.TH LLAPI_LAYOUT_COMP_ADD_PARITY_ONLY 3 2026-08-25 "Lustre User API" "Lustre Library Functions"
.SH NAME
llapi_layout_comp_add_parity_only \- add an EC parity component to a layout
.SH SYNOPSIS
.nf
.B #include <lustre/lustreapi.h>
.PP
.BI "int llapi_layout_comp_add_parity_only(struct llapi_layout *" layout ,
.BI "                                      uint16_t " target_mirror_id ,
.BI "                                      uint64_t " start ,
.BI "                                      uint64_t " end ,
.BI "                                      uint8_t " dstripe_count ,
.BI "                                      uint8_t " cstripe_count ,
.BI "                                      uint64_t " stripe_size ,
.BI "                                      uint64_t " data_stripe_count ,
.BI "                                      const char *" pool_name ");"
.fi
.SH DESCRIPTION
.BR llapi_layout_comp_add_parity_only (3)
appends an Erasure Coding (EC) parity component to the in-memory
.IR layout ,
bound to the data mirror
.I target_mirror_id
of the file that
.I layout
will be merged into. The
.I layout
is turned into a composite layout if it was plain before the component was
added, and its current component pointer is left on the newly added
component.
.PP
The data mirror is not part of
.IR layout ,
so its attributes are passed in by the caller rather than read out of
.IR layout ,
and the parity component is bound to
.I target_mirror_id
directly instead of through the transient link id that is resolved while a
layout is created. Use
.B llapi_layout_comp_add_ec()
instead when the data component being protected is in the same
.IR layout .
.PP
.I target_mirror_id
is the mirror id of the data mirror to protect, and must be non-zero.
.PP
.I start
and
.I end
give the extent of the parity component, which must match the extent of the
data component it protects.
.PP
.I dstripe_count
and
.I cstripe_count
are the number of data stripes (K) and parity stripes (P) of each RAID Set.
.PP
.I stripe_size
is the stripe size of the data component being protected. A parity component
uses the same stripe size as its data component.
.PP
.I data_stripe_count
is the stripe count of the data component being protected. Besides bounding
K+P, it is stored as the parity component's own stripe count, from which the
MDS derives the number of RAID Sets when the data component is not part of
the same layout.
.PP
.I pool_name
is the OST pool of the data component being protected, so that parity objects
are allocated alongside the data they protect, or NULL to leave the parity
component unbound.
.PP
One parity component is added per call, so a data mirror made of several
components needs one call per component.
.SH RETURN VALUES
.BR llapi_layout_comp_add_parity_only (3)
returns 0 on success, or -1 if an error occurred (in which case,
.B errno
is set appropriately). On failure
.I layout
is left unchanged.
.SH ERRORS
.TP 15
.B EINVAL
An invalid argument was specified:
.I layout
was NULL,
.I target_mirror_id
was zero,
.I start
was not below
.IR end ,
or the EC stripe counts exceeded the EC limits or did not fit
.IR data_stripe_count .
.TP 15
.B ENOMEM
Insufficient memory to complete the operation.
.SH EXAMPLES
Build a layout holding one 4+2 EC parity component for mirror 1 of a file
whose data component has 8 stripes of 1MiB, to be merged into that file:
.EX
        struct llapi_layout *layout;
        int rc;
\&
        layout = llapi_layout_alloc();
        if (layout == NULL)
                /* error, out of memory */
\&
        /* K=4, P=2 per RAID Set over the 8-stripe data mirror */
        rc = llapi_layout_comp_add_parity_only(layout, 1, 0, LUSTRE_EOF,
                                               4, 2, 1048576, 8, NULL);
        if (rc != 0)
                /* error, errno is set */
\&
        llapi_layout_free(layout);
.EE
.SH AVAILABILITY
.B llapi_layout_comp_add_parity_only()
is part of the
.BR lustre (7)
user application interface library since release 2.18.0.
.\" Added in commit v2_17_56-51-gxxx
.SH SEE ALSO
.BR lfs-mirror-extend (1),
.BR lfs-setstripe (1),
.BR llapi_layout_alloc (3),
.BR llapi_layout_comp_add (3),
.BR llapi_layout_file_comp_add (3),
.BR llapi_layout_free (3),
.BR llapi_layout (7),
.BR lustreapi (7)