Blame Manuals/en/Texinfo/Concepts/CentOS/release.texi

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The upstream vendor has released 4 versions of enterprise Linux that
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CentOS rebuilds the freely available SRPMS for. So, the major CentOS
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releases are CentOS 2, CentOS 3, CentOS 4 and CentOS 5.  The upstream
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vendor releases security updates as required by circumstances. CentOS
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releases rebuilds of security updates as soon as possible. Usually
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within 24 hours (our stated goal is with 72 hours, but we are usually
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much faster).
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The upstream vendor also releases numbered update sets for Version 3,
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Version 4 and Version 5 of their product (Currently EL 3 update 9, EL
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4 update 6 and EL 5 update 1) 2 to 4 times per year. There are new
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ISOs from the upstream vendor provided for these update sets. Update
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sets will be completed as soon as possible after the vendor releases
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their version ... generally within 2 weeks. CentOS follows these
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conventions as well, so CentOS 3.9 correlates with EL 3 update 9 and
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CentOS 4.6 correlates with EL 4 update 6, CentOS 5.1 correlates to EL
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5 update 1, etc.
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One thing some people have problems understanding is that if you have
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any CentOS-3 product and update it, you will be updated to the latest
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CentOS-3.x version.
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The same is true for CentOS-4 and CentOS 5. If you update any CentOS-4
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product, you will be updated to the latest CentOS-4.x version, or to
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the latest CentOS 5.x version if you are updating a CentOS 5 system.
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This is exactly the same behavior as the upstream product. Let's
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assume that the latest EL4 product is update 6. If you install the
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upstream original EL4 CDs (the ones before any update set) and upgrade
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via their up2date, you will have latest update set installed (EL4
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update 6 in our example). Since all updates within a major release
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(CentOS 2, CentOS 3, CentOS 4, CentOS 5) always upgrade to the latest
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version when updates are performed (thus mimicking upstream behavior),
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only the latest version is maintained in each main tree on the CentOS
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mirrors.
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There is a CentOS Vault containing old CentOS trees. This vault is a
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picture of the older tree when it was removed from the main tree, and
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does not receive updates. It should only be used for reference. 
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