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Diffstat (limited to 'man/pvcreate.8_des')
| -rw-r--r-- | man/pvcreate.8_des | 78 |
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diff --git a/man/pvcreate.8_des b/man/pvcreate.8_des new file mode 100644 index 0000000..69bd133 --- /dev/null +++ b/man/pvcreate.8_des @@ -0,0 +1,78 @@ +pvcreate initializes a Physical Volume (PV) on a device so the device is +recognized as belonging to LVM. This allows the PV to be used in a Volume +Group (VG). An LVM disk label is written to the device, and LVM metadata +areas are initialized. A PV can be placed on a whole device or partition. +.P +Use \fBvgcreate\fP(8) to create a new VG on the PV, or \fBvgextend\fP(8) +to add the PV to an existing VG. Use \fBpvremove\fP(8) to remove the LVM +disk label from the device. +.P +The force option will create a PV without confirmation. Repeating the +force option (\fB-ff\fP) will forcibly create a PV, overriding checks that +normally prevent it, e.g. if the PV is already in a VG. +.P +.B Metadata location, size, and alignment +.P +The LVM disk label begins 512 bytes from the start of the device, and is +512 bytes in size. +.P +The LVM metadata area begins at an offset (from the start of the device) +equal to the page size of the machine creating the PV (often 4 KiB.) The +metadata area contains a 512 byte header and a multi-KiB circular buffer +that holds text copies of the VG metadata. +.P +With default settings, the first physical extent (PE), which contains LV +data, is 1 MiB from the start of the device. This location is controlled +by \fBdefault_data_alignment\fP in lvm.conf, which is set to 1 (MiB) by +default. The pe_start will be a multiple of this many MiB. This location +can be checked with: +.br +.B pvs -o pe_start +.I PV +.P +The size of the LVM metadata area is the space between the the start of +the metadata area and the first PE. When metadata begins at 4 KiB and the +first PE is at 1024 KiB, the metadata area size is 1020 KiB. This can be +checked with: +.br +.B pvs -o mda_size +.I PV +.P +The mda_size cannot be increased after pvcreate, so if larger metadata is +needed, it must be set during pvcreate. Two copies of the VG metadata +must always fit within the metadata area, so the maximum VG metadata size +is around half the mda_size. This can be checked with: +.br +.B vgs -o mda_free +.I VG +.P +A larger metadata area can be set with --metadatasize. The resulting +mda_size may be larger than specified due to default_data_alignment +placing pe_start on a MiB boundary, and the fact that the metadata area +extends to the first PE. With metadata starting at 4 KiB and +default_data_alignment 1 (MiB), setting --metadatasize 2048k results in +pe_start of 3 MiB and mda_size of 3068 KiB. Alternatively, --metadatasize +2044k results in pe_start at 2 MiB and mda_size of 2044 KiB. +.P +The alignment of pe_start described above may be automatically overridden +based on md device properties or device i/o properties reported in sysfs. +These automatic adjustments can be enabled/disabled using lvm.conf +settings md_chunk_alignment and data_alignment_offset_detection. +.P +To use a different pe_start alignment, use the --dataalignment option. +The --metadatasize option would also typically be used in this case +because the metadata area size also determines the location of pe_start. +When using these two options together, pe_start is calculated as: +metadata area start (page size), plus the specified --metadatasize, +rounded up to the next multiple of --dataalignment. +With metadata starting at 4 KiB, --metadatasize 2048k, and --dataalignment 128k, +pe_start is 2176 KiB and mda_size is 2172 KiB. +The pe_start of 2176 KiB is the nearest even multiple of 128 KiB that +provides at least 2048 KiB of metadata space. +Always check the resulting alignment and metadata size when using +these options. +.P +To shift an aligned pe_start value, use the --dataalignmentoffset option. +The pe_start alignment is calculated as described above, and then the +value specified with --dataalignmentoffset is added to produce the final +pe_start value. |