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As far as I understand: Recoveries (e.g. ClockworkMod) can be used, among other things, to install new versions of the Android OS, like custom ROMs (e.g. CyanogenMod) by updating the ROM section which contains the Android OS firmware. Recoveries are entered by shutting down the phone and powering it up by pressing special button combinations, so the phone doesn't enter the regular OS, but instead activates the recovery section.

Now, from that I expecte that the Recovery is something like the boot system on a PC (e.g. GRUB), that is separate from the actual OS, and activated when there is trouble getting into the OS. And because it's not a part of the OS, it can be used to update the OS itself.

However, what I'm confused about is this: The guide to installing CyanogenMod on GT-S5570 says that "The official Android 2.3 update from Samsung must be applied before following this guide." Another guide for flashing the ClockworkMod recovery on GT-S5570 also says that if you have 2.2 on the phone, you have to use a different version of ClockworkMod recovery, than if you have 2.3.

What is the relationship between these two, a recovery and the Android OS?

Further: Is a different version of the recovery really needed for different OS versions? Is this something specific to ClockworkMod, or is it how Andoroid's recovery system works always? If a recovery can be used to update the OS to a different version, does the recovery then become invalid after the update?

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Is a different version of the recovery really needed for different OS versions?

No. In case of updating custom roms you're completely right in that the recovery is mostly* independent of the rom used.
However phone manufacturers sometimes push out updates that change some vital parts of the phone. For example repartition the phone. This is why sometimes you need a different recovery for the phone after major software updates.

Is this something specific to ClockworkMod, or is it how Andoroid's recovery system works always?

This is something that depends on the phone and the the manufacturer's customizations.

If a recovery can be used to update the OS to a different version, does the recovery then become invalid after the update?

Factory roms are not always in the update.zip format thus they're not always applied using the recovery. In these cases the recovery is commonly among the parts that are updated. Furthermore recovery can be updated from the recovery just like the boot image (kernel.)

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Indeed, partitioning and possibly mount points of the file system dictate which recovery you need. –  Matthew Read May 7 '12 at 17:37
    
So the update.zip format never changes the recovery? What happens if I flash a recovery that isn't compatible with the phone OS version? –  Ilari Kajaste May 8 '12 at 10:06
    
@Ilari no the recovery can be updates from the recovery, read the last sentence. Well it depends on the phone and the way the update changed. The severity can change from it simply not working (because they changed mount points) to not being able to flash (eg. They changed the partition size or even removed it altogether). It's important to always flash the right version. The good thing is that the chances of permanently bricking your device this way are almost nonexistent. –  Richard Borcsik May 8 '12 at 11:03
    
@RichardBorcsik Yes I did notice that sentence, I just assumed that if you update recovery from recovery, that update would then be in a different format than update.zip. –  Ilari Kajaste May 8 '12 at 12:26
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@Ilari Well it depends on why it's incompatible. There are numerous reasons and I'm not familiar with your phone so it's hard to tell. In case of my phone (ZTE Blade) they changed the partition layout and the baseband so kernel changes were required. ( The recovery rolls it's own kernel so that's why it needs to be updated to.) If you try flashing a new rom from an old recovery it will possibly just fail. Worst case is that you get a boot loop but you can still access the recovery. –  Richard Borcsik May 8 '12 at 13:17
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