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Wed Feb 17 17:53:07 2021 UTC (23 months, 2 weeks ago) by gdt
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zfs: describe mountpoint conventions

    1: # ZFS on NetBSD
    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.
    8: Many things are marked with \todo because they need a better
    9: explanation, and some have question marks
   11: # Status of ZFS in NetBSD
   13: ## NetBSD 8
   15: NetBSD 8 has an old version of ZFS, and it is not recommended for use
   16: at all.  There is no evidence that anyone is interested in helping
   17: with ZFS on 8.  Those wishing to use ZFS on NetBSD 8 should therefore
   18: update to NetBSD 9.
   20: ## NetBSD 9
   22: NetBSD-9 has ZFS that is considered to work well.  There have been
   23: fixes since 9.0_RELEASE.  As always, people running NetBSD 9 are
   24: likely best served by the most recent version of the netbsd-9 stable
   25: branch.  As of 2021-02, ZFS in the NetBSD 9.1 release is very close to
   26: netbsd-9.
   28: ## NetBSD-current
   30: NetBSD-current (as of 2021-02) has similar ZFS code to 9.
   32: There is initial support for [[ZFS root|wiki/RootOnZFS]], via booting from
   33: ffs and pivoting.
   35: ## NetBSD/xen special issues
   37: Summary: if you are using NetBSD, xen and zfs, use NetBSD-current.
   39: In NetBSD-9, MAXPHYS is 64KB in most places, but because of xbd(4) it
   40: is set to 32KB for XEN kernels.  Thus the standard zfs kernel modules
   41: do not work under xen.  In NetBSD-current, xbd(4) supports 64 KB
   42: MAXPHYS and this is no longer an issue.  Xen and zfs on current are
   43: reported to work well together, as of 2021-02.
   45: ## Architectures
   47: Most people seem to be using amd64.
   49: To build zfs, one puts MKZFS=yes in mk.conf.  This is default on amd64
   50: and aarch64 on netbsd-9.  In current, it is also default on sparc64.
   52: More or less, zfs can be enabled on an architecture when it is known
   53: to build and run reliably.  (Of course, users are welcome to build it
   54: and report.)
   56: # Quick Start
   58: See the [FreeBSD Quickstart
   59: Guide](; only
   60: the first item is NetBSD specific.
   62:   - Put zfs=YES in rc.conf.
   64:   - Create a pool as "zpool create pool1 /dev/dk0".
   66:   - df and see /pool1
   68:   - Create a filesystem mounted on /n0 as "zfs create -o
   69:     mountpoint=/n0 pool1/n0".
   71:   - Read the documentation referenced in the next section.
   73: ## Documentation Pointers
   75: See the man pages for zfs(8), zpool(8).  Also see zdb(8), if only for
   76: seeing pool config info when run with no arguments.
   78:   - [OpenZFS Documentation](
   79:   - [OpenZFS admin docs index page](
   80:   - [FreeBSD Handbook ZFS Chapter](
   81:   - [Oracle ZFS Administration Manual](
   82:   - [Wikipedia](
   84: # NetBSD-specific information
   86: ## rc.conf
   88: The main configuration is to put zfs=YES in rc.conf, so that the rc.d
   89: scripts bring up ZFS and mount ZFS file systems.
   91: ## pool locations
   93: One can add disks or parts of disks into pools.  Methods of specifying
   94: areas to be included include:
   96:   - entire disks (e.g., /dev/wd0d on amd64, or /dev/wd0 which has the same major/minor)
   97:   - disklabel partitions (e.g., /dev/sd0e)
   98:   - wedges (e.g., /dev/dk0)
  100: Information about created or imported pools is stored in
  101: /etc/zfs/zpool.cache.
  103: ## pool native blocksize mismatch
  105: ZFS attempts to find out the native blocksize for a disk when using it
  106: in a pool; this is almost always 512 or 4096.  Somewhere between 9.0
  107: and 9.1, at least some disks on some controllers that used to report
  108: 512 now report 4096.  This provokes a blocksize mismatch warning.
  110: Given that the native blocksize of the disk didn't change, and things
  111: seemed OK using the 512 emulated blocks, the warning is likely not
  112: critical.  However, it is also likely that rebuilding the pool with
  113: the 4096 blocksize is likely to result in better behavior because ZFS
  114: will only try to do 4096-byte writes.  \todo Verify this and find the
  115: actual change and explain better.
  117: ## pool importing problems
  119: While one can "zpool pool0 /dev/wd0f" and have a working pool, this
  120: pool cannot be exported and imported straigthforwardly.  "zpool
  121: export" works fine, and deletes zpool.cache.  "zpool import", however,
  122: only looks at entire disks (e.g. /dev/wd0), and might look at slices
  123: (e.g. /dev/dk0).  It does not look at partitions like /dev/wd0f, and
  124: there is no way on the command line to ask that specific devices be
  125: examined.  Thus, export/import fails for pools with disklabel
  126: partitions.
  128: One can make wd0 be a link to wd0f temporarily, and the pool will then
  129: be importable.  However, "wd0" is stored in zpool.cache and on the
  130: next boot that will attempt to be used.  This is obviously not a good
  131: approach.
