mirror of https://github.com/schoebel/mars
doc: section on cloud storage
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Thomas Schoebel-Theuer
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@ -418,6 +418,191 @@ reference "chap:LV-Football"
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with each of the fundamental models mentioned here.
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\end_layout
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\begin_layout Section
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What is
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\emph on
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Cloud Storage
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:Requirements-for-Cloud"
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\end_inset
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\end_layout
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\begin_layout Standard
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According to a popular definition from
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\begin_inset Flex URL
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status open
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\begin_layout Plain Layout
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https://en.wikipedia.org/wiki/Cloud_storage
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\end_layout
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\end_inset
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(retrieved June 2018), cloud storage is
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\end_layout
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\begin_layout Description
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(1) Made up of many
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\series bold
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distributed resources
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\series default
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, but still
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\series bold
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act as one
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\series default
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.
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\end_layout
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\begin_layout Description
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(2) Highly
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\series bold
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fault tolerant
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\series default
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through redundancy and distribution of data.
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\end_layout
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\begin_layout Description
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(3) Highly
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\series bold
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durable
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\series default
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through the creation of versioned copies.
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\end_layout
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\begin_layout Description
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(4) Typically
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\series bold
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eventually consistent
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\series default
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with regard to data replicas.
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\end_layout
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\begin_layout Standard
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There are some consequences from this definition:
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\end_layout
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\begin_layout Enumerate
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Distributed Storage, in particular BigCluster architectures (see section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "sec:Distributed-vs-Local:"
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\end_inset
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): many of them (with few exceptions) are conforming to all of these requirement
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s.
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Typical granularity are objects, or chunks, or other relatively small units
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of data.
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\end_layout
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\begin_layout Enumerate
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Centralized Storage: does not conform to (1) and to (4) by definition
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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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Notice that sharding on top of CentralStorage is no longer a CentralStorage
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model by definition, but a RemoteSharding model according to section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "subsec:Variants-of-Sharding"
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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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.
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By introduction of synchronous or asynchronous replication, it can be made
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to
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\emph on
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almost
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\emph default
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conform, except for (1) where some concept mismatches remain (probably
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resolvable by going to a RemoteSharding model on top of CentralStorage,
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where CentralStorage is only a
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\emph on
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sub-component
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\emph default
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).
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Typical granularity is replication of whole storage pools, or of LVs, or
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of filesystem data.
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\end_layout
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\begin_layout Enumerate
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LocalStorage, and some further models like RemoteSharding (see section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "subsec:Variants-of-Sharding"
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\end_inset
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):
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\end_layout
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\begin_deeper
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\begin_layout Description
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(1) can be achieved at LV granularity with Football (see chapter
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "chap:LV-Football"
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\end_inset
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), which creates a
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\series bold
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Big Virtual LVM Pool
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\series default
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.
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\end_layout
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\begin_layout Description
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(2) can be achieved at disk granularity with local RAID, and at LV granularity
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with DRBD or MARS.
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\end_layout
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\begin_layout Description
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(3) can be achieved at LV granularity with LVM snapshots, and/or ZFS (or
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other filesystem) snapshots, and/or above filesystem layer by addition
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of classical backup.
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\end_layout
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\begin_layout Description
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(4) can be achieved by MARS, which provides two different consistency guarantees
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at different levels,
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\emph on
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both at the same time
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\emph default
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:
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\end_layout
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\begin_deeper
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\begin_layout Description
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locally: Strict local consistency at LV granularity, also
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\emph on
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within
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\emph default
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any LV replica.
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\end_layout
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\begin_layout Description
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globally: Eventually consistent
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\emph on
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between
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\emph default
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different LV replicas.
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\end_layout
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\end_deeper
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\end_deeper
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\begin_layout Section
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Local vs Centralized Storage
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\begin_inset CommandInset label
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