Re: Hole-punching, TRIM, etc

On Tue, Nov 13, 2018 at 3:10 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.
>
> 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).
>
> The greatest beneficiaries of this work would be type 2 hypervisors like
> QEMU and VirtualBox with guests that use TRIM, and userland filesystems
> such as fusefs-ext2 and fusefs-exfat.  High-performance storage systems
> using SPDK would also benefit.  The last item, aio_freesp(2), may seem
> unnecessary but it would really benefit my application.
>
> Questions, objections, flames?
>

So the fcntl would deallocate blocks from a filesystem only. The filesystem
may issue BIO_DELETE as a result, but that's up to the filesystem, correct?

On a raw device it would be translated into a BIO_DELETE command directly,
correct?

Warner