mirror of https://github.com/schoebel/mars
arch-guide: split backup from replication and rework
This commit is contained in:
Thomas Schoebel-Theuer
Thomas Schoebel-Theuer
parent
b2092e3d51
commit
1f67d8833d
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@ -1233,7 +1233,183 @@ reach
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\end_layout
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\begin_layout Section
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Replication vs Backup
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What is Backup
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:What-is-Backup"
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\end_inset
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\end_layout
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\begin_layout Standard
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A Backup is a
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\series bold
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copy of your data
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\series default
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at a
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\series bold
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different location
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\series default
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.
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There are two distinct operations associated with backup:
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\end_layout
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\begin_layout Enumerate
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\series bold
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Creation
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\series default
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of backup.
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This creates a
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\series bold
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copy
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\series default
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, or a new version of a copy.
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It involves some network traffic over various distances, e.g in simplest
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case over a USB cable, or from the application datacenter to a backup datacente
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r.
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Typically, this is done at
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\series bold
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regular time intervals
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\series default
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, e.g.
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daily.
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\end_layout
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\begin_layout Enumerate
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\series bold
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Restore
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\series default
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from backup.
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This does the
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\emph on
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opposite
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\emph default
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of backup creation.
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It also involves network traffic, but in
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\series bold
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reverse direction
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\series default
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.
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The
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\series bold
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roles
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\series default
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of application datacenter and backup datacenter
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\series bold
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do not change
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\series default
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.
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Restore is typically
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\series bold
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triggered manually
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\series default
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, and only after some incident which led to
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\series bold
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data loss
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\series default
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.
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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/MatieresCorrosives.png
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lyxscale 50
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scale 17
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\end_inset
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It would be a
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\emph on
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bad idea
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\emph default
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to restore a backup although there is no data loss.
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This would likely overwrite your newest application data with an elder
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version, likely leading to
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\emph on
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data loss
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\emph default
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.
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Therefore, restore is
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\series bold
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potentially dangerous operation
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\series default
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!
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\end_layout
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\begin_layout Standard
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\begin_inset Flex Custom Color Box 3
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status open
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\begin_layout Plain Layout
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\begin_inset Argument 1
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status open
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\begin_layout Plain Layout
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\series bold
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Summary: structural properties of backup
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\end_layout
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\end_inset
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Backup is
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\series bold
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asymmetrical
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\series default
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.
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It involves two non-exchangable roles / locations, application location
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vs backup location.
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\end_layout
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\begin_layout Plain Layout
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\noindent
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\begin_inset Graphics
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filename images/MatieresCorrosives.png
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lyxscale 50
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scale 17
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\end_inset
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Confusion of these roles, or triggering an unnecessary restore is a
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\series bold
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risk
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\series default
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for
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\series bold
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data integrity
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\series default
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.
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\end_layout
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\begin_layout Plain Layout
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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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Conversely, having no reasonable backup at all is an even higher risk.
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Backup is a
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\series bold
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best practice
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\series default
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.
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\end_layout
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\end_inset
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\end_layout
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\begin_layout Section
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What is Replication
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:Replication-vs-Backup"
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@ -1278,7 +1454,9 @@ https://en.wikipedia.org/wiki/Replication_(computing)
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\end_layout
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\begin_layout Standard
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For a rough comparison of
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For this guide, we want a clearer discrimination, for better orientation
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in the solution jungle.
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As a rough comparison of
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\emph on
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typical
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\emph default
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@ -1293,7 +1471,7 @@ typical
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\noindent
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\align center
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\begin_inset Tabular
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<lyxtabular version="3" rows="6" columns="3">
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<lyxtabular version="3" rows="7" columns="3">
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<features tabularvalignment="middle">
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<column alignment="left" valignment="top">
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<column alignment="center" valignment="top">
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@ -1331,6 +1509,35 @@ Replication
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<cell alignment="left" valignment="top" topline="true" leftline="true" usebox="none">
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\begin_inset Text
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\begin_layout Plain Layout
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Timely pattern
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\end_layout
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\end_inset
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</cell>
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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\begin_inset Text
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\begin_layout Plain Layout
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intervals
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\end_layout
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\end_inset
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</cell>
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<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
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\begin_inset Text
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\begin_layout Plain Layout
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continuously
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\end_layout
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\end_inset
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</cell>
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</row>
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<row>
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<cell alignment="left" valignment="top" topline="true" leftline="true" usebox="none">
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\begin_inset Text
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\begin_layout Plain Layout
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Fast handover (planned)
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\end_layout
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@ -1428,7 +1635,7 @@ Protect for logical data corruption
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\begin_inset Text
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\begin_layout Plain Layout
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yes (partly)
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yes
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\end_layout
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\end_inset
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@ -1448,7 +1655,11 @@ typically no
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\begin_inset Text
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\begin_layout Plain Layout
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Disaster Recovery Time (MTTR)
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Disaster Recovery Time (
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\series bold
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MTTR
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\series default
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)
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\end_layout
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\end_inset
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@ -1457,7 +1668,9 @@ Disaster Recovery Time (MTTR)
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\begin_inset Text
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\begin_layout Plain Layout
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typically (very) slow
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typically
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\series bold
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very slow
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\end_layout
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\end_inset
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@ -1466,6 +1679,8 @@ typically (very) slow
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\begin_inset Text
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\begin_layout Plain Layout
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\series bold
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fast
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\end_layout
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@ -1481,12 +1696,109 @@ fast
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\begin_layout Standard
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\noindent
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There are some solutions implementing a
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\emph on
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mixture
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\emph default
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, by different combinations of some of these typical properties.
