COMP 3000 Essay 1 2010 Question 10

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

1. Limited number of writes/erasures
2. Bytes cannot be individually flipped

Traditionally Optimized File Systems

With the previous section in mind, it should come as no surprise that a conventional hard disk drive (HDD) file-system is not optimized to work with flash memory. The reason for this is that conventional hard-disks have different constraints from those of flash memory - their primary problem is to reduce seeking time, while the primary problem when working with flash memory is to erase in a minimal and balanced way.

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 in order 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. This is also why defragmentaion, a procedure used by HDDs to put files into more convenient configurations and thus minimize seeking times, loses its purpose in a flash memory context. Indeed, the unnecessary erasures that it entails are both inefficient and harmful for a flash memory unit.

This comes direclty out of flash memory's aforementioned constraints: the slow block-sized erasures and the limited number of erase-cycles. Because of these, a flash optimal file system needs to minimize its erase operations and also to spread out its erasures in such a way as to avoid the formation of hot-spots: sections of memory which have undergone a disproportionately high number of erasures and are thus in danger of burning out. To minimize these, a system using flash memory would have to write new data to empty memory blocks. This method would also call for some sort of garbage collection to conduct necessary erasure operations while the system is idle. It makes better sense to do these at this time because of the slow nature of erasures in the flash memory context. Of course, there is no such feature in a traditional HDD file-system.

Flash Optimized File Systems

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

References

I'm going to start making up some proper citations for the websites we have. the order they're going in probably won't line up with the actual essay so I put the number for the link I'm actually citing next to each link, renumber as appropriate:

also, add in the right page number

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old

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