Tested also the snapshot of today 2018/12/17 on same hardware but with 500Gb disk with single big "a" partition, it works:
OpenBSD 6.4-current (GENERIC.MP) #522: Mon Dec 17 11:59:44 MST 2018 dera...@amd64.openbsd.org:/usr/src/sys/arch/amd64/compile/GENERIC.MP real mem = 2130313216 (2031MB) avail mem = 2056507392 (1961MB) mpath0 at root scsibus0 at mpath0: 256 targets mainbus0 at root bios0 at mainbus0: SMBIOS rev. 2.6 @ 0x9f400 (48 entries) bios0: vendor American Megatrends Inc. version "080015" date 06/02/2009 bios0: ECS GF8100VM-M5 acpi0 at bios0: rev 0 acpi0: sleep states S0 S3 S4 S5 acpi0: tables DSDT FACP APIC MCFG OEMB HPET INFO NVHD SSDT acpi0: wakeup devices SMB0(S4) USB0(S4) USB2(S3) US15(S4) US12(S3) NMAC(S5) P0P1(S4) HDAC(S4) MXR0(S4) BR11(S4) BR12(S4) BR13(S4) BR14(S4) BR15(S4) BR16(S4) BR17(S4) [...] acpitimer0 at acpi0: 3579545 Hz, 24 bits acpimadt0 at acpi0 addr 0xfee00000: PC-AT compat cpu0 at mainbus0: apid 0 (boot processor) cpu0: AMD Athlon(tm) II X2 250 Processor, 3000.53 MHz, 10-06-02 cpu0: FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CFLUSH,MMX,FXSR,SSE,SSE2,HTT,SSE3,MWAIT,CX16,POPCNT,NXE,MMXX,FFXSR,PAGE1GB,RDTSCP,LONG,3DNOW2,3DNOW,LAHF,CMPLEG,SVM,EAPICSP,AMCR8,ABM,SSE4A,MASSE,3DNOWP,OSVW,IBS,SKINIT,ITSC cpu0: 64KB 64b/line 2-way I-cache, 64KB 64b/line 2-way D-cache, 1MB 64b/line 16-way L2 cache cpu0: ITLB 32 4KB entries fully associative, 16 4MB entries fully associative cpu0: DTLB 48 4KB entries fully associative, 48 4MB entries fully associative cpu0: AMD erratum 721 detected and fixed cpu0: smt 0, core 0, package 0 mtrr: Pentium Pro MTRR support, 8 var ranges, 88 fixed ranges cpu0: apic clock running at 199MHz cpu0: mwait min=64, max=64, IBE cpu1 at mainbus0: apid 1 (application processor) cpu1: AMD Athlon(tm) II X2 250 Processor, 3000.01 MHz, 10-06-02 cpu1: FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CFLUSH,MMX,FXSR,SSE,SSE2,HTT,SSE3,MWAIT,CX16,POPCNT,NXE,MMXX,FFXSR,PAGE1GB,RDTSCP,LONG,3DNOW2,3DNOW,LAHF,CMPLEG,SVM,EAPICSP,AMCR8,ABM,SSE4A,MASSE,3DNOWP,OSVW,IBS,SKINIT,ITSC cpu1: 64KB 64b/line 2-way I-cache, 64KB 64b/line 2-way D-cache, 1MB 64b/line 16-way L2 cache cpu1: ITLB 32 4KB entries fully associative, 16 4MB entries fully associative cpu1: DTLB 48 4KB entries fully associative, 48 4MB entries fully associative cpu1: AMD erratum 721 detected and fixed cpu1: smt 0, core 1, package 0 ioapic0 at mainbus0: apid 2 pa 0xfec00000, version 11, 24 pins acpimcfg0 at acpi0 acpimcfg0: addr 0xe0000000, bus 0-255 acpihpet0 at acpi0: 25000000 Hz acpiprt0 at acpi0: bus 0 (PCI0) acpiprt1 at acpi0: bus 1 (P0P1) acpiprt2 at acpi0: bus -1 (IXVE) acpiprt3 at acpi0: bus 2 (MXR0) acpiprt4 at acpi0: bus -1 (BR11) acpiprt5 at acpi0: bus 3 (BR12) acpiprt6 at acpi0: bus 4 (BR13) acpiprt7 at acpi0: bus -1 (BR14) acpiprt8 at acpi0: bus -1 (BR15) acpiprt9 at acpi0: bus -1 (BR16) acpiprt10 at acpi0: bus -1 (BR17) acpicpu0 at acpi0: C1(@1 halt!), PSS acpicpu1 at acpi0: C1(@1 halt!), PSS acpitz0 at acpi0: critical temperature is 110 degC acpipci0 at acpi0 PCI0: 0x00000010 0x00000011 0x00000000 acpicmos0 at acpi0 "*pnp0c14" at acpi0 not configured acpibtn0 at acpi0: PWRB cpu0: 3000 MHz: speeds: 3000 2300 1800 800 MHz pci0 at mainbus0 bus 0 "NVIDIA MCP77 Memory" rev 0xa2 at pci0 dev 0 function 0 not configured pcib0 at pci0 dev 1 function 0 "NVIDIA MCP77 ISA" rev 0xa2 nviic0 at pci0 dev 1 function 1 "NVIDIA MCP77 SMBus" rev 0xa1 iic0 at nviic0 spdmem0 at iic0 addr 0x50: 1GB DDR2 SDRAM non-parity PC2-5300CL5 spdmem1 at iic0 addr 0x51: 1GB DDR2 SDRAM non-parity PC2-5300CL5 iic1 at nviic0 "NVIDIA MCP77 Memory" rev 0xa1 at pci0 dev 1 function 2 not configured "NVIDIA MCP77 Co-processor" rev 0xa2 at pci0 dev 1 function 3 not configured "NVIDIA MCP77 Memory" rev 0xa1 at pci0 dev 1 function 4 not configured ohci0 at pci0 dev 2 function 0 "NVIDIA MCP77 USB" rev 0xa1: apic 2 int 15, version 1.0, legacy support ehci0 at pci0 dev 2 function 1 "NVIDIA MCP77 USB" rev 0xa1: apic 2 int 10 usb0 at ehci0: USB revision 2.0 uhub0 at usb0 configuration 1 interface 0 "NVIDIA EHCI root hub" rev 2.00/1.00 addr 1 ohci1 at pci0 dev 4 function 0 "NVIDIA MCP77 USB" rev 0xa1: apic 2 int 10, version 1.0, legacy support ehci1 at pci0 dev 4 function 1 "NVIDIA MCP77 USB" rev 0xa1: apic 2 int 11 usb1 at ehci1: USB revision 2.0 uhub1 at usb1 configuration 1 interface 0 "NVIDIA EHCI root hub" rev 2.00/1.00 addr 1 pciide0 at pci0 dev 6 function 0 "NVIDIA MCP77 IDE" rev 0xa1: DMA, channel 0 configured to compatibility, channel 1 configured to compatibility pciide0: channel 0 disabled (no drives) pciide0: channel 1 ignored (disabled) azalia0 at pci0 dev 7 function 0 "NVIDIA MCP77 HD Audio" rev 0xa1: apic 2 int 15 azalia0: codecs: VIA/0xe721 audio0 at azalia0 ppb0 at pci0 dev 8 function 0 "NVIDIA MCP77 PCI" rev 0xa1 pci1 at ppb0 bus 1 ahci0 at pci0 dev 9 function 0 "NVIDIA MCP77 AHCI" rev 0xa2: apic 2 int 9, AHCI 1.2 ahci0: port 0: 3.0Gb/s scsibus1 at ahci0: 32 targets sd0 at scsibus1 targ 0 lun 0: <ATA, ST500DM002-1BD14, HP73> SCSI3 0/direct fixed naa.5000c5006469f66a sd0: 476940MB, 512 bytes/sector, 976773168 sectors nfe0 at pci0 dev 10 function 0 "NVIDIA MCP77 LAN" rev 0xa2: apic 2 int 10, address 00:25:11:c2:87:93 rlphy0 at nfe0 phy 1: RTL8201L 10/100 PHY, rev. 1 ppb1 at pci0 dev 16 function 0 "NVIDIA MCP77 PCIE" rev 0xa1: msi pci2 at ppb1 bus 2 radeondrm0 at pci2 dev 0 function 0 "ATI Radeon HD 3450" rev 0x00 drm0 at radeondrm0 radeondrm0: msi