) along with a controller.•Provides a standardized physical interface, socket and command set.•Basic variants of SD and MMC have identical pinouts and almost identical command set.•SD has a SPI compatible mode (MMC: opt.).

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Secure Digital/MultiMediaCard

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Card Type MMC MMCplus SD microSD eMMC

Form Factor Standard Standard Standard Micro IC

Pins 7 13 9 8 -

Removable Yes Yes Yes Yes No

1-bit bus mode Yes Yes Yes Yes Yes

4-bit bus mode No Opt. Opt. Opt. Opt.

8-bit bus mode No Opt. No No Opt.

SPI mode Opt. Opt. Yes Yes Opt.

Max bus rate 20 Mb/s 416 Mb/s 832 Mb/s 832 Mb/s 1600 Mb/s

Secure Digital

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• Maximum capacity is 2GB for SD cards, 32 GB for SDHC cards and 2TB for SDXC cards.• SDA has recommended exFAT as FS for SDXC cards.

• Speed class: minimum sustained sequential read/write speed in MB/s with a card in a fragmented state (class 10: non-fragmented).• Random read/write performance not controlled.

• Ultra High Speed (UHS): bus modes increasing max bus bandwidth (DDR, clk rate, 1.8V).

e•MMCManaged flash memory in a non-removable package, compatible with MMCplus, convenient for integrated embedded system storage.

• Higher throughput bus modes,• TRIM (background erase),• Secure erase (purge),• Sanitize (purge unmapped),• Reliable writes,• Boot areas…

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Secure Digital/MultiMediaCardPerformance of SD/MMC cards are highly dependent on embedded controller and storage device used.

• Sequential read/write throughput is usually good, but may be limited by max bus rate.

• Random read performance is usually middling, depending on controller latency.

• Random write performance is usually poor to mediocre, depending on controller.

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USB Mass Storage Class DevicesUSB standard has a device class specifying a protocol for controlling a logical block addressed storage device through USB.Most common types:

• External HDD: similar performance to internal HDDs, but may be limited in throughput by bus.

• Flash memory (USB Flash drives): similar performance to SD/MMC cards, dependent on embedded controller and storage device.

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Storage Media Comparison

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Type Hard Disk NOR NAND SD/MMC

Addressable Unit Logical block Byte/Word Page Logical block

Sequential Write Very Good Very Slow Good Avg-Good

Random Write Good Slow Good Average

Sequential Read Outstanding Good Very Good Good

Random Read Poor Good Good Average-Good

Capacity High Low Average Average

Cost per Bit Very Low High Low Low

Reliability Fair Good Good Good

Expected life 5-15 yrs 10k-100k P/E (SLC)

100k-1M P/E (SLC)10k-100k P/E (MLC) Device dependent

USB HDD: similar to Hard Disk, USB Flash Drive: similar to SD/MMC