  133: One an mkdir e.g. /etc/zfs/pool0 and in it have a symlink to
  134: /dev/wd0f.  Then, zpool import -d /etc/zfs/pool0 will scan
  135: /etc/zfs/pool0/wd0f and succeed.  The resulting zpool.cache will have
  136: that path, but having symlinks in /etc/zfs/POOLNAME seems acceptable.
  138: \todo Determine a good fix, perhaps man page changes only, fix it
  139: upstream, in curent, and in 9, before removing this discussion.
  141: ## mountpoint conventions
  143: By default, datasets are mounted as /poolname/datasetname.  One can
  144: also set a mountpoint; see zfs(8).
  146: There does not appear to be any reason to choose explicit mountpoints
  147: vs the default (and either using data in place or symlinking to it).
  149: ## mount order
  151: NetBSD 9 mounts other file systems and then ZFS file systems.  This can
  152: be a problem if /usr/pkgsrc is on ZFS and /usr/pkgsrc/distfiles is on
  153: NFS.  A workaround is to use noauto and do the mounts in
  154: /etc/rc.local.
  156: NetBSD current after 20200301 mounts ZFS first.  The same issues and
  157: workarounds apply in different circumstances.
  159: ## NFS
  161: zfs filesystems can be exported via NFS, simply by placing them in
  162: /etc/exports like any other filesystem.
  164: The "zfs share" command adds a line for each filesystem with the
  165: sharenfs property set to /etc/zfs/exports, and "zfs unshare" removes
  166: it.  This file is ignored on NetBSD-9 and current before 20210216; on
  167: current after 20210216 those filesystems should be exported (assuming
  168: NFS is enabled).  It does not appear to be possible to set options
  169: like maproot and network restrictions via this method.
  171: On current before 20210216, a remote mkdir of a filesystem mounted via
  172: -maproot=0:10 causes a kernel NULL pointer dereference.  This is now
  173: fixed.
  175: ## zvol
  177: Within a ZFS pool, the standard approach is to have file systems, but
  178: one can also create a zvol, which is a block device of a certain size.
  180: \todo The zvol will appear as /dev/???? and can be used in many
  181: respects like a slice.  However, the system will not read disklabels
  182: and gpt labels from a zvol; in this respect it is more like a disklabel
  183: partition or wedge than a disk drive.
  185: \todo Explain that one can export a zvol via iscsi.
  187: \todo Explain if one can swap on a zvol.
  189: \todo Explain that one can use ccd to create a normal-looking disk
  190: from a zvol.  This allows reading a GPT label from the zvol, which is
  191: useful in case the zvol had been exported via iscsi and some other
  192: system created a label.
  194: # Memory usage
  196: Basically, ZFS uses lots of memory and most people run it on systems
  197: with large amounts of memory.  NetBSD works well on systems with
  198: comparatively small amounts of memory.  So a natural question is how
  199: well ZFS works on one's VAX with 2M of RAM :-) More seriously, one
  200: might ask if it is reasonable to run ZFS on a RPI3 with 1G of RAM, or
  201: if it is reasonable on a system with 4G.
  203: The prevailing wisdom is more or less that ZFS consumes 1G plus 1G per
  204: 1T of disk.  32-bit architectures are viewed as too small to run ZFS.
  206: Besides RAM, zfs requires that architecture kernel stack size is at
  207: least 12KB or more -- some operations cause stack overflow with 8KB
  208: kernel stack. On NetBSD, the architectures with 16KB kernel stack are
  209: amd64, sparc64, powerpc, and experimental ia64, hppa. mac68k and sh3
  210: have 12KB kernel stack. All others use only 8KB stack, which is not
  211: enough to run zfs.
  213: NetBSD has many statistics provided via sysctl; see "sysctl
  214: kstat.zfs".
  216: FreeBSD has tunables that NetBSD does not seem to have, described in
  217: [FreeBSD Handbook ZFS Advanced
  218: section](
  220: # Interoperability with other systems
  222: Modern ZFS uses pool version 5000 and feature flags.
  224: It is in general possible to export a pool and them import the pool on
  225: some other system, as long as the other system supports all the used
  226: features.
  228: \todo Explain how to do this and what is known to work.
  230: \todo Explain feature flags relationship to FreeBSD, Linux, iIllumos,
  231: macOS.
  233: # Sources of ZFS code
  235: Currently, there are multiple ZFS projects and codebases:
  237:   - [OpenZFS](
  238:   - [openzfs repository](
  239:   - [zfsonlinux](
  240:   - [OpenZFS on OS X ]( [repo](
  241:   - proprietary ZFS in Solaris (not relevant in open source)
  242:   - ZFS as released under the CDDL (common ancestor, now of historical interest)
  244: OpenZFS is a coordinating project to align open ZFS codebases.  There
  245: is a notion of a shared core codebase and OS-specific adaptation code.
  247:   - [zfsonlinux relationship to OpenZFS](
  248:   - FreeBSD more or less imports code from openzfs and pushes back fixes. \todo Verify this.
  249:   - NetBSD has imported code from FreeBSD.
  250:   - The status of ZFS on macOS is unclear (2021-02).

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