On Tuesday 22 March 2011, Andreas Dilger wrote: > On 2011-03-21, at 8:05 PM, Arnd Bergmann wrote: > > On Monday 21 March 2011 19:03:09 Andreas Dilger wrote: > >> Note that mballoc was specifically designed to handle allocation > >> requests that are aligned on RAID stripe boundaries, so it should > >> be able to handle this for MMC as well. What is needed is to tell > >> the filesystem what the underlying alignment is. That can be done > >> at format time with mke2fs or afterward with tune2fs by using the > >> "-E stripe_width" option. > > > > Ah, that sounds useful. So would I set the stripe_width to the > > erase block size, and the block group size to a multiple of that? > > When you write "block group size" do you mean the ext4 block group? Yes. > Then yes it would help. You could also consider setting the flex_bg > size to a multiple of this, so that the bitmap blocks are grouped as > a multiple of this size. However, they may not be aligned correctly, > which needs extra effort that isn't obvious. > > I think it would be nice to have mke2fs take the stripe_width and/or > flex_bg factor into account when sizing/aligning the bitmaps, but it > doesn't yet. A few more questions: * On cards that can only write to a single erase block at a time, should I make the block group size the same as the as the erase block? I suppose writing both block bitmaps, inode and data to separate erase blocks would create multiple eraseblock read-modify-write cycles for every single file otherwise. * Is it guaranteed that inode bitmap, inode, block bitmap and blocks are always written in low-to-high sector order within one ext4 block group? A lot of the drives will do a garbage-collect step (adding hundreds of miliseconds) every time you move back inside of the eraseblock. * Is there any way to make ext4 use effective blocks larger than 4 KB? The most common size for a NAND flash page is 16 KB right (effectively, ignoring what the hardware does), so it would be good to never write smaller. * Calling TRIM on SD cards is probably counterproductive unless you trim entire erase blocks. Is that even possible with ext4, assuming that we use block group == erase block? * Is there a way to put the journal into specific parts of the drive? Almost all SD cards have an area in the second 4 MB (more for larger cards) that can be written using random access without forcing garbage collection on other parts. > > Does this also work in (rare) cases where the erase block size is > > not a power of two? > > It does (or is supposed to), but that isn't code that is exercised > very much (most installations use a power-of-two size). Ok. Recently, cheap TLC (three-level cell, 3-bit MLC) NAND is becoming popular. I've seen erase block sizes of 6 MiB, 1376 KiB (4096 / 3, rounded up) and 4128 KiB (1376 * 3) because of this, in place of the common 4096 KiB. The SD card standard specifies values of 12 MB and 24 MB aside from the usual power-of-two values up to 64 MB for large cards (>32GB), while smaller cards are allowed only up to 4 MB erase blocks and need to be power-of-two. Many cards do not use the size they claim in their registers. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