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Here we focus on fundamental principles.
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\end_layout
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\begin_layout Standard
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Although
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\series bold
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replication
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\series default
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as defined here
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\series bold
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has much better properties
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\series default
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from a risk viewpoint on enterprise-critical data, there remains a gap
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in favour of backup: backup is typically implemented as a
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\emph on
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logical copy
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\emph default
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, which lowers risks from certain types of
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\series bold
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data corruption
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\series default
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, such as filesystem corruption, for which only risky repair workarounds
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like
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\family typewriter
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fsck
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\family default
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are the last resort when you don't have a backup
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\emph on
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in addition
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\emph default
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\begin_inset Foot
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status open
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\begin_layout Plain Layout
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An integrated solution for continuous replication via logical copies would
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be difficult.
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There is a
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\emph on
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concept mismatch
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\emph default
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between logical copies and strict consistency requirements posed by fast
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handover, while at the same time compensation of logical data corruption
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would require the
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\emph on
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opposite
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\emph default
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of strict consistency.
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Notice that logical copies are residing at higher layers, e.g.
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filesystems or database records, while pure replication is easier done
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at block layer.
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See also section
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\begin_inset CommandInset ref
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LatexCommand nameref
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reference "sec:Performance-Arguments-from"
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plural "false"
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caps "false"
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noprefix "false"
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\end_inset
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.
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Notice that snapshots at block layer cannot
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\emph on
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reliably
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\emph default
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protect against long-lasting
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\series bold
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silent corruptions
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\series default
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.
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Even higher-layer ZFS snapshots treated in section
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\begin_inset CommandInset ref
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LatexCommand nameref
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reference "subsec:Example:-ZFS-Replication"
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plural "false"
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caps "false"
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noprefix "false"
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\end_inset
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cannot provide the same protection level as a classical per-inode multi-generat
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ion backup onto a different filesystem type, thus lowering systematical
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risks from software bugs in filesystem code.
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\end_layout
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\end_inset
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to replication.
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\end_layout
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\begin_layout Standard
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Because of these typical differences, enterprise-critical data typically
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deserves
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\emph on
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both
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\emph default
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solution classes.
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solution classes at the same time.
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\end_layout
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\begin_layout Standard
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@ -1494,9 +1806,126 @@ both
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status open
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\begin_layout Plain Layout
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Confusion of solution classes and/or their corresponding problem classes
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/ properties can be harmful to enterprises and to carreers of responsible
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persons.
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\noindent
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\begin_inset Argument 1
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status open
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\begin_layout Plain Layout
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\series bold
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Important requirements for replication
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\end_layout
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\end_inset
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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 good replication solution is
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\series bold
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symmetrical
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\series default
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.
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There a two (or more) copies at different locations.
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They are either active at the same time (which works reliably only rack-to-rack
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over crossover cables, see section
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\begin_inset CommandInset ref
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LatexCommand nameref
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reference "sec:Explanation-via-CAP"
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plural "false"
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caps "false"
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noprefix "false"
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\end_inset
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), and/or they need to
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\series bold
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switch their roles quickly
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\series default
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.
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Switching should have two different triggers:
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\series bold
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planned handover
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\series default
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, vs
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\series bold
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unplanned failover
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\series default
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in case of an incident.
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\end_layout
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\begin_layout Plain Layout
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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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Symmetry is an important precondition for
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\series bold
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fast reaction
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\series default
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onto incidents.
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For
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\series bold
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enterprise-critical data
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\series default
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, this is important for drastically
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\series bold
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lowering
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\series default
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the expectance value of
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\series bold
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losses by incidents
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\series default
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.
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\end_layout
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\begin_layout Plain Layout
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\begin_inset Graphics
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filename images/MatieresCorrosives.png
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lyxscale 50
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scale 17
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\end_inset
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Confusion of solution classes replication vs backup and/or their corresponding
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problem classes / properties can be harmful to enterprises and to carreers
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of responsible persons.
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\end_layout
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\begin_layout Plain Layout
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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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Hint: the
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\emph on
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granularity
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\emph default
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of replication handover / failover is important for maximum flexibility.
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See section
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\begin_inset CommandInset ref
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LatexCommand nameref
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reference "subsec:Flexibility-of-Failover"
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plural "false"
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caps "false"
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noprefix "false"
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\end_inset
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.
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\end_layout
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\end_inset
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