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lvconvert changes the LV type and includes utilities for LV data
maintenance. The LV type controls data layout and redundancy.
The LV type is also called the segment type or segtype.
.P
To display the current LV type, run the command:
.P
.B lvs -o name,segtype
.I LV
.P
In some cases, an LV is a single device mapper (dm) layer above physical
devices. In other cases, hidden LVs (dm devices) are layered between the
visible LV and physical devices. LVs in the middle layers are called sub LVs.
A command run on a visible LV sometimes operates on a sub LV rather than
the specified LV. In other cases, a sub LV must be specified directly on
the command line.
.P
Sub LVs can be displayed with the command:
.P
.B lvs -a
.P
The
.B linear
type is equivalent to the
.B striped
type when one stripe exists.
In that case, the types can sometimes be used interchangeably.
.P
In most cases, the
.B mirror
type is deprecated and the
.B raid1
type should be used. They are both implementations of mirroring.
.P
Striped raid types are
\fBraid0/raid0_meta\fP,
\fBraid5\fP (an alias for raid5_ls),
\fBraid6\fP (an alias for raid6_zr) and
\fBraid10\fP (an alias for raid10_near).
.P
As opposed to mirroring, raid5 and raid6 stripe data and calculate parity
blocks. The parity blocks can be used for data block recovery in case
devices fail. A maximum number of one device in a raid5 LV may fail, and
two in case of raid6. Striped raid types typically rotate the parity and
data blocks for performance reasons, thus avoiding contention on a single
device. Specific arrangements of parity and data blocks (layouts) can be
used to optimize I/O performance, or to convert between raid levels. See
\fBlvmraid\fP(7) for more information.
.P
Layouts of raid5 rotating parity blocks can be: left-asymmetric
(raid5_la), left-symmetric (raid5_ls with alias raid5), right-asymmetric
(raid5_ra), right-symmetric (raid5_rs) and raid5_n, which doesn't rotate
parity blocks. Layouts of raid6 are: zero-restart (raid6_zr with alias
raid6), next-restart (raid6_nr), and next-continue (raid6_nc).
.P
Layouts including _n allow for conversion between raid levels (raid5_n to
raid6 or raid5_n to striped/raid0/raid0_meta). Additionally, special raid6
layouts for raid level conversions between raid5 and raid6 are:
raid6_ls_6, raid6_rs_6, raid6_la_6 and raid6_ra_6. Those correspond to
their raid5 counterparts (e.g. raid5_rs can be directly converted to
raid6_rs_6 and vice-versa).
.P
raid10 (an alias for raid10_near) is currently limited to one data copy
and even number of sub LVs. This is a mirror group layout, thus a single
sub LV may fail per mirror group without data loss.
.P
Striped raid types support converting the layout, their stripesize and
their number of stripes.
.P
The striped raid types combined with raid1 allow for conversion from
linear \[->] striped/raid0/raid0_meta and vice-versa by e.g. linear \[<>] raid1
\[<>] raid5_n (then adding stripes) \[<>] striped/raid0/raid0_meta.
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