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Expand on explanation of loop-lvm mode and use as test

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DanHam
2018-04-14 00:54:20 +01:00
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storage/storagedriver/device-mapper-driver.md
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@@ -42,10 +42,18 @@ Before following these procedures, you must first meet all the
### Configure `loop-lvm` mode for testing
This configuration is only appropriate for testing. Loopback devices are slow
and resource-intensive, and they require you to create file on disk at specific sizes.
They can also introduce race conditions. They are available for testing because
the setup is easier.
This configuration is only appropriate for testing. The `loop-lvm` mode makes
use of a 'loopback' mechanism that allows files on the local disk to be
read from and written to as if they were an actual physical disk or block
device.
However, the addition of the loopback mechanism, and interaction with the OS
filesystem layer, means that IO operations can be slow and resource-intensive.
Use of loopback devices can also introduce race conditions.
However, setting up `loop-lvm` mode can help identify basic issues (such as
missing user space packages, kernel drivers etc) ahead of attempting the more
complex set up required to enable `direct-lvm` mode. `loop-lvm` mode should
therefore only be used to perform rudimentary testing prior to configuring
`direct-lvm`.
For production systems, see
[Configure direct-lvm mode for production](#configure-direct-lvm-mode-for-production).