Re: when does btrfs create sparse extents?

[Marek Behun <marek.behun@nic.cz>]

On Thu, 23 Apr 2020 11:49:16 +0100
Filipe Manana <fdmanana@gmail.com> wrote:

> On Wed, Apr 22, 2020 at 10:00 PM Marek Behun <marek.behun@nic.cz> wrote:
> >
> > On Wed, 22 Apr 2020 14:44:46 -0600
> > Chris Murphy <lists@colorremedies.com> 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?

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