On 09/29/2010 10:08 AM, Nicholas A. Bellinger wrote: > On Tue, 2010-09-28 at 08:46 +0200, Boaz Harrosh wrote: >>> + printk(KERN_WARNING "Ignoring UNMAP for non BD" >>> + " backend for struct file\n"); >> >> TODO: Lots of extent based filesystems could enjoy if you punch an hole >> in the file. (Which will also send a proper discard). >> So how to punch an hole in a file via vfs? > > Good question, as all of the current block discard support assumes a > struct block_device * pointer currently. > > I know that hch has mentioned that accesing a struct block_device from > struct file is very dangerous, but I am assuming for the TCM/FILEIO case > that this code will be protected by igrab() and iput(). > > hch and jaxboe, any comments here..? > No you did not understand. FILEIO (working on a filesystem's real file), should *never* attempt a discard on the blocks even if it was to read the file's map and figure out these blocks. It should always call a filesystem specific API that puntches-an-hole in the file. Fore stupid filesystem that means fill with zero's. For a smart extent-based file system it means splitting up the extents the range belongs to and freeing up the block that where allocated for that range. But it is an heavy meta-data operation that only the filesystem can do. With really smart FS like xfs the unallocated blocks also get discarded, for when working with SANs with over provisioning. > Thanks! > > --nab Boaz -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
