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zfs: Add 9 woes

    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: There is a crash with mkdir over NFS with maproot.   See
   30: There is a credible rumor, but apparently no PR, that rm spuriously
   31: commits the ZIL, which hurts performance.
   33: There has been a report of an occasional panic somewhere in
   34: zfs_putpages.
   36: ## NetBSD-current
   38: NetBSD-current (as of 2021-02) has similar ZFS code to 9.
   40: There is initial support for [[ZFS root|wiki/RootOnZFS]], via booting from
   41: ffs and pivoting.
   43: ## NetBSD/xen special issues
   45: Summary: if you are using NetBSD, xen and zfs, use NetBSD-current.
   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.  Xen and zfs on current are
   51: reported to work well together, as of 2021-02.
   53: ## Architectures
   55: Most people seem to be using amd64.
   57: To build zfs, one puts MKZFS=yes in mk.conf.  This is default on amd64
   58: and aarch64 on netbsd-9.  In current, it is also default on sparc64.
   60: More or less, zfs can be enabled on an architecture when it is known
   61: to build and run reliably.  (Of course, users are welcome to build it
   62: and report.)
   64: # Quick Start
   66: See the [FreeBSD Quickstart
   67: Guide](; only
   68: the first item is NetBSD specific.
   70:   - Put zfs=YES in rc.conf.
   72:   - Create a pool as "zpool create pool1 /dev/dk0".
   74:   - df and see /pool1
   76:   - Create a filesystem mounted on /n0 as "zfs create -o
   77:     mountpoint=/n0 pool1/n0".
   79:   - Read the documentation referenced in the next section.
   81: ## Documentation Pointers
   83: See the man pages for zfs(8), zpool(8).  Also see zdb(8), if only for
   84: seeing pool config info when run with no arguments.
   86:   - [OpenZFS Documentation](
   87:   - [OpenZFS admin docs index page](
   88:   - [FreeBSD Handbook ZFS Chapter](
   89:   - [Oracle ZFS Administration Manual](
   90:   - [Wikipedia](
   92: # NetBSD-specific information
   94: ## rc.conf
   96: The main configuration is to put zfs=YES in rc.conf, so that the rc.d
   97: scripts bring up ZFS and mount ZFS file systems.
   99: ## pool locations
  101: One can add disks or parts of disks into pools.  Methods of specifying
  102: areas to be included include:
  104:   - entire disks (e.g., /dev/wd0d on amd64, or /dev/wd0 which has the same major/minor)
  105:   - disklabel partitions (e.g., /dev/sd0e)
  106:   - wedges (e.g., /dev/dk0)
  108: Information about created or imported pools is stored in
  109: /etc/zfs/zpool.cache.
  111: Conventional wisdom is that a pool that is more than 80% used gets
  112: unhappy; so far there is not NetBSD-specific wisdom to confirm or
  113: refute that.
  115: ## pool native blocksize mismatch
  117: ZFS attempts to find out the native blocksize for a disk when using it
  118: in a pool; this is almost always 512 or 4096.  Somewhere between 9.0
  119: and 9.1, at least some disks on some controllers that used to report
  120: 512 now report 4096.  This provokes a blocksize mismatch warning.
  122: Given that the native blocksize of the disk didn't change, and things
  123: seemed OK using the 512 emulated blocks, the warning is likely not
  124: critical.  However, it is also likely that rebuilding the pool with
  125: the 4096 blocksize is likely to result in better behavior because ZFS
  126: will only try to do 4096-byte writes.  \todo Verify this and find the
  127: actual change and explain better.
  129: ## pool importing problems
  131: While one can "zpool pool0 /dev/wd0f" and have a working pool, this
  132: pool cannot be exported and imported straigthforwardly.  "zpool
  133: export" works fine, and deletes zpool.cache.  "zpool import", however,
  134: only looks at entire disks (e.g. /dev/wd0), and might look at slices
  135: (e.g. /dev/dk0).  It does not look at partitions like /dev/wd0f, and
  136: there is no way on the command line to ask that specific devices be
  137: examined.  Thus, export/import fails for pools with disklabel
  138: partitions.
  140: One can make wd0 be a link to wd0f temporarily, and the pool will then
  141: be importable.  However, "wd0" is stored in zpool.cache and on the
  142: next boot that will attempt to be used.  This is obviously not a good
  143: approach.
  145: One an mkdir e.g. /etc/zfs/pool0 and in it have a symlink to
  146: /dev/wd0f.  Then, zpool import -d /etc/zfs/pool0 will scan
  147: /etc/zfs/pool0/wd0f and succeed.  The resulting zpool.cache will have
  148: that path, but having symlinks in /etc/zfs/POOLNAME seems acceptable.
  150: \todo Determine a good fix, perhaps man page changes only, fix it
  151: upstream, in curent, and in 9, before removing this discussion.
  153: ## mountpoint conventions
  155: By default, datasets are mounted as /poolname/datasetname.  One can
  156: also set a mountpoint; see zfs(8).
