Hi, On Wed, Jan 19, 2011 at 3:57 PM, Arnd Bergmann <a...@arndb.de> wrote: > On Wednesday 19 January 2011, Dave Martin wrote: >> It works well enough that e2fsck passes after a writing an image to a >> card which was previously full of random data. >> >> Anyway, it's there if anyone wants to play with it -- comments welcome. > > * You write with a block size of just 4kb here, which is really suboptimal > when talking to SD cards. The performance should be best if you write only > aligned 64kb blocks or larger. In particular, it's faster to fill 64kb > with zeros when writing just one non-zero byte than it is to skip the > remaining space.
Good points -- initially I kept things very simple just to see if I got valid cards out. And it wasn't initially SD specific. I guess it might wake sense to wait until your investigations are concluded and roll the results into a really good utiliy for writing images ... how close do you reckon you are to the end? > * At least for the output, I'd do O_DIRECT, since you don't care if the > block device ends up in your page cage or not. For the input, you might > want to mmap the image file into memory and do madvise(MADV_DONTNEED) > on the data you have already written out, to evict it from your page cache. Both good ideas :) > * In the cases where we have an actual SD card reader, it would be really > cool to use ioctl(BLKDISCARD) on the medium to erase it before writing to > it. This has two effects: > 1. For data you want to write immediately, the following write is faster, > especially if you don't write 4MB at a time. > 2. The card's wear levelling works much better if some of the space has > been marked as erased. You know which parts are zero, so erasing them > would be a signficant speedup for the life-time of the system. > Note that some cards fill erased space with 0xff bytes, while others fill > the card with 0x00. Erase does not work if you are dealing with a USB > card reader. Sounds interesting ... does that only apply to SD cards, or does it work for other media? Does this work for card readers and flash storage on USB? > * I'd like to see this program do a ioctl(HDIO_GETGEO) to verify that the > partition is aligned. You should probably print a warning if the alignment > of the ext3 partition is less than 4MB, and bail out if it is less than 64kb, > as that is the point where it gets really slow. Well, currently this just copies a whole image, with no knowledge of what partition layout (if any) is inside. I may experiment with some changes, but patches are definitely welcome if you feel energetic :) Cheers ---Dave _______________________________________________ linaro-dev mailing list linaro-dev@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-dev
