[Nexus 5X Original Android Development] kdh2834:
First take out fstab.bullhead from jollakernel/bullhead/ramdisk folder.

open fstab with notepad or notepad++(preferred, more easy to see)

and change the code from this

# Android fstab file.
#
# The filesystem that contains the filesystem checker binary (typically /system) cannot
# specify MF_CHECK, and must come before any filesystems that do specify MF_CHECK

/dev/block/platform/soc.0/f9824900.sdhci/by-name/system /system ext4 ro,barrier=0 wait
/dev/block/platform/soc.0/f9824900.sdhci/by-name/vendor /vendor ext4 ro,barrier=1 wait
/dev/block/platform/soc.0/f9824900.sdhci/by-name/userdata /data f2fs rw,noatime,nodiratime,nosuid,nodev,nobarrier,discard,inline_xattr wait,check,encryptable=/dev/block/platform/soc.0/f9824900.sdhci/by-name/metadata
/dev/block/platform/soc.0/f9824900.sdhci/by-name/userdata /data ext4 rw,noatime,nodiratime,nosuid,nodev,barrier=0,data=ordered,nomblk_io_submit,noauto_da_alloc,errors=panic wait,check,encryptable=/dev/block/platform/soc.0/f9824900.sdhci/by-name/metadata
/dev/block/platform/soc.0/f9824900.sdhci/by-name/cache /cache f2fs rw,noatime,nodiratime,nosuid,nodev,nobarrier,discard,inline_xattr wait,check
/dev/block/platform/soc.0/f9824900.sdhci/by-name/cache /cache ext4 rw,noatime,nodiratime,nosuid,nodev,barrier=0,data=ordered,nomblk_io_submit,noauto_da_alloc,errors=panic wait,check
/dev/block/platform/soc.0/f9824900.sdhci/by-name/persist /persist ext4 nosuid,nodev,barrier=1,data=ordered,nodelalloc,nomblk_io_submit,errors=panic wait,notrim
/dev/block/platform/soc.0/f9824900.sdhci/by-name/modem /firmware vfat ro,shortname=lower,uid=1000,gid=1000,dmask=227,fmask=337,context=u:object_r:firmware_file:s0 wait
/dev/block/platform/soc.0/f9824900.sdhci/by-name/boot /boot emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/recovery /recovery emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/misc /misc emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/modem /radio emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/sbl1 /sbl1 emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/tz /tz emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/rpm /rpm emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/sdi /sdi emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/aboot /aboot emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/imgdata /imgdata emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/pmic /pmic emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/hyp /hyp emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/sec /sec emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/keymaster /keymaster emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/cmnlib /cmnlib emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/persistent /persistent emmc defaults defaults
/devices/soc.0/f9200000.ssusb/f9200000.dwc3/xhci-hcd.0.auto/usb* auto auto defaults voldmanaged=usb:auto
/dev/block/zram0 none swap defaults zramsize=533413200,notrim
/dev/block/platform/soc.0/f9824900.sdhci/by-name/sbl1bak /sbl1bak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/tzbak /tzbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/rpmbak /rpmbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/abootbak /abootbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/pmicbak /pmicbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/hypbak /hypbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/keymasterbak /keymasterbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/cmnlibbak /cmnlibbak emmc defaults defaults

To this

# Android fstab file.
#
# The filesystem that contains the filesystem checker binary (typically /system) cannot
# specify MF_CHECK, and must come before any filesystems that do specify MF_CHECK

/dev/block/platform/soc.0/f9824900.sdhci/by-name/system /system f2fs rw,noatime,nodiratime,nosuid,nodev,nobarrier,discard,inline_xattr wait
/dev/block/platform/soc.0/f9824900.sdhci/by-name/system /system ext4 ro,barrier=0 wait
/dev/block/platform/soc.0/f9824900.sdhci/by-name/vendor /vendor ext4 ro,barrier=1 wait
/dev/block/platform/soc.0/f9824900.sdhci/by-name/userdata /data f2fs rw,noatime,nodiratime,nosuid,nodev,nobarrier,discard,inline_xattr wait,check,encryptable=/dev/block/platform/soc.0/f9824900.sdhci/by-name/metadata
/dev/block/platform/soc.0/f9824900.sdhci/by-name/userdata /data ext4 rw,noatime,nodiratime,nosuid,nodev,barrier=0,data=ordered,nomblk_io_submit,noauto_da_alloc,errors=panic wait,check,encryptable=/dev/block/platform/soc.0/f9824900.sdhci/by-name/metadata
/dev/block/platform/soc.0/f9824900.sdhci/by-name/cache /cache f2fs rw,noatime,nodiratime,nosuid,nodev,nobarrier,discard,inline_xattr wait,check
/dev/block/platform/soc.0/f9824900.sdhci/by-name/cache /cache ext4 rw,noatime,nodiratime,nosuid,nodev,barrier=0,data=ordered,nomblk_io_submit,noauto_da_alloc,errors=panic wait,check
/dev/block/platform/soc.0/f9824900.sdhci/by-name/persist /persist ext4 nosuid,nodev,barrier=1,data=ordered,nodelalloc,nomblk_io_submit,errors=panic wait,notrim
/dev/block/platform/soc.0/f9824900.sdhci/by-name/modem /firmware vfat ro,shortname=lower,uid=1000,gid=1000,dmask=227,fmask=337,context=u:object_r:firmware_file:s0 wait
/dev/block/platform/soc.0/f9824900.sdhci/by-name/boot /boot emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/recovery /recovery emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/misc /misc emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/modem /radio emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/sbl1 /sbl1 emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/tz /tz emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/rpm /rpm emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/sdi /sdi emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/aboot /aboot emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/imgdata /imgdata emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/pmic /pmic emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/hyp /hyp emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/sec /sec emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/keymaster /keymaster emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/cmnlib /cmnlib emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/persistent /persistent emmc defaults defaults
/devices/soc.0/f9200000.ssusb/f9200000.dwc3/xhci-hcd.0.auto/usb* auto auto defaults voldmanaged=usb:auto
/dev/block/zram0 none swap defaults zramsize=533413200,notrim
/dev/block/platform/soc.0/f9824900.sdhci/by-name/sbl1bak /sbl1bak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/tzbak /tzbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/rpmbak /rpmbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/abootbak /abootbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/pmicbak /pmicbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/hypbak /hypbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/keymasterbak /keymasterbak emmc defaults defaults
/dev/block/platform/soc.0/f9824900.sdhci/by-name/cmnlibbak /cmnlibbak emmc defaults defaults

Next step will be your ROM

Go to META-INF/com/google/android and open updater-script and change all the ext4 to f2fs

For eg.

mount("ext4", "EMMC", "/dev/block/platform/soc.0/f9824900.sdhci/by-name/system", "/system", "");

To

mount("f2fs", "EMMC", "/dev/block/platform/soc.0/f9824900.sdhci/by-name/system", "/system", "");

Put the edited files back to the folders where you took them out from in the zip. When I'm doing this on the computer i drag and drop the file out and put them back the same way.

Since I've not experimented f2fs on 5x yet I cant' ascertain you if this will work or not but this is what i used to do when I had my N5 so it will probably work.

This should be done everytime you change your rom or kernel.

Let me know if this works :)