LU-10239 osc: limit chunk number of write submit

[fs/lustre-release.git] / lustre / osc / osc_internal.h

diff --git a/lustre/osc/osc_internal.h b/lustre/osc/osc_internal.h
index d1bf9d2..62b5c2b 100644 (file)
--- a/lustre/osc/osc_internal.h
+++ b/lustre/osc/osc_internal.h
@@ -173,6 +173,35 @@ extern unsigned long osc_cache_shrink_count(struct shrinker *sk,
                                            struct shrink_control *sc);
 extern unsigned long osc_cache_shrink_scan(struct shrinker *sk,
                                           struct shrink_control *sc);
+static inline unsigned int osc_max_write_chunks(const struct client_obd *cli)
+{
+       /*
+        * LU-8135:
+        *
+        * The maximum size of a single transaction is about 64MB in ZFS.
+        * #define DMU_MAX_ACCESS (64 * 1024 * 1024)
+        *
+        * Since ZFS is a copy-on-write file system, a single dirty page in
+        * a chunk will result in the rewrite of the whole chunk, therefore
+        * an RPC shouldn't be allowed to contain too many chunks otherwise
+        * it will make transaction size much bigger than 64MB, especially
+        * with big block size for ZFS.
+        *
+        * This piece of code is to make sure that OSC won't send write RPCs
+        * with too many chunks. The maximum chunk size that an RPC can cover
+        * is set to PTLRPC_MAX_BRW_SIZE, which is defined to 16MB. Ideally
+        * OST should tell the client what the biggest transaction size is,
+        * but it's good enough for now.
+        *
+        * This limitation doesn't apply to ldiskfs, which allows as many
+        * chunks in one RPC as we want. However, it won't have any benefits
+        * to have too many discontiguous pages in one RPC.
+        *
+        * An osc_extent won't cover over a RPC size, so the chunks in an
+        * osc_extent won't bigger than PTLRPC_MAX_BRW_SIZE >> chunkbits.
+        */
+       return PTLRPC_MAX_BRW_SIZE >> cli->cl_chunkbits;
+}
 
 static inline void osc_set_io_portal(struct ptlrpc_request *req)
 {