Date: Tue, 13 Nov 2018 14:51:36 -0800 From: Conrad Meyer <cem@freebsd.org> To: Alan Somers <asomers@freebsd.org> Cc: "freebsd-arch@freebsd.org" <freebsd-arch@freebsd.org>, freebsd-fs <freebsd-fs@freebsd.org>, freebsd-current <freebsd-current@freebsd.org> Subject: Re: Hole-punching, TRIM, etc Message-ID: <CAG6CVpVcr=e=Dmg3JKD0BVQ9wiEWUujThAwy=PXyoyoRr_R7Og@mail.gmail.com> In-Reply-To: <CAOtMX2jgb_Pf9-MqirM=xihVpyRmAGZKx2VRnvA_1Fx6kMYXXg@mail.gmail.com> References: <CAOtMX2jgb_Pf9-MqirM=xihVpyRmAGZKx2VRnvA_1Fx6kMYXXg@mail.gmail.com>
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Hi Alan, On Tue, Nov 13, 2018 at 2:10 PM Alan Somers <asomers@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
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