mirror of https://github.com/schoebel/mars
doc: clarify future way of symlink updates
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Thomas Schoebel-Theuer
Thomas Schoebel-Theuer
parent
bda94f439f
commit
03523a61fc
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@ -13023,12 +13023,12 @@ reference "sub:Predefined-Trivial-Macros"
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\end_layout
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\begin_layout Standard
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The
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The current
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\family typewriter
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/mars/
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\family default
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filesystem contains not only transaction logfiles, but also acts as a generic
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storage for (persistent) state information.
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filesystem container format contains not only transaction logfiles, but
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also acts as a generic storage for (persistent) state information.
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Both configuration information and runtime state information are currently
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stored in symlinks.
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Symlinks are
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@ -13066,9 +13066,40 @@ key -> value
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\end_inset
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Therefrom results a fundamentally different behaviour than DRBD.
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When your DRBD primary crashed before and now comes up again, you have
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to setup DRBD again by a sequence of commands like
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It is not yet clear / decided, but there is a
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\emph on
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chance
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\emph default
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that the
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\emph on
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concept
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\emph default
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of
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\family typewriter
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key -> value
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\family default
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pairs will be retained in future versions of MARS.
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Instead of being represented by symlinks, another representation will be
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used, such that hopefully the
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\family typewriter
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key
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\family default
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part will remain in the form of a pathname, even if there were no longer
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a physical representation in an actual filesystem.
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\end_layout
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\begin_layout Standard
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\noindent
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\begin_inset Graphics
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filename images/lightbulb_brightlit_benj_.png
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lyxscale 12
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scale 7
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\end_inset
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A fundamentally different behaviour than DRBD: when your DRBD primary crashed
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some time ago, and now comes up again, you have to setup DRBD again by
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a sequence of commands like
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\family typewriter
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modprobe drbd; drbdadm up all; drbdadm primary all
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\family default
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@ -13161,17 +13192,8 @@ global namespace
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\end_layout
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\begin_layout Standard
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Since the symlink replication works generically, you may use the
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\family typewriter
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/mars/userspace/
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\family default
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directory in order to place your own symlink there (for whatever purpose,
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which need not have to do with MARS).
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\end_layout
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\begin_layout Standard
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In order to avoid name clashes, each symlink created at node A should have
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the name A in its path name.
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In order to avoid name clashes, each pathname created at node A follows
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a convention: the node name A should be a suffix of the pathname.
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Typically, internal MARS names follow the scheme
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\family typewriter
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/mars/
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@ -13180,7 +13202,14 @@ something
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\emph default
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/myname-A
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\family default
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, and you should follow the best practice of systematically using
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.
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When using the expert command
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\family typewriter
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marsadm {get,set}-link
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\family default
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(which will likely be replaced by something else in future MARS releases),
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you should follow the best practice of systematically using pathnames like
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\family typewriter
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/mars/userspace/myname-A
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\family default
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@ -13194,6 +13223,33 @@ something
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(context-dependent names).
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\end_layout
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\begin_layout Standard
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\noindent
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\begin_inset Graphics
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filename images/lightbulb_brightlit_benj_.png
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lyxscale 12
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scale 7
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\end_inset
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Experts only: the symlink replication works generically.
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You might use the
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\family typewriter
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/mars/userspace/
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\family default
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directory in order to place your own symlink there (for whatever purpose,
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which need not have to do with MARS).
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However, the symlinks are likely to disappear.
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Use
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\family typewriter
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marsadm {get,set}-link
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\family default
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instead.
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There is a chance that these abstract commands (or variants thereof) will
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be retained, by acting on the new data representation in future, even if
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the old symlink format will vanish some day.
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\end_layout
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\begin_layout Standard
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\noindent
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\begin_inset Graphics
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@ -13207,7 +13263,7 @@ Important: the convention of placing the
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\series bold
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creator host name
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\series default
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inside your symlink names should be used wherever possible.
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inside your pathnames should be used wherever possible.
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The name part is a kind of
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\begin_inset Quotes eld
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\end_inset
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@ -13226,18 +13282,18 @@ belonging
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\end_inset
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to him.
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Other hosts may read foreign symlinks as often as they want, but never
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Other hosts may read foreign information as often as they want, but never
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modify them.
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This way, your cluster nodes are able to
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\emph on
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communicate
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\emph default
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with each other via symlink updates.
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with each other via symlink / information updates.
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\end_layout
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\begin_layout Standard
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Although you may create (and change) your symlinks with userspace tools
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like
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Although experts might create (and change) the current symlinks with userspace
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tools like
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\family typewriter
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ln -s
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\family default
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@ -13257,7 +13313,7 @@ marsadm delete-file /mars/userspace/mykey-A
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\end_layout
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\begin_layout Standard
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There are two reasons for this: first, the
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There are many reasons for this: first, the
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\family typewriter
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marsadm set-link
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\family default
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@ -13284,20 +13340,30 @@ marsadm delete-file
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whole cluster
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\emph default
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.
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And finally, there is a chance that this will work in future versions of
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MARS even after the symlinks have vanished.
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\end_layout
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\begin_layout Standard
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What's the difference? If you try to remove your symlink locally by hand
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via
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What's the difference? If you would try to remove your symlink locally by
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hand via
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\family typewriter
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rm -f
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\family default
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, you will be surprised: since the symlink has been replicated to other
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, you will be surprised: since the symlink has been replicated to the other
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cluster nodes, it will be re-transferred from there and will be resurrected
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locally after some short time.
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This way, you cannot delete any object reliably, because your whole cluster
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(which may consist of many nodes) remembers all your state information
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and will resurrect it whenever
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and will
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\begin_inset Quotes eld
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\end_inset
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correct
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\begin_inset Quotes erd
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\end_inset
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it whenever
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\begin_inset Quotes eld
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\end_inset
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@ -13316,7 +13382,7 @@ In order to solve the deletion problem, MARS Light uses some internal deletion
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\family default
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The deletion protocol ensures that all replicas get deleted in the whole
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cluster, and only after that the auxiliary symlinks in
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cluster, and only thereafter the auxiliary symlinks in
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\family typewriter
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/mars/todo-global/
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\family default
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@ -13324,12 +13390,13 @@ In order to solve the deletion problem, MARS Light uses some internal deletion
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\end_layout
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\begin_layout Standard
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You may change your already existing symlink via
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You may update your already existing symlink via
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\family typewriter
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marsadm set-link some-other-value /mars/userspace/mykey-A
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\family default
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.
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The new value will be propagated in the cluster according to a
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The new value will be propagated throughout the cluster according to a
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\series bold
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timestamp comparison protocol
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\series default
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@ -13359,7 +13426,7 @@ by hand
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via
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\family typewriter
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ln -s
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ln -sf
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\family default
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may get lost when the local system clock is much more earlier than the
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Lamport clock.
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@ -13387,18 +13454,18 @@ temporarily inconsistent
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\end_layout
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\begin_layout Standard
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The meaning of the internal MARS Light symlinks residing in
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The meaning of some internal MARS Light symlinks residing in
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\family typewriter
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/mars/
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\family default
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is documented in section
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will be hopefully documented in section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "sec:Documentation-of-the"
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\end_inset
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.
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some day.
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\end_layout
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\begin_layout Section
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