On Mon, Nov 04, 2013 at 02:03:43AM -0800, Christoph Hellwig wrote: > > Besides that I really miss an explanation what the intended use cases > are. What does this buy us over punching a hole on an actual real > workload? Where is the overhead? Is it our shitty discard > implementation? If so there's tons of low hanging fruit to fix there > anyway that we shouldn't work around by interfaces taking shortcuts. > Is it problems in ext4's extent management on hole punch? Is the > bit of metadata created when doing an actual hole punch too much for > that very specific workload? The an application in question wants to treat a large file as if it were a block device --- that's hardly unprecedented; enterprise databases tend to prefer using raw block devices (at least for benchmarking purposes), but system administrators like to administrative convenience of using a file system. The goal here is get the performace as close to a raw block device as possible. Especially if you are using fast flash, the overhead of deallocating blocks using punch, only to reallocate the blocks when we later write into them, is just unnecessary overhead. Also, if you deallocate the blocks, they could end up getting grabbed by some other block allocation, which means the file can end up getting very fragmented --- which doesn't matter that much for flash, I suppose, but it means the extent tree could end up growing and getting nasty over time. The bottom line is why bother doing extra work when it's not necessary? If people don't like exposing this via FIEMAP, we could just simply make it available via the BLKDISCARD ioctl, which already exists (although currently it is only implemented for block devices). - Ted -- 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
