Viewing: mdc.max_pages_per_rpc.4
.TH MDC.MAX_PAGES_PER_RPC 4 2026-09-04 "Lustre" "Lustre Kernel Interfaces"
.SH NAME
mdc.max_pages_per_rpc, mdc.max_mb_per_rpc_read, mdc.max_mb_per_rpc_write, osc.max_pages_per_rpc, osc.max_mb_per_rpc_read, osc.max_mb_per_rpc_write \- configure RPC size limits for Lustre clients
.SH SYNOPSIS
.nf
lctl set_param \fIobd_type\fR.\fIobd_name\fR.max_pages_per_rpc=\fIPAGE_COUNT\fR
lctl set_param \fIobd_type\fR.\fIobd_name\fR.max_mb_per_rpc_read=\fIREAD_SIZE\fR
lctl set_param \fIobd_type\fR.\fIobd_name\fR.max_mb_per_rpc_write=\fIWRITE_SIZE\fR
.sp
lctl get_param \fIobd_type\fR.\fIobd_name\fR.max_pages_per_rpc
lctl get_param \fIobd_type\fR.\fIobd_name\fR.max_mb_per_rpc_read
lctl get_param \fIobd_type\fR.\fIobd_name\fR.max_mb_per_rpc_write
.fi
.SH DESCRIPTION
The
.BR max_pages_per_rpc ,
.BR max_mb_per_rpc_read ,
and
.BR max_mb_per_rpc_write
parameters configure the maximum RPC size used by Lustre clients when
performing bulk data transfers to OST and MDT targets.
.sp
Lustre performs file I/O by issuing RPC requests from the client to
metadata and object storage targets. Each RPC may transfer multiple
memory pages of data. These parameters limit the size of those RPC
requests and therefore influence the amount of data transmitted in a
single network operation.
.sp
On an
.B osc
device these parameters bound the file data RPCs sent to an OST. An
.B mdc
device uses them for two kinds of bulk transfer: the readdir RPCs that
fetch directory pages from the MDT, and the file data RPCs for files
with a Data-on-MDT layout.
.sp
The
.B max_pages_per_rpc
parameter defines the maximum number of memory pages,
.IR PAGE_COUNT ,
that may be included in a single RPC. The effective RPC size is
determined by multiplying the system page size by this value. Setting
this parameter is equivalent to setting both
.B max_mb_per_rpc_read
and
.B max_mb_per_rpc_write
to the same RPC size.
.sp
The page size depends on the system architecture. The commonly-used x86
(Intel/AMD) client CPUs use only a page size of 4 KiB, but aarch64 (ARM)
and other CPUs may use 4KiB, 16KiB, 64KiB or other page sizes. As a
result, the effective RPC size configured via
.B max_pages_per_rpc
may differ across platforms. For this reason, the
.B max_mb_per_rpc_read
and
.B max_mb_per_rpc_write
parameters are generally preferred over
.BR max_pages_per_rpc .
.sp
The
.B max_mb_per_rpc_read
and
.B max_mb_per_rpc_write
parameters specify limits,
.I READ_SIZE
and
.IR WRITE_SIZE ,
for read and write RPC requests issued by the client. These values may
be specified as sizes using whole or fractional numbers with
.BR K ,
.BR M ,
or
.BR G
suffixes.
.sp
When queried using
.BR lctl (8),
the
.B max_pages_per_rpc
parameter reports the value in pages, while
.B max_mb_per_rpc_read
and
.B max_mb_per_rpc_write
report the RPC size in megabytes. Where the read and write limits differ,
.B max_pages_per_rpc
reports the smaller of the two.
.SH EXAMPLES
Display the current value:
.EX
.B # lctl get_param mdc.lustre-MDT0000-mdc-ffffa010de31f000.max_pages_per_rpc
.B # lctl get_param osc.lustre-OST0001-osc-ffffa010de31f000.max_mb_per_rpc_read
.B # lctl get_param osc.lustre-OST0000-osc-ffffa010de31f000.max_mb_per_rpc_write
.B # lctl get_param osc.*.max_pages_per_rpc
.B # lctl get_param *.*.max_mb_per_rpc_write
.EE
.sp
Set the parameter:
.EX
.B # lctl set_param osc.lustre-OST0000-osc-ffffa010de31f000.max_pages_per_rpc=512
.B # lctl set_param osc.lustre-OST0001-osc-ffffa010de31f000.max_mb_per_rpc_read=2M
.B # lctl set_param osc.lustre-OST0001-osc-ffffa010de31f000.max_mb_per_rpc_write=512K
.EE
.sp
Tune the read RPC size for every MDT connection on the client, which bounds
readdir and Data-on-MDT reads:
.EX
.B # lctl set_param mdc.*.max_mb_per_rpc_read=512K
.EE
.SH AVAILABILITY
.B osc.max_pages_per_rpc
is part of the
.BR lustre (7)
filesystem package since release 1.0.0
.\" Added in commit 0.0.0-3210-gc5050e4125
.br
.B mdc.max_pages_per_rpc
is part of the
.BR lustre (7)
filesystem package since release 2.1.0, and settable since release 2.10.0
.\" Added in commit v2_0_65_0-9-g58f3eb928f
.\" Made settable in commit v2_9_56_0-68-g664bad91b5
.br
.B mdc.max_mb_per_rpc_read, osc.max_mb_per_rpc_read,
.B mdc.max_mb_per_rpc_write, osc.max_mb_per_rpc_write
are part of the
.BR lustre (7)
filesystem package since release 2.18.0
.\" Added in commit v2_17_51-55-g04f6831096
.SH SEE ALSO
.BR lctl (8),
.BR lctl-get_param (8),
.BR lctl-set_param (8)
