md ftl

Multiple Device Driver and Flash FTL

Sarah Diesburg

COP 5641

Introduction

Kernel uses logical remapping layers over storage to hide complexity and add functionality

Two examples

Multiple device drivers

Flash Translation Layer (FTL)

The md driver

Provides virtual devices

Created from one or more independent underlying devices

The basic mechanism to support

RAIDs

Full-disk encryption (software)

LVM

Secure deletion (TrueErase)

The md driver

File systems mounted on top of device mapper virtual device

Virtual device can

Abstract multiple devices

Perform encryption

Other things

User/Kernel Applications

DM

File System

Simple Device Mappers

Linear Maps a linear range of a device

Delay delays reads and/or writes and maps them to different

devices

Zero provides a block-device that always returns zero'd data on

reads and silently drops writes similar behavior to /dev/zero, but as a block-device instead

of a character-device.

Flakey Used for testing only, simulates intermittent, catastrophic

device failure

http://lxr.linux.no/#linux+v3.2/Documentation/device-mapper

Loading a device mapper

#!/bin/sh

# Create an identity mapping for a device

echo "0 `blockdev --getsize $1` linear $1 0" \

| dmsetup create identity

Loading a device mapper

#!/bin/sh

# Create an identity mapping for a device

echo "0 `blockdev --getsize $1` linear $1 0" \

| dmsetup create identity

Logical start sector

Loading a device mapper

#!/bin/sh

# Create an identity mapping for a device

echo "0 `blockdev --getsize $1` linear $1 0" \

| dmsetup create identity

Command to get number of sectors of a device (like /dev/sda1)

Loading a device mapper

#!/bin/sh

# Create an identity mapping for a device

echo "0 `blockdev --getsize $1` linear $1 0" \

| dmsetup create identity

Type of device mapper device we want. Linear is a one-to-one logical to physical sector mapping.

Loading a device mapper

#!/bin/sh

# Create an identity mapping for a device

echo "0 `blockdev --getsize $1` linear $1 0" \

| dmsetup create identity

Linear parameters: base device (like /dev/sda1)

Loading a device mapper

#!/bin/sh

# Create an identity mapping for a device

echo "0 `blockdev --getsize $1` linear $1 0" \

| dmsetup create identity

Linear parameters: starting offset within the device

Loading a device mapper

#!/bin/sh

# Create an identity mapping for a device

echo "0 `blockdev --getsize $1` linear $1 0" \

| dmsetup create identity

Pipe the command to dmsetup, acts like “table_file” parameter

Loading a device mapper

#!/bin/sh

# Create an identity mapping for a device

echo "0 `blockdev --getsize $1` linear $1 0" \

| dmsetup create identity

dmsetup command manages logical devices that use the device mapper driver. See ‘man dmsetup’ for more information.

Loading a device mapper

#!/bin/sh

# Create an identity mapping for a device

echo "0 `blockdev --getsize $1` linear $1 0" \

| dmsetup create identity

We wish to “create” a new logical device mapper device.

Loading a device mapper

#!/bin/sh

# Create an identity mapping for a device

echo "0 `blockdev --getsize $1` linear $1 0" \

| dmsetup create identity

We name the new device “identity”.

Loading a device mapper

Can then mount file system directly on top of virtual device

#!/bin/bash

mount /dev/mapper/identity /mnt

Unloading a device mapper

#!/bin/bash

umount /mnt

dmsetup remove identity

Unloading a device mapper

#!/bin/bash

umount /mnt

dmsetup remove identity

First unmount the file system

Unloading a device mapper

#!/bin/bash

umount /mnt

dmsetup remove identity

Then use dmsetup to remove the device called identity

dm-linear.c

Documentation

http://lxr.linux.no/#linux+v3.2/Documentation/device-mapper/linear.txt

Code

http://lxr.linux.no/#linux+v3.2/drivers/md/dm-linear.c

dm-linear.c

static struct target_type linear_target = {

.name = "linear",

.version = {1, 1, 0},

.module = THIS_MODULE,

.ctr = linear_ctr,

.dtr = linear_dtr,

.map = linear_map,

.status = linear_status,

.ioctl = linear_ioctl,

.merge = linear_merge,

.iterate_devices = linear_iterate_devices,

};

linear_map

static int linear_map(struct dm_target *ti, struct bio *bio,

union map_info *map_context)

{

struct linear_c *lc = (struct linear_c *) ti->private;

bio->bi_bdev = lc->dev->bdev;

bio->bi_sector = lc->start + (bio->bi_sector - ti->begin);

return DM_MAPIO_REMAPPED;

}

(**Note – this is a simpler function from an earlier kernel version.

