Re: Hole-punching, TRIM, etc

Hi Alan,

On Tue, Nov 13, 2018 at 2:10 PM Alan Somers <asomers_at_freebsd.org> wrote:
>
> Hole-punching has been discussed on these lists before[1].  It basically
> means to turn a dense file into a sparse file by deallocating storage for
> some of the blocks in the middle.  There's no standard API for it.  Linux
> uses fallocate(2); Solaris and OSX add a new opcode to fcntl(2).
>
> A related concept is telling a block device that some blocks are no longer
> used.  SATA calls this "TRIM", SCSI calls it "UNMAP", NVMe calls it
> "Deallocate", ZBC and ZAC call it "Reset Write Pointer".  They all do
> basically the same thing, and it's analogous to hole-punching for regular
> files.  They are also all inaccessible from FreeBSD's userland except by
> using pass(4), which is inconvenient and protocol-specific.

Geom devices have the DIOCGDELETE ioctl, which translates into
BIO_DELETE (which is TRIM, as I understand it).  It's available in
libgeom as g_delete() and used by hastd, newfs_nandfs, and nandtool.

> Linux has a BLKDISCARD ioctl for issuing TRIM-like commands from userland,
> but it's totally undocumented and doesn't work on regular files.
>
> I propose adding support for all of these things using the fcntl(2) API.
> Using the same syntax that Solaris defined, you would be able to punch a
> hole in a regular file or TRIM blocks from an SSD.  ZFS already supports it
> (though FreeBSD's port never did, and the code was deleted in r303763).
> Here's what I would do:
>
> 1) Add the F_FREESP command to fcntl(2).
> 2) Add a .fo_space field for struct fileops
> 3) Add a devfs_space method that implements .fo_space
> 4) Add a .d_space field to struct cdevsw
> 5) Add a g_dev_space method for GEOM that implements .d_space using
> BIO_DELETE.
> 6) Add a VOP_SPACE vop
> 7) Implement VOP_SPACE for tmpfs
> 8) Add aio_freesp(2), an asynchronous version of fcntl(F_FREESP).

Why not just add DIOCGDELETE support to various VOP_IOCTL
implementations?  The file objects forward correctly through vn_ioctl
to VOP_IOCTL for both regular files and devfs VCHR nodes.

We can emulate the Linux API if we want to be compatible there, but I
wouldn't bother with Solaris.

Best,
Conrad