azalia1 at pci2 dev 0 function 1 "ATI Radeon HD 34xx HD Audio" rev 0x00: msi azalia1: no supported codecs ppb2 at pci0 dev 18 function 0 "NVIDIA MCP77 PCIE" rev 0xa1: msi pci3 at ppb2 bus 3 ppb3 at pci0 dev 19 function 0 "NVIDIA MCP77 PCI" rev 0xa1: msi pci4 at ppb3 bus 4 pchb0 at pci0 dev 24 function 0 "AMD AMD64 10h HyperTransport" rev 0x00 pchb1 at pci0 dev 24 function 1 "AMD AMD64 10h Address Map" rev 0x00 pchb2 at pci0 dev 24 function 2 "AMD AMD64 10h DRAM Cfg" rev 0x00 km0 at pci0 dev 24 function 3 "AMD AMD64 10h Misc Cfg" rev 0x00 pchb3 at pci0 dev 24 function 4 "AMD AMD64 10h Link Cfg" rev 0x00 isa0 at pcib0 isadma0 at isa0 com0 at isa0 port 0x3f8/8 irq 4: ns16550a, 16 byte fifo pckbc0 at isa0 port 0x60/5 irq 1 irq 12 pckbd0 at pckbc0 (kbd slot) wskbd0 at pckbd0: console keyboard pcppi0 at isa0 port 0x61 spkr0 at pcppi0 lpt0 at isa0 port 0x378/4 irq 7 it0 at isa0 port 0x2e/2: IT8718F rev 4, EC port 0xa10 usb2 at ohci0: USB revision 1.0 uhub2 at usb2 configuration 1 interface 0 "NVIDIA OHCI root hub" rev 1.00/1.00 addr 1 usb3 at ohci1: USB revision 1.0 uhub3 at usb3 configuration 1 interface 0 "NVIDIA OHCI root hub" rev 1.00/1.00 addr 1 vmm0 at mainbus0: SVM/RVI uhidev0 at uhub2 port 2 configuration 1 interface 0 "Microsoft Microsoft Wheel Mouse Optical\M-." rev 1.10/1.21 addr 2 uhidev0: iclass 3/1 ums0 at uhidev0: 3 buttons, Z dir wsmouse0 at ums0 mux 0 uhidev1 at uhub3 port 2 configuration 1 interface 0 "Chicony USB Keyboard" rev 2.00/1.65 addr 2 uhidev1: iclass 3/1 ukbd0 at uhidev1: 8 variable keys, 6 key codes wskbd1 at ukbd0 mux 1 uhidev2 at uhub3 port 2 configuration 1 interface 1 "Chicony USB Keyboard" rev 2.00/1.65 addr 2 uhidev2: iclass 3/0, 3 report ids uhid0 at uhidev2 reportid 1: input=1, output=0, feature=0 uhid1 at uhidev2 reportid 2: input=2, output=0, feature=2 uhid2 at uhidev2 reportid 3: input=3, output=0, feature=0 vscsi0 at root scsibus2 at vscsi0: 256 targets softraid0 at root scsibus3 at softraid0: 256 targets root on sd0a (d1b9eadf722d2aa4.a) swap on sd0b dump on sd0b initializing kernel modesetting (RV620 0x1002:0x95C5 0x1043:0x01E2). radeondrm0: 1280x1024, 32bpp wsdisplay0 at radeondrm0 mux 1: console (std, vt100 emulation), using wskbd0 wskbd1: connecting to wsdisplay0 wsdisplay0: screen 1-5 added (std, vt100 emulation) disklabel sd0 # /dev/rsd0c: type: SCSI disk: SCSI disk label: ST500DM002-1BD14 duid: d1b9eadf722d2aa4 flags: bytes/sector: 512 sectors/track: 63 tracks/cylinder: 255 sectors/cylinder: 16065 cylinders: 60801 total sectors: 976773168 boundstart: 64 boundend: 976768065 drivedata: 0 16 partitions: # size offset fstype [fsize bsize cpg] a: 976768000 64 4.2BSD 4096 32768 26062 # / c: 976773168 0 unused tnx for the patch! :)