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79 lines
3.6 KiB
Plaintext
79 lines
3.6 KiB
Plaintext
pvcreate initializes a Physical Volume (PV) on a device so the device is
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recognized as belonging to LVM. This allows the PV to be used in a Volume
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Group (VG). An LVM disk label is written to the device, and LVM metadata
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areas are initialized. A PV can be placed on a whole device or partition.
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.P
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Use \fBvgcreate\fP(8) to create a new VG on the PV, or \fBvgextend\fP(8)
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to add the PV to an existing VG. Use \fBpvremove\fP(8) to remove the LVM
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disk label from the device.
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.P
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The force option will create a PV without confirmation. Repeating the
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force option (\fB-ff\fP) will forcibly create a PV, overriding checks that
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normally prevent it, e.g. if the PV is already in a VG.
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.P
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.B Metadata location, size, and alignment
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.P
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The LVM disk label begins 512 bytes from the start of the device, and is
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512 bytes in size.
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.P
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The LVM metadata area begins at an offset (from the start of the device)
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equal to the page size of the machine creating the PV (often 4 KiB.) The
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metadata area contains a 512 byte header and a multi-KiB circular buffer
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that holds text copies of the VG metadata.
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.P
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With default settings, the first physical extent (PE), which contains LV
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data, is 1 MiB from the start of the device. This location is controlled
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by \fBdefault_data_alignment\fP in lvm.conf, which is set to 1 (MiB) by
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default. The pe_start will be a multiple of this many MiB. This location
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can be checked with:
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.br
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.B pvs -o pe_start
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.I PV
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.P
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The size of the LVM metadata area is the space between the the start of
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the metadata area and the first PE. When metadata begins at 4 KiB and the
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first PE is at 1024 KiB, the metadata area size is 1020 KiB. This can be
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checked with:
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.br
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.B pvs -o mda_size
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.I PV
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.P
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The mda_size cannot be increased after pvcreate, so if larger metadata is
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needed, it must be set during pvcreate. Two copies of the VG metadata
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must always fit within the metadata area, so the maximum VG metadata size
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is around half the mda_size. This can be checked with:
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.br
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.B vgs -o mda_free
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.I VG
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.P
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A larger metadata area can be set with --metadatasize. The resulting
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mda_size may be larger than specified due to default_data_alignment
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placing pe_start on a MiB boundary, and the fact that the metadata area
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extends to the first PE. With metadata starting at 4 KiB and
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default_data_alignment 1 (MiB), setting --metadatasize 2048k results in
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pe_start of 3 MiB and mda_size of 3068 KiB. Alternatively, --metadatasize
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2044k results in pe_start at 2 MiB and mda_size of 2044 KiB.
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.P
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The alignment of pe_start described above may be automatically overridden
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based on md device properties or device i/o properties reported in sysfs.
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These automatic adjustments can be enabled/disabled using lvm.conf
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settings md_chunk_alignment and data_alignment_offset_detection.
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.P
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To use a different pe_start alignment, use the --dataalignment option.
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The --metadatasize option would also typically be used in this case
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because the metadata area size also determines the location of pe_start.
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When using these two options together, pe_start is calculated as:
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metadata area start (page size), plus the specified --metadatasize,
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rounded up to the next multiple of --dataalignment.
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With metadata starting at 4 KiB, --metadatasize 2048k, and --dataalignment 128k,
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pe_start is 2176 KiB and mda_size is 2172 KiB.
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The pe_start of 2176 KiB is the nearest even multiple of 128 KiB that
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provides at least 2048 KiB of metadata space.
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Always check the resulting alignment and metadata size when using
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these options.
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.P
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To shift an aligned pe_start value, use the --dataalignmentoffset option.
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The pe_start alignment is calculated as described above, and then the
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value specified with --dataalignmentoffset is added to produce the final
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pe_start value.
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