.TH LLAPI_LAYOUT_GET_BY_FD 3 2025-03-19 "Lustre User API" "Lustre Library Functions" .SH NAME llapi_layout_get_by_fd, llapi_layout_get_by_fid, llapi_layout_get_by_path, llapi_layout_get_by_xattr, \- obtain the layout of a Lustre file .SH SYNOPSIS .nf .B #include .PP .BI "struct llapi_layout *llapi_layout_get_by_fd(int " fd , .BI " enum llapi_layout_get_flags " flags ); .PP .BI "struct llapi_layout *llapi_layout_get_by_fid(const char *"lustre_path , .BI " const struct lu_fid *"fid , .BI " enum llapi_layout_get_flags " flags ); .PP .BI "struct llapi_layout *llapi_layout_get_by_path(const char *"path , .BI " enum llapi_layout_get_flags " flags ); .PP .BI "struct llapi_layout *llapi_layout_get_by_xattr(void *"lov_xattr , .BI " ssize_t " lov_xattr_size , .BI " enum llapi_layout_get_flags " flags ); .fi .SH DESCRIPTION The functions .BR llapi_layout_get_by_xattr() , .BR llapi_layout_get_by_fd() , .BR llapi_layout_get_by_fid() , and .BR llapi_layout_get_by_path() return a pointer to a newly-allocated .B struct llapi_layout containing the layout information for the file referenced by .IR lov_xattr , .IR fd , .IR fid , or .IR path , respectively. The .B struct llapi_layout is an opaque entity containing the layout information for a file in a Lustre filesystem. Its internal structure should not be directly accessed by an application. See .BR llapi_layout (7). The pointer should be freed with .B llapi_layout_free() when it is no longer needed. .PP For .BR llapi_layout_get_by_xattr() , .I lov_xattr is a Lustre layout extended attribute (LOV EA) from a file or directory in a Lustre filesystem. The .I lov_xattr should be the raw xattr without being byte-swapped, since this function will swap it to the local machine endianness properly. .PP For .BR llapi_layout_get_by_fd() , .I fd is a valid open file descriptor for a file or directory in a Lustre filesystem. .PP For .BR llapi_layout_get_by_fid() , .I lustre_path identifies the Lustre filesystem containing the file represented by .IR fid . It is typically the filesystem root directory, but may also be any path beneath the root. Use the function .BR llapi_path2fid (3) to obtain a .B struct lu_fid associated with a given path. .PP The function .B llapi_layout_get_by_path() accepts a .I path argument that names a file or directory in a Lustre filesystem. .PP Zero or more flags may be bitwise-or'd together in .I flags to control how a layout is retrieved. Currently .B llapi_layout_get_by_fd() and .B llapi_layout_get_by_fid() do not accept any values for .IR flags , while .B llapi_layout_get_by_path() accepts only one flag as follows: .TP 5 .B LLAPI_LAYOUT_GET_EXPECTED Unspecified attribute values are replaced by the literal default values that will be assigned when the file is created or first written to. A default value is inherited from the parent directory if the attribute is specified there, otherwise it is inherited from the filesystem root. This flag is only recognized by .BR llapi_layout_get_by_path() . Unspecified attributes may belong to directories and never-written-to files. .P By default, layouts report the abstract value .B LLAPI_LAYOUT_DEFAULT to indicate an unspecified attribute. Use .B LLAPI_LAYOUT_GET_EXPECTED to discover the expected literal values for new files in a given directory. Do not use it if you need to distinguish between specified and unspecified attributes. The flag has no effect if .I path names a file or directory with a fully specified layout. .P For concreteness, consider a Lustre filesystem with a default stripe size of 1048576 and a default stripe count of 1. A user sets the stripe count for directory D to 2 (thus overriding the filesystem-wide default) but leaves the stripe size unspecified. Newly created files in D inherit a stripe count of 2 from D and a stripe size of 1048576 from the filesystem default. The layout of D returned by .B llapi_layout_get_by_path(D, 0) has the abstract stripe size value .BR LLAPI_LAYOUT_DEFAULT , since stripe size is unspecified, while .B llapi_layout_get_by_path(D, LLAPI_LAYOUT_GET_EXPECTED) reports the literal value 1048576. Both forms report a stripe count of 2, since that attribute is specified. .PP Valid arguments for .I flags with .B llapi_layout_get_by_xattr() are: .TP 5 .B LLAPI_LAYOUT_GET_CHECK If the .B LLAPI_LAYOUT_GET_CHECK flag is set, this function will check whether the objects count in lum is consistent with the stripe count in lum. This check only apply to regular files, so it should not be passed if the xattr is a default layout from a directory. .TP .B LLAPI_LAYOUT_GET_COPY If the .B LLAPI_LAYOUT_GET_COPY flag is set, this function will use a temporary buffer for byte swapping when necessary, leaving .I lov_xattr unmodified. Otherwise, the byte swapping will be done to the fields of the .I lov_xattr buffer directly. .SH RETURN VALUES These functions return a valid pointer on success or .B NULL on failure with .B errno set to an appropriate error code. .SH ERRORS .TP 15 .B ENOMEM Insufficient storage space is available. .TP .B ENOTTY File does not reside on a Lustre filesystem. .TP .B ENOENT .I path does not exist. .TP .B EINVAL An invalid argument was specified. .TP .B EINTR The kernel returned less than the expected amount of data. .SH NOTES When using these functions to copy an existing file's layout to create a new file with .B llapi_layout_file_open (3) for mirroring, migration, or as the template for a new file, .BR llapi_layout_ost_index_reset (3) should be called to reset the OST index values for each component, so that the file copy is not created on exactly the same OSTs as the original file. .SH AVAILABILITY .BR llapi_layout_get_by_fd(), .BR llapi_layout_get_by_fid(), .B llapi_layout_get_by_path() and .B llapi_layout_get_by_xattr() are part of the .BR lustre (7) user application interface library since release 2.7.0 .\" Added in commit v2_6_51_0-23-g3d3a37c9c8 .SH SEE ALSO .BR llapi_layout_file_open (3), .BR llapi_layout_ost_index_reset (3), .BR llapi_path2fid (3), .BR llapi_layout (7), .BR lustreapi (7)