  158: There does not appear to be any reason to choose explicit mountpoints
  159: vs the default (and either using data in place or symlinking to it).
  161: ## mount order
  163: NetBSD 9 mounts other file systems and then ZFS file systems.  This can
  164: be a problem if /usr/pkgsrc is on ZFS and /usr/pkgsrc/distfiles is on
  165: NFS.  A workaround is to use noauto and do the mounts in
  166: /etc/rc.local.
  168: NetBSD current after 20200301 mounts ZFS first.  The same issues and
  169: workarounds apply in different circumstances.
  171: ## NFS
  173: zfs filesystems can be exported via NFS, simply by placing them in
  174: /etc/exports like any other filesystem.
  176: The "zfs share" command adds a line for each filesystem with the
  177: sharenfs property set to /etc/zfs/exports, and "zfs unshare" removes
  178: it.  This file is ignored on NetBSD-9 and current before 20210216; on
  179: current after 20210216 those filesystems should be exported (assuming
  180: NFS is enabled).  It does not appear to be possible to set options
  181: like maproot and network restrictions via this method.
  183: On current before 20210216, a remote mkdir of a filesystem mounted via
  184: -maproot=0:10 causes a kernel NULL pointer dereference.  This is now
  185: fixed.
  187: ## zvol
  189: Within a ZFS pool, the standard approach is to have file systems, but
  190: one can also create a zvol, which is a block device of a certain size.
  192: As an example, "zfs create -V 16G tank0/xen-netbsd-9-amd64" creates a
  193: zvol (intended to be a virtual disk for a domU).
  195: The zvol in the example will appear as
  196: /dev/zvol/rdsk/tank0/xen-netbsd-9-amd64 and
  197: /dev/zvol/dsk/tank0/xen-netbsd-9-amd64 and can be used like a
  198: disklabel partition or wedge.  However, the system will not read
  199: disklabels and gpt labels from a zvol.
  201: Doing "swapctl -a" on a zvol device node fails.  \todo Is it really
  202: true that NetBSD can't swap on a zvol?  (When using a zvol for swap,
  203: standard advice is to avoid the "-s" option which avoids reserving the
  204: allocated space.  Standard advice is also to consider using a
  205: dedicated pool.)
  207: \todo Explain that one can export a zvol via iscsi.
  209: One can use ccd to create a normal-looking disk from a zvol.  This
  210: allows reading a GPT label from the zvol, which is useful in case the
  211: zvol had been exported via iscsi and some other system created a
  212: label.
  214: # Memory usage
  216: Basically, ZFS uses lots of memory and most people run it on systems
  217: with large amounts of memory.  NetBSD works well on systems with
  218: comparatively small amounts of memory.  So a natural question is how
  219: well ZFS works on one's VAX with 2M of RAM :-) More seriously, one
  220: might ask if it is reasonable to run ZFS on a RPI3 with 1G of RAM, or
  221: if it is reasonable on a system with 4G.
  223: The prevailing wisdom is more or less that ZFS consumes 1G plus 1G per
  224: 1T of disk.  32-bit architectures are viewed as too small to run ZFS.
  226: Besides RAM, zfs requires that architecture kernel stack size is at
  227: least 12KB or more -- some operations cause stack overflow with 8KB
  228: kernel stack. On NetBSD, the architectures with 16KB kernel stack are
  229: amd64, sparc64, powerpc, and experimental ia64, hppa. mac68k and sh3
  230: have 12KB kernel stack. All others use only 8KB stack, which is not
  231: enough to run zfs.
  233: NetBSD has many statistics provided via sysctl; see "sysctl
  234: kstat.zfs".
  236: FreeBSD has tunables that NetBSD does not seem to have, described in
  237: [FreeBSD Handbook ZFS Advanced
  238: section](
  240: # Interoperability with other systems
  242: Modern ZFS uses pool version 5000 and feature flags.
  244: It is in general possible to export a pool and them import the pool on
  245: some other system, as long as the other system supports all the used
  246: features.
  248: \todo Explain how to do this and what is known to work.
  250: \todo Explain feature flags relationship to FreeBSD, Linux, iIllumos,
  251: macOS.
  253: # Sources of ZFS code
  255: Currently, there are multiple ZFS projects and codebases:
  257:   - [OpenZFS](
  258:   - [openzfs repository](
  259:   - [zfsonlinux](
  260:   - [OpenZFS on OS X ]( [repo](
  261:   - proprietary ZFS in Solaris (not relevant in open source)
  262:   - ZFS as released under the CDDL (common ancestor, now of historical interest)
  264: OpenZFS is a coordinating project to align open ZFS codebases.  There
  265: is a notion of a shared core codebase and OS-specific adaptation code.
  267:   - [zfsonlinux relationship to OpenZFS](
  268:   - FreeBSD more or less imports code from openzfs and pushes back fixes. \todo Verify this.
  269:   - NetBSD has imported code from FreeBSD.
  270:   - The status of ZFS on macOS is unclear (2021-02).

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