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doc: add filter info to scanning

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David Teigland 2018-05-09 11:14:05 -05:00
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commit 38f33251b1
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doc/lvm-disk-reading.txt
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@ -187,3 +187,45 @@ For each VG name:
command-specific work
Filter i/o
----------
Some filters must be applied before reading a device, and other filters
must be applied after reading a device. In all cases, the filters must be
applied before lvm processes the device, i.e. before it looks for an lvm
label.
1. Some filters need to be applied prior to reading any devices
because the purpose of the filter is to avoid submitting any
io on the excluded devices. The regex filter is the primary
example. Other filters benefit from being applied prior to
reading devices because they can tell which devices to
exclude without doing io to the device. An example of this
is the mpath filter.
2. Some filters need to be applied after reading a device because
they are based on data/signatures seen on the device.
The partitioned filter is an example of this; lvm needs to
read a device to see if it has a partition table before it can
know whether to exclude the device from further processing.
We apply filters from 1 before reading devices, and we apply filters from
2 after populating bcache, but before processing the device (i.e. before
checking for an lvm label, which is the first step in processing.)
The current implementation of this makes filters return -EAGAIN if they
want to read the device, but bcache data is not yet available. This will
happen when filtering runs prior to populating bcache. In this case the
device is flagged. After bcache is populated, the filters are reapplied
to the flagged devices. The filters which need to look at device content
are now able to get it from bcache. Devices that do not pass filters at
this point are excluded just like devices which were excluded earlier.
(Some filters from 2 can be skipped by consulting udev for the information
instead of reading the device. This is not entirely reliable, so it is
disabled by default with the config setting external_device_info_source.
It may be worthwhile to change the filters to use the udev info as a hint,
or only use udev info for filtering in reporting commands where
inaccuracies are not a big problem.)