Analytical View of Garbage Collection in Solid State Media File Systems
1.0 Overview
Solid State media's such as flash memory are non-volatile computer memory that can be electrically erased and reprogrammed. These are specific types of EEPROM's that are erased and programmed in large blocks.
Flash memory is non-volatile, which means that it does not need power to maintain the information stored in the chip. In addition, flash memory offers fast read access times and better kinetic shock resistance than hard disks (Table 1).
Though flash memory has many advantages, its special hardware characteristics impose design challenges on storage systems.
Media Access Time
Read (512B) Write (512B) Erase
DRAM 2.56µs 2.56µs -
NOR Flash 14.4µs 3.53ms 1.2s (128kB)
NAND Flash 135.9µs 226µs 2-3ms (16kB)
Disk 12.4ms 12.4ms -
Table 1. Characteristics of different storage media. (NOR Flash: Intel 28F128JF3A-150, NAND Flash: K9F5608U0M)
1.1 Architecture
1.1.1 Partitions
Flash memory is divided into one or many sections of memory called partitions. A multi-partition architecture allows system processor to multi-task the I/O operations with the flash memory. While the processor can read from one partition, it can write/erase in another.
Figure 1. Partitions, Blocks and Pages
1.1.2 Blocks
In addition to partitions, flash memory is further divided into sections of memory called blocks (Figure 1). Flash memory devices in which all blocks are of the same size are symmetrically-blocked, while devices which are asymmetrically-blocked generally have several blocks that are significantly smaller than main array of flash blocks. Small blocks are typically used for storing small data or boot code. Block sizes vary 64kB to 256kB.
1.1.3 Pages
Each block in flash memory comprises of fixed number of pages (Figure 1). A page is typically of size 512B to 2kB. While erase operations can be done only on blocks, I/O operations can be done on every page.
1.2 Programming Data
Flash devices allow programming values from logical "1" to "0", but not from "0" to "1" (Figure 2). To program values back to "1"s requires erasing a full block. In most cases when data is data edited, it can either be re-written to the same block by caching the original, erasing the block and re-writing over it, or by writing the edited file to a new location in flash and the old one invalidated. Eventually, invalid data needs to be reclaimed which is usually done as a background process.
1 1 1 1 1 1 1 1 1 1
1 0 1 0 0 1 1 0 1 0
1 1 1 1 1 1 1 1 1 1
Figure 2. Flash Programming Limitations
2.0 Flash File System Functions
While flash file systems have many functions in common with file systems for other media, there are many that are unique to file systems for flash devices.
2.1 Wear Leveling
Each block in a flash memory device has a finite number of erase-write cycles (~ 10,000 to 100,000).
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