LU-9348 proc: describe brw_size limits better

Add a description of the brw_size limits (minimum and maximum values).

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I03ea6bfedd971a510d2861deae8dafaa6fbe4f82
Reviewed-on: https://review.whamcloud.com/26726
Tested-by: Jenkins
Reviewed-by: Joseph Gmitter <joseph.gmitter@intel.com>

diff --git a/LustreTuning.xml b/LustreTuning.xml
index 7caf0e6..b3854f0 100644 (file)
--- a/LustreTuning.xml
+++ b/LustreTuning.xml
@@ -1495,15 +1495,26 @@ ldlm.namespaces.filter-<replaceable>fsname</replaceable>-*.
           <para>Beginning with Lustre 2.9, Lustre is extended to support RPCs up
           to 16MB in size. By enabling a larger RPC size, fewer RPCs will be
           required to transfer the same amount of data between clients and
           <para>Beginning with Lustre 2.9, Lustre is extended to support RPCs up
           to 16MB in size. By enabling a larger RPC size, fewer RPCs will be
           required to transfer the same amount of data between clients and

-          servers.  With a larger RPC size, the OST can submit more data to the
+          servers.  With a larger RPC size, the OSS can submit more data to the

           underlying disks at once, therefore it can produce larger disk I/Os
           to fully utilize the increasing bandwidth of disks.</para>
           underlying disks at once, therefore it can produce larger disk I/Os
           to fully utilize the increasing bandwidth of disks.</para>

-          <para>At client connecting time, clients will negotiate with
-          servers for the RPC size it is going to use.</para>
-          <para>A new parameter, <literal>brw_size</literal>, is introduced on
-          the OST to tell the client the preferred IO size.  All clients that
+          <para>At client connection time, clients will negotiate with
+          servers what the maximum RPC size it is possible to use, but the
+         client can always send RPCs smaller than this maximum.</para>
+          <para>The parameter <literal>brw_size</literal> is used on the OST
+         to tell the client the maximum (preferred) IO size.  All clients that

           talk to this target should never send an RPC greater than this size.
           talk to this target should never send an RPC greater than this size.

+         Clients can individually set a smaller RPC size limit via the
+         <literal>osc.*.max_pages_per_rpc</literal> tunable.

           </para>
           </para>

+         <note>
+         <para>The smallest <literal>brw_size</literal> that can be set for
+         ZFS OSTs is the <literal>recordsize</literal> of that dataset.  This
+         ensures that the client can always write a full ZFS file block if it
+         has enough dirty data, and does not otherwise force it to do read-
+         modify-write operations for every RPC.
+          </para>
+         </note>

       </section>
       <section><title>Usage</title>
           <para>In order to enable a larger RPC size,
       </section>
       <section><title>Usage</title>
           <para>In order to enable a larger RPC size,


@@ -1530,7 +1541,7 @@ ldlm.namespaces.filter-<replaceable>fsname</replaceable>-*.
           <screen>client$ lctl conf_param <replaceable>fsname</replaceable>-OST*.osc.max_pages_per_rpc=16M</screen>
           <caution><para>The <literal>brw_size</literal> of an OST can be
           changed on the fly.  However, clients have to be remounted to
           <screen>client$ lctl conf_param <replaceable>fsname</replaceable>-OST*.osc.max_pages_per_rpc=16M</screen>
           <caution><para>The <literal>brw_size</literal> of an OST can be
           changed on the fly.  However, clients have to be remounted to

-          renegotiate the new RPC size.</para></caution>
+          renegotiate the new maximum RPC size.</para></caution>

       </section>
   </section>
   <section xml:id="dbdoclet.50438272_80545">
       </section>
   </section>
   <section xml:id="dbdoclet.50438272_80545">