Version 3.2 does the same, but with a few more helper functions)

Memory Technology Device

Different than a character or block device

Exports a special character device with extra ioctls and operations to access flash storage

For raw flash devices (not USB sticks)

Embedded chips

http://www.linux-mtd.infradead.org/

NAND Flash Characteristics

Flash has different constraints than hard drives or character devices

Exports read, write, and erase operations

NAND Flash Characteristics

Can only write to a freshly-erased location

If you want to write again to same physical location, you must first erase the area

Reads and writes are to smaller flash pages

Erasures are performed in flash blocks

Holds many flash pages

NAND Flash Characteristics

Each storage location can be erased only 10K-1M times

Writing is slower than reading

Erasures can be 10x slower than writing

Each NAND page has a small, non-addressable out-of-bounds area to hold state and mapping information

Accessed by ioctls

NAND Flash Characteristics

We need a way to not wear out the flash and have good performance with a minimum of writes and erases

Flash Translation Layer

The solution is to stack a flash translation layer (FTL) on top of the raw flash device

Exports a block device

Takes care of the flash operations of reads, writes, and erases

Evenly wears writes to all flash locations

Marks old pages as invalid until they can be erased later

Data Path

Virtual file system (VFS)

File system

Multi-device drivers

Ext3

Disk driver Disk driver MTD driver MTD driver

JFFS2

FTL

Apps

Flash Translation Layer

Rotates the usage of pages

OS

Logical Address Physical Address

0 0

1 1

Write random

bits to 1

data Flash

0 1 2 3 4 5 6

data

Flash Translation Layer

Overwrites go to new page

Logical Address Physical

Address 0 0

1 2

Write random

bits to 1

data Flash

0 1 2 3 4 5 6

random data

OS

FTL Example

INFTL – Inverse Nand Flash Translation Layer

Open-source FTL in linux kernel for DiskOnChip flash

Somewhat out-dated

INFTL

Broken into two files

inftlmount.c – load/unload functions

inftlcore.c – flash and wear-leveling operations

http://lxr.linux.no/linux+*/drivers/mtd/inftlmount.c

http://lxr.linux.no/linux+*/drivers/mtd/inftlcore.c

INFTL

Stack-based algorithm to provide the illusion of updates

Each stack (or chain) corresponds to a virtual address with sequentially-addressed pages

INFTL “Chaining”

INFTL “Chaining”

Chains can grow to any length

Once there are no more freshly-erased erase blocks, some old ones must be garbage-collected

Chain is “folded” so that all valid data is copied into top erase block

Lower erase blocks in chain are erased and put back into the pool

inftlcore.c

static struct mtd_blktrans_ops inftl_tr = {

.name = "inftl",

.major = INFTL_MAJOR,

.part_bits = INFTL_PARTN_BITS,

.blksize = 512,

.getgeo = inftl_getgeo,

.readsect = inftl_readblock,

.writesect = inftl_writeblock,

.add_mtd = inftl_add_mtd,

.remove_dev = inftl_remove_dev,

.owner = THIS_MODULE,

};

inftl_writeblock

static int inftl_writeblock(struct mtd_blktrans_dev *mbd, unsigned long block,

char *buffer)

{

struct INFTLrecord *inftl = (void *)mbd;

unsigned int writeEUN;

unsigned long blockofs = (block * SECTORSIZE) & (inftl->EraseSize - 1);

size_t retlen;

struct inftl_oob oob;

char *p, *pend;

inftl_writeblock

/* Is block all zero? */

pend = buffer + SECTORSIZE;

for (p = buffer; p < pend && !*p; p++);

if (p < pend) {

writeEUN = INFTL_findwriteunit(inftl, block);

if (writeEUN == BLOCK_NIL) {

printk(KERN_WARNING "inftl_writeblock():cannot find"

"block to write to\n");

/*

* If we _still_ haven't got a block to use, we're screwed.

*/

return 1;

}

memset(&oob, 0xff, sizeof(struct inftl_oob));

inftl_writeblock

memset(&oob, 0xff, sizeof(struct inftl_oob));

oob.b.Status = oob.b.Status1 = SECTOR_USED;

inftl_write(inftl->mbd.mtd, (writeEUN * inftl->EraseSize) +

blockofs, SECTORSIZE, &retlen, (char *)buffer,

(char *)&oob);

} else {

INFTL_deleteblock(inftl, block);

}

return 0;

}