---------- Forwarded message ---------- From: Tiago Marques <tiago...@gmail.com> Date: Tue, Oct 6, 2009 at 1:26 PM Subject: Re: Woodhouse on flash storage To: Mitch Bradley <w...@laptop.org>
On Tue, Oct 6, 2009 at 11:10 AM, Mitch Bradley <w...@laptop.org> wrote: > Tiago Marques wrote: >> >> Trying to find datasheets of the flash chips to know what their erase >> block size and page size(and number of erase cycles) has been a >> nightmare for me, the manufacturer just doesn't care if your >> partitioning choice ends up sending the SSD/SD/MMC sooner than the >> warranty expires. >> Have you had the same experience? >> > > It depends on who "you" are. If "you" is Quanta (the manufacturer of OLPC > machines), chip vendors will tell you whatever you ask, because it > determines whether or not they sell tens of thousands of parts per month. > Quanta can pass that information on to us at OLPC under certain conditions. > So the answer is that OLPC can get the information with some amount of > effort, at least for parts that are actual candidates for use in our > machines. With some work I managed to get a datasheet from Samsung, which is what I asked, but I'm an individual. It said much about the flash, 5000 erase cycles down from 10k in a previous revision, although they have spare flash on each chip for redundancy. > > As an invidual or small-scale manufacturer, getting information can be very > difficult bordering on impossible. There is just no incentive for chip > vendors to spend the time to talk to individuals. The way that the > distribution model works, by the time you get out near the edges of the > distribution channel graph, the detailed knowledge has dispersed too much. > "This one is blue and it says 4 GB. The price is $14.95. Are you going to > buy it or not? Would you like to buy our extended warranty for an extra > $10? Next customer, please." > > One way to get a clue is to look at the factory partition map and use what > > As regards partitioning choice, my current thinking is: > > a) If you can, don't repartition it - use the factory format > Going to ext2 doesn't screw that up? Since they align the FAT table to be on a single block? > b) If you must repartition and you have some flexibility, look at the > factory partition map and align your partitions on the granularity of the > existing first partition's starting block number. As an extra clue, look to > see where the primary and secondary FATs start; their alignment granularity > is likely to be important. > > b) If you must construct a fixed partition layout for use on multiple > different devices, align each partition on at least a 4MiB boundary. That > means that you "waste" 4M for the partition map (one 512-byte sector padded > out to a 4 MiB boundary, but oh well). For devices 2 GiB or less - the > previous generation - maybe 1 MiB alignment would be okay, as such devices > are likely to have smaller erase blocks. Those numbers seem "safe" to me. > Perhaps you could use smaller alignment granularity with knowledge of > specific parts, but as we have noted, such knowledge can be hard to obtain. > Thanks, I'll look into that. I've refrained myself from using SWAP partitions and now use a swap file but it's still confusing to me to get the alignment right with linux's fdisk. I mostly follow ted tso's article but align it myself because I'm creating just one partition. General rule of thumb, when the flash is good, I can do all kind of nasty to it and it sill keeps going. Bought a TakeMS CF card some time ago that keeps corruptiing the filesystem for no good reason. I rather keep it aligned but it's more as a good than anything else, as I don't have the money or need to stress test flash. Best regards, Tiago Marques _______________________________________________ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel