On Thu, Apr 23, 2020 at 12:42 PM Marek Behun <marek.behun@xxxxxx> wrote: > > On Thu, 23 Apr 2020 11:49:16 +0100 > Filipe Manana <fdmanana@xxxxxxxxx> wrote: > > > On Wed, Apr 22, 2020 at 10:00 PM Marek Behun <marek.behun@xxxxxx> wrote: > > > > > > On Wed, 22 Apr 2020 14:44:46 -0600 > > > Chris Murphy <lists@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > e.g. from a 10m file created with truncate on two Btrfs file systems > > > > > > > > original holes format (default) > > > > > > > > item 6 key (257 EXTENT_DATA 0) itemoff 15768 itemsize 53 > > > > generation 7412 type 1 (regular) > > > > extent data disk byte 0 nr 0 > > > > extent data offset 0 nr 10485760 ram 10485760 > > > > extent compression 0 (none) > > > > > > > > On a file system with no-holes feature set, this item simply doesn't > > > > exist. I think basically it works by inference. Both kinds of files > > > > have size in the INODE_ITEM, e.g. > > > > > > > > item 4 key (257 INODE_ITEM 0) itemoff 32245 itemsize 160 > > > > generation 889509 transid 889509 size 10485760 nbytes 0 > > > > > > > > Sparse extents are explicitly stated in the original format with disk > > > > byte 0 in an EXTENT_DATA item; whereas in the newer format, sparse > > > > extents exist whenever EXTENT_DATA items don't completely describe the > > > > file's size. > > > > > > Ok this means that U-Boot currently gained support for the original > > > sparse extents. > > > > To clear any confusion, what you mean by sparse extents is actually holes. > > The concept of sparse files exists (files with holes, regions of a > > file for which there is no allocated extent), but not sparse extents. > > > > > > > > I fear that current u-boot does not handle the new no-holes feature. > > > > The no-holes feature has been around since 2013, not exactly new, but > > it's not the default yet when creating a new filesystem. > > > > As it has been mentioned earlier by Chris, it just removes the need > > for explicitly having metadata representing holes. > > When not using the no-holes feature, there is an explicit file extent > > item pointing to a disk location of 0 (disk_bytenr field has a value > > of 0) for each file hole. > > When using no-holes, there's no such file extent item - btrfs knows > > about the hole by checking that there is a gap between two consecutive > > file extent items (both having a disk_bytenr > 0). > > This I already understand. My main question though is: does kernel or > btrfs do checking (at least sometimes) when writing a block of data onto > disk if this block is all zero, and if yes, then this block is written > as a hole (either by writing hole item or not writing anything)? > > Or does this happen ONLY when requested by userspace? There's nothing in btrfs that converts a sequence of zeroes automatically to a hole. It always has to be done by user space, either by writes that leave holes intentionally (e.g. create file, write 64K to offset 0, write 4K to offset 128, leaves a hole from range 64K to 128K) or by hole punching through fallocate(). > > Because for the love of god I cannot find why our kernel is being > written this way onto disk - the installer doesn't explicitly request > for PUNCH_HOLES nor anything, as far as I looked. > > Marek > > Marek -- Filipe David Manana, “Whether you think you can, or you think you can't — you're right.”
