Flash 比傳統的 HardDisk 會遇上以下幾個問題,

• block allocation (NAND flash chips pages of 2-4KB, multi-channel access 後: 8-16KB)
• garbage collection(Erasing blocks) (idle 時要自動去清空可用的 block, 一般成 2 MB 大 !!, allocation group 後, 4 MB)
• Wear leveling [寫入次數 Flash10^3 (for MLC NAND), 10^5 (NOR, SLC NAND)]
• Bad eraseblocks 管理

不過它們全由 controller 解決

Memory Technology Devices

MTD 是 Linux 系統用來 Flash Controller 的軟件層來

把 HardDisk 看成 mtd

modprobe block2mtd
echo "/dev/loop0" > /sys/module/block2mtd/parameters/block2mtd
cat /proc/mtd

OR

modprobe block2mtd block2mtd=/dev/sdb1

軟件包:

mtd-utils
/usr/sbin/mkfs.jffs2
/usr/sbin/mtdinfo

logfs-tools (/sbin/mklogfs, man 8 mklogfs)
(replace JFFS2 for most uses)

-c, --compress

LogFS

特點:

• stores the inode tree on the drive; JFFS2 does not.
  (which requires it to scan the entire drive at mount and cache the entire tree in RAM)
• packing multiple file-system blocks into a single flash-memory block.
  (A "sum" entry at the end of the flash-memory block records what data is stored in it.)

Format 它:

mkfs.logfs /dev/sdb1

Will create filesystem with the following details:
              hex:   decimal:
fssize=   1e940000  513015808
segsize=     40000     262144
blocksize=    1000       4096
writesize=       1          1

Do you wish to continue (yes/no)

奇怪的是在 Ubuntu 12.04 是 mount 不到 logfs 的 ....

grep -i logfs /boot/config-3.*

/boot/config-3.2.0-33-generic:# CONFIG_LOGFS is not set
/boot/config-3.2.0-34-generic:# CONFIG_LOGFS is not set
/boot/config-3.2.0-35-generic:# CONFIG_LOGFS is not set

注意

USB-Sticks, SD-Card 它們是 block devices 來 !! 原因是它有 FTL (Flash Translation Layer) 在硬件裡面

所以對它們而言, 用 ext2 及 "noatime,relatime" mount option 是最優化的.

• atime: 會記錄最後讀取時間
• noatime: 不記錄最後讀取時間
• relatime: 當最後讀取時間比最後更改時間早時才更新