Flash Storage Devices

SSD, MMC & SD

NOR & NAND

Generally come in two flavors:

• NOR
  • Used to store firmware images on embedded devices
  • Connected to the processor via address and data lines (like RAM)
  • Can be eXecuted In Place (XIP) by a processor
• NAND
  • Used for solid-state mass storage media (USB sticks, SD cards)
  • Large, dense, cheap, but imperfect storage
  • Interfaces uses IO and control lines
  • Has to be copied to RAM before execution
  • Requires some sort of flash memory management

NAND typically presented to the system as ATA disk

• FTL (Flash Translation Layer)
  • Handle all internal flash memory operations
  • Presents illusion of ‘standard’ block device
  • Logical to physical address mapping
  • Block size translation
  • Power off recovery
  • Wear leveling (erase/write counts)
  • Error correction (single/multi bit errors)
  • Bad-block management (remap failing blocks)
  • Write RAM buffer (tends to scale with price/size)
• Typically embedded in hardware (categorized as managed NAND)

NAND characteristics:

• Erased in blocks at a time, before re-write is possible
• Erase blocks are divided into pages (for example 64 per erase block)
• Pages usually a multiple of 512 bytes
• “Spare area” bytes used to store meta data (i.e. worn out marker, ECC data)
• Usually has bad blocks already when leaving the factory (to save costs)

Flash memory is unreliable:

• Data stored as probabilistic approximations
• Workaround: computational error correction (ECC)

Contain an internal controller

SSD

Storage

Memory cells…

• SLC (Single Level Cell)…1 Bit per memory cell
  • Write/erase 100ns…100,000 cycles
  • Low density…higher cost…more endurance
• MLC (Multi Level Cell)…2 Bits per memory cell
  • Write/erase 120ns…10,000 cycles
  • Medium denisty…less cost…less endurance
• TLC (Triple Level Cell)…3 Bits per memory cell
• 16LC (16 Level Cell)…4 Bits per memory cell

Layout…

• Pages…multiple memory cells
  • …smallest structure is one page
  • …can be read/written
• Blocks…multiple pages
  • …smallest structure than can be erased
  • 512 KB block…128x pages each 16.384 cells MLC (4KB)
• Planes…multiple blocks
• Dies…multiple planes
• TSOP (Thin Small Outline Package)
  • …multiple dies (typically two)
  • …device build from multiple TSOPs

Endurance

Sparse area…

• 7% to 25% of net capacity
• Cells have limited erase cycles..erase before overwrite required
• Wear leveling…
  • …wear-out distributed over all cells
  • …bad block replacement from sparse area
• Bad blocks…
  • …fails to erase…
  • …controller keeps track and replaces…
  • …as long there is enough capacity
• Write errors…
  • RBER (Raw Bit Error Rate)…
  • …increase over time by cell aging
  • …repaired buy ECC
  • UBER (Uncorrectable Bit Error Rate)
    • …should stay very low
    • …errors <1 for every 10^15 accesses

MMC & eMMC

MMC (MultiMedia Card)

• Non-volatile, removable, portable flash (NAND) base storage
• Developed by Joint Electron Device Engineering Council (JEDEC)
• SD (Secure digital) cards are an improvement to MMC

eMMC (Embedded MultiMedia Card)

• Not removable
• Optimized for low cost, low power, small form factor
• Typically single silicon dye for controller, interface, and memory

UFS (Universal Flash Storage)…meant to replace SD cards and eMMCs

SD Cards

SD Card (Secure Digital Card)

• Non-volatile memory for consumer hand-held and portable electronics
• Based on MMC (Multi Media Cars), adding more features
• SD Specifications is controlled by the SD Card Association

Dimensions:

• Standard 32x24mm (2g)
• Mini 21x20mm (800mg)
• Micro 15x11mm (250mg)

Families	Size	Description
SDSC	1 MB to 2 GB	Standard-Capacity
SDHC	2 GB to 32 GB	High-Capacity
SDXC	32 GB to 2 TB	eXtended-Capacity
SDUC	2 TB to 128 TB	Ultra-Capacity

Speed classes: U1,U3 (10/30 MB/s) - V{1,3,6,9}0 MB/s

Bus	Speed max.	Spec
UHS-I	≤ 104MB/s	3.01
UHS-II	≤ 312MB/s	4.0
UHS-III	≤ 624MB/s	6.0
Express	≤ 985MB/s	7.0

Vendors: SanDisk, Samsung, Lexar, Kingston, Toshiba

Model	Read MB/s	Write MB/s	Size GB
SanDisk Extrem PRO	<275	<100	64,128
Lexar 1800x	<270	<150	64,128
Toshiba M502	<270	<150	16,32,64
SanDisk Extrem PLUS	<170	<90	32,64,128,256,400,512
SanDisk Extrem	<160	<90	32,64,128,256,400,512
Lexar 1000x	<150	<90	32,64,128,256
Samsung EVO Plus	<100	<90	32,64,128,256,512
Samsung EVO Select	<100	<90	32,64,128,256,512
Kingston Canvas React	<100	<80	32,64,128,256,512
Lexar 633x	<100	<70	16,32,64,128,256,512
Samsung PRO Endurance	<100	<30	32,64,128
SanDisk Ultra PLUS	<100		16,32,64,128,256,512
Samsung EVO	<100		32,64,128,256
Toshiba M402	<95	<95	16,32,64,128
Toshiba M401	<95	<80	16,32,64
Toshiba M303	<95	<65	64,128,256
Toshiba M302	<90		16,32,64,128
Kingston Canvas Go	<90	<45	32,64,128
SanDisk Ultra	<80		16,32,64
Kingston Canvas Select	<80	<10	16,32,64,128,256
Lexar 300x	<45	<45	16,32,65

/dev/mmc*

SD cards almost always pre-formatted, typically FAT32 of exFAT (on SDXC cards)

• Most controllers optimises for FAT
• First partition starts on an erase boundary (segment-aligned)

Potential file-systems to use on MMC devices

• FAT (default on SD cards)
  • Lacks features of modern file-systems
• F2FS (Flash Friendly File-System)
  • Create by Samsung, integrated into the Linux kernel (2013)
  • Aims to create a NAND flash aware file-system
  • Atomic operations, defragmentation, TRIM support

Supported in Linux though the mmc subsystem

• Code located in drivers/mmc and headers in include/linux/mmc/
• Block device /dev/mmc*

# delete a DOS/MBR boot sector
dd if=/dev/zero bs=512 count=1 of=/dev/mmcblk0
# write a new boot sector
parted /dev/mmcblk0 mklabel msdos
# create a new partition
parted -a optimal /dev/mmcblk0 mkpart primary 0% 100%
# write a FAT32 file-system
mkfs.fat -F 32 /dev/mmcblk0p1
# mount the device
pmount /dev/mmcblk0p1 flash && ls /media/flash