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zfs: add wd0 notion

    1: # ZFS on NetBSD
    2: 
    3: This page attempts to do two things: provide enough orientation and
    4: pointers to standard ZFS documentation for NetBSD users who are new to
    5: ZFS, and to describe NetBSD-specific ZFS information.  It is
    6: emphatically not a tutorial or an introduction to ZFS.
    7: 
    8: Many things are marked with \todo because they need a better
    9: explanation, and some have question marks
   10: 
   11: # Documentation Pointers
   12: 
   13: See the man pages for zfs(8) and zpool(8).
   14: 
   15:   - [OpenZFS Documentation](https://openzfs.github.io/openzfs-docs/)
   16:   - [OpenZFS admin docs index page](https://github.com/openzfs/zfs/wiki/Admin-Documentation)
   17:   - [FreeBSD Handbook ZFS Chapter](https://www.freebsd.org/doc/handbook/zfs.html)
   18:   - [Oracle ZFS Administration Manual](https://docs.oracle.com/cd/E26505_01/html/E37384/index.html)
   19:   - [Wikipedia](https://en.wikipedia.org/wiki/ZFS)
   20: 
   21: # Status of ZFS in NetBSD
   22: 
   23: ## NetBSD 8
   24: 
   25: NetBSD 8 has an old version of ZFS, and it is not recommended for use
   26: at all.  There is no evidence that anyone is interested in helping
   27: with ZFS on 8.  Those wishing to use ZFS on NetBSD 8 should therefore
   28: update to NetBSD 9.
   29: 
   30: ## NetBSD 9
   31: 
   32: NetBSD-9 has ZFS that is considered to work well.  There have been
   33: fixes since 9.0_RELEASE.  As always, people running NetBSD 9 are
   34: likely best served by the most recent version of the netbsd-9 stable
   35: branch.  As of 2021-02, ZFS in the NetBSD 9.1 release is very close to
   36: netbsd-9.
   37: 
   38: ## NetBSD-current
   39: 
   40: NetBSD-current (as of 2021-02) has similar ZFS code to 9.
   41: 
   42: There is initial support for [[ZFS root|wiki/RootOnZFS]], via booting from
   43: ffs and pivoting.
   44: 
   45: ## NetBSD/xen special issues
   46: 
   47: In NetBSD-9, MAXPHYS is 64KB in most places, but because of xbd(4) it
   48: is set to 32KB for XEN kernels.  Thus the standard zfs kernel modules
   49: do not work under xen.  In NetBSD-current, xbd(4) supports 64 KB
   50: MAXPHYS and this is no longer an issue.
   51: 
   52: Xen and zfs on current are reported to work well together, as of 2021-02.
   53: 
   54: ## Architectures
   55: 
   56: Most people seem to be using amd64.
   57: 
   58: To build zfs, one puts MKZFS=yes in mk.conf.  This is default on amd64
   59: and aarch64 on netbsd-9.  In current, it is also default on sparc64.
   60: 
   61: More or less, zfs can be enabled on an architecture when it is known
   62: to build and run reliably.  (Of course, users are welcome to build it
   63: and report.)
   64: 
   65: # Quick Start
   66: 
   67: See the [FreeBSD Quickstart
   68: Guide](https://www.freebsd.org/doc/handbook/zfs-quickstart.html); only
   69: the first item is NetBSD specific.
   70: 
   71:   - Put zfs=YES in rc.conf.
   72: 
   73:   - Create a pool as "zpool create pool1 /dev/dk0".
   74: 
   75:   - df and see /pool1
   76: 
   77:   - Create a filesystem mounted on /n0 as "zfs create -o
   78:     mountpoint=/n0 pool1/n0".
   79: 
   80:   - Go back and read the documentation and start over.
   81: 
   82: # NetBSD-specific information
   83: 
   84: ## rc.conf
   85: 
   86: The main configuration is to put zfs=YES in rc.conf, so that the rc.d
   87: scripts bring up ZFS and mount ZFS file systems.
   88: 
   89: ## pool locations
   90: 
   91: One can add disks or parts of disks into pools.  Methods of specifying
   92: areas to be included include:
   93: 
   94:   - entire disks (e.g., /dev/wd0d on amd64, or /dev/wd0 which has the same major/minor)
   95:   - disklabel partitions (e.g., /dev/sd0e)
   96:   - wedges (e.g., /dev/dk0)
   97: 
   98: ## mount order
   99: 
  100: NetBSD 9 mounts other file systems and then ZFS file systems.  This can
  101: be a problem if /usr/pkgsrc is on ZFS and /usr/pkgsrc/distfiles is on
  102: NFS.  A workaround is to use noauto and do the mounts in
  103: /etc/rc.local.
