Question

How do the constraints of flash storage affect the design of flash-optimized file systems? Explain by contrasting with hard disk-based file systems.

Answer

First introduced in the late 80s, Flash-memory is a light, energy-independent, compact, shock-resistent and efficiently readable type of storage. Because of the particular limitations of this kind of memory, flash file systems require a fundamentally different system architecture than disk-based file-systems: these systems need to be designed in light of flash-memory’s limited number of erase-cycles and its need to conduct erasures a block at a time. These constraints are a direct result of the same design that gives flash its advantages with reguard to [ TO WHAT?] as both are due to [TO WHAT?] . Thus, a typical disk-based file-system is not suitable for working with flash memory as it erases far too frequently and indiscriminently while being simultaneously optimized for other constraints that do not affect flash memory. This means that a different solution is necessary and that is the log-based file-system: this type of system is far better suited to working with flash memory because it optimizes erasures by [WHAT?].

Flash Memory

Flash memory is Nonvolatile(meaning digital storage that does not require power to retain its memory.) storage space that has become more popular recently due to its fast fetch times.

Constraints

Flash memory has two major constraints

Limited number of writes/erasures
Bytes cannot be individually flipped

Hard Disk drives

basic info on HDDs

Constraints

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Traditionally Optimized File Systems

things traditional files systems do to optimize HDD read/write/etc

Most conventional file systems are designed to me implemented on hard disk drives. This fact does not mean they cannot be implemented on a solid state drive (file storage that uses flash memory instead of magnetic discs). It would however, in many ways, defeat the purpose of using flash memory. The most consuming process for an HDD is seeking data by relocating the read-head and spinning the magnetic disk. A traditional file system optimizes the way it stores data by placing related blocks close-by on the disk to minimize mechanical movement within the HDD. One of the great advantages of flash memory, which accounts for its fast read speed, is that there is no need to seek data physically so there is no need to waste resources laying out the data in close proximity. A traditional HDD file system will also attempt to defragment itself, moving blocks of data around for closer proximity on the magnetic disk. This process, although beneficial for HDD's, is harmful and inefficient for flash based storage. A flash optimal file system needs to reduce the amount of erase operations, since flash memory only has a limited amount of erase cycles as well as having very slow erase speeds. When an HDD rewrites data to a physical location there is no need for it to erase the previously occupying data first, so a traditional disk based file system doesn't worry about erasing data from unused memory blocks. In contrast flash memory needs to first erase the data block before it can modify any of it contents. Since the erase procedure is extremely slow, its not practical to overwrite old data every time. It is also decremental to the life span of flash memory. To maximize the potential of flash based memory the file system would have to write new data to empty memory blocks. This method would also call for some sort of garbage collection to erase unused blocks when the system is idle, which does not get implemented in conventional file systems since it is not needed.

Flash Optimized File Systems

Flash Optimized files systems do to optimize HDD read/write/etc

Similarities

this could probably be titled better

Differences

ditto for this one

Questions

References

External links

Hey guys,

This is what I've got so far... mostly based on wikipedia:

Flash memory has two limitations: it can only be erased in blocks and and it wears out after a certain number of erase cycles. Furthermore, a particular kind of Flash memory (NAND) is not able to provide random access. As a result of these Flash based file-systems cannot be handled in the same way as disk-based file systems. Here are a few of the key differences:

- Because memory must be erased in blocks, its erasure tends to take up time. Consequently, it is necessary to time the erasures in a way so as not to interfere with the efficiency of the system’s other operations. This is is not a real concern with disk-based file-systems. - A disk file-system needs to minimize the seeking time, but Flash file-system does not concern itself with this as it doesn’t have a disk. - A flash system tries to distribute memory in such a way so as not to make a particular block of memory subject to a disproportionally large number of erasures. The purpose of this is to keep the block from wearing out prematurely. The result of it is that memory needs to be distributed differently than in a disk based file-system. Log-sturctured file systems are thus best suited to dealing with flash memory (they apparently do all of the above things).

For the essay form, I'm thinking of doing a section about traditional hard-disk systems, another about flash-memory and a third about flash systems. At this point, I am imagining the thesis as something like, "Flash systems require a fundamentally different system architecture than disk-based systems due to their need to adapt to the constraints inherent in flash memory: specifically, due to that memory's limited life-span and block-based erasures." The argument would then talk about how these two differences directly lead to a new FS approach.

That's how I see it at the moment. Honestly, I don't like doing research about this kind of stuff, so my data isn't very deep. That said, if you guys could find more info and summarize it, I'm pretty sure that I could synthesize it all into a coherent essay.

Fedor