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I want to use google's compute engine as a database. When using the calculator, I got confused about what the boot disk is. Google's documentation didn't help either. Is the boot disk only for the OS or is it where everything is stored? If I don't specify the boot disk size, will I be able to set up my database in the compute engine? If yes, what would be the storage limit for the database?

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Boot disk is simply the first disk attached to the instance. It will have a boot loader and OS installed to it.

It has same performance characteristics as any other Persistent Disk volumes you may attach to a VM (with a few minor exceptions). You can also install database and keep all the data on it.

But many people prefer to separate their data disk and OS disk. This allows apply different disaster recovery policies to those disks. It is also not uncommon to treat boot devices as ephemeral. Meaning people can just throw away boot disk, keep data disk and rebuild VM from scratch (e.g. during upgrade, or as a mitigation to an outage) while still keeping the data.

Separation of OS and data can be used for efficiency/price optimization. Boot devices rarely require high performance so those can be created as Balanced or even Standard PD, while it could make sense to put database files to higher performing SSD PD or even Extreme PD. If durability does not matter much data can be placed to local SSD.

The one caveat that pops to mind is that you can't have Regional PD as your boot disk. Meaning if you want to use it you must have zonal device attached as a boot disk and regional device as 2nd, or 3rd, or Nth device.

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  • Does that mean that boot disks are always equivalent to zonal disks with respect to data redundancy characteristics?
    – Jens Moser
    Jan 17, 2023 at 17:05
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    Yes. And it is currently not possible to use Regional disk as a first/boot device. Note that zonal devices still provide redundancy against regular hardware failures.
    – rvs
    Jan 20, 2023 at 10:21

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