  104: 
  105: NetBSD current after 20200301 mounts ZFS first.  The same issues and
  106: workarounds apply in different circumstances.
  107: 
  108: ## NFS
  109: 
  110: zfs filesystems are reported to be exportable over NFS.
  111: 
  112: The "zfs share" command adds a line to /etc/zfs/exports, and "zfs
  113: unshare" removes it.
  114: 
  115: \todo Explain how /etc/zfs/exports is used.
  116: 
  117: This is reported to work on 9.0 STABLE, but to cause a panic on
  118: current (20200302).  See [misc/55042](http://gnats.netbsd.org/55042).
  119: 
  120: ## zvol
  121: 
  122: Within a ZFS pool, the standard approach is to have file systems, but
  123: one can also create a zvol, which is a block device of a certain size.
  124: 
  125: \todo The zvol will appear as /dev/???? and can be used in many
  126: respects like a slice.  However, the system will not read disklabels
  127: and gpt labels from a zvol; in this respect it is more like a disklabel
  128: partition or wedge than a disk drive.
  129: 
  130: \todo Explain that one can export a zvol via iscsi.
  131: 
  132: \todo Explain if one can swap on a zvol.
  133: 
  134: \todo Explain that one can use ccd to create a normal-looking disk
  135: from a zvol.  This allows reading a GPT label from the zvol, which is
  136: useful in case the zvol had been exported via iscsi and some other
  137: system created a label.
  138: 
  139: # Memory usage
  140: 
  141: Basically, ZFS uses lots of memory and most people run it on systems
  142: with large amounts of memory.  NetBSD works well on systems with
  143: comparatively small amounts of memory.  So a natural question is how
  144: well ZFS works on one's VAX with 2M of RAM :-) More seriously, one
  145: might ask if it is reasonable to run ZFS on a RPI3 with 1G of RAM, or
  146: if it is reasonable on a system with 4G.
  147: 
  148: The prevailing wisdom is more or less that ZFS consumes 1G plus 1G per
  149: 1T of disk.  32-bit architectures are viewed as too small to run ZFS.
  150: 
  151: Besides RAM, zfs requires that architecture kernel stack size is at
  152: least 12KB or more -- some operations cause stack overflow with 8KB
  153: kernel stack. On NetBSD, the architectures with 16KB kernel stack are
  154: amd64, sparc64, powerpc, and experimental ia64, hppa. mac68k and sh3
  155: have 12KB kernel stack. All others use only 8KB stack, which is not
  156: enough to run zfs.
  157: 
  158: NetBSD has many statistics provided via sysctl; see "sysctl
  159: kstat.zfs".
  160: 
  161: FreeBSD has tunables that NetBSD does not seem to have, described in
  162: [FreeBSD Handbook ZFS Advanced
  163: section](https://docs.freebsd.org/en/books/handbook/zfs/#zfs-advanced).
  164: 
  165: # Interoperability with other systems
  166: 
  167: Modern ZFS uses pool version 5000 and feature flags.
  168: 
  169: It is in general possible to export a pool and them import the pool on
  170: some other system, as long as the other system supports all the used
  171: features.
  172: 
  173: \todo Explain how to do this and what is known to work.
  174: 
  175: \todo Explain feature flags relationship to FreeBSD, Linux, iIllumos,
  176: macOS.
  177: 
  178: # Sources of ZFS code
  179: 
  180: Currently, there are multiple ZFS projects and codebases:
  181: 
  182:   - [OpenZFS](http://www.open-zfs.org/wiki/Main_Page)
  183:   - [openzfs repository](https://github.com/openzfs/zfs}
  184:   - [zfsonlinux](https://zfsonlinux.org/)
  185:   - [OpenZFS on OS X ](https://openzfsonosx.org/) [repo](https://github.com/openzfsonosx)
  186:   - proprietary ZFS in Solaris (not relevant in open source)
  187:   - ZFS as released under the CDDL (common ancestor, now of historical interest)
  188: 
  189: OpenZFS is a coordinating project to align open ZFS codebases.  There
  190: is a notion of a shared core codebase and OS-specific adaptation code.
  191: 
  192:   - [zfsonlinux relationship to OpenZFS](https://github.com/openzfs/zfs/wiki/OpenZFS-Patches)
  193:   - FreeBSD more or less imports code from openzfs and pushes back fixes. \todo Verify this.
  194:   - NetBSD has imported code from FreeBSD.
  195:   - The status of ZFS on macOS is unclear (2021-02).

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