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doc: storage: provide instructions to use Linux' device mapper

Patrick Steinhardt requested to merge pks/docs-storage-dm into master

What does this Merge Request do?

The current instructions for simulating delays for storage depends on the use of sshfs. One obvious benefit is that it's cross platform and doesn't require any kernel support except for FUSE, but on the other hand it's not all that useful due to not being as flexible as other solutions.

On Linux, we can instead use some device mapper targets, most importantly the targets "delay", "dust" and "flaky". These allow you to exactly tweak several parameters like read/write delays, bad disk sectors and the likelihood of I/O operations causing failures. This flexibility is not possible with sshfs.

Document the setup of these device mapper targets to help others discover this superior, but more complex, option.

Merge Request checklist

  • Tests added for new functionality. If not, please raise Issue to follow-up.
  • This change is backward compatible. If not, please include steps to communicate to our users.
  • Documentation added/updated, if needed.
  • gdk doctor test added, if needed.

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