new "nosharetransport" option for NFS mounts.

This patch adds a "nosharetransport" option to allow two different mounts from the same server to use different transports. If the mounts use NFSv4, or are of the same filesystem, then "nosharecache" must be used as well.

There are at least two circumstances where it might be desirable to use separate transports:

1/ If the NFS server can get into a state where it will ignore
requests for one filesystem while servicing request for another, then using separate connections for the separate filesystems can stop problems with one affecting access to the other.

This is particularly relevant for NetApp filers where one filesystem has been "suspended". Requests to that filesystem will be dropped (rather than the more correct NFS3ERR_JUKEBOX). This currently interferes with other filesystems.

2/ If a very fast network is used with a many-processor client, a
single TCP connection can present a bottle neck which reduces total throughput. Using multiple TCP connections (one per mount) removes the bottleneck.
An alternate workaround is to configure multiple virtual IP addresses on the server and mount each filesystem from a different IP. This is effective (throughput goes up) but an unnecessary administrative burden.

---
Is this a good idea? Bad idea? Have I missed something important?

NeilBrown

index c513b0c..64e3f39 100644
@@ -403,8 +403,13 @@ static struct nfs_client *nfs_match_client(const struct nfs_client_initdata *dat
 	const struct sockaddr *sap = data->addr;
 	struct nfs_net *nn = net_generic(data->net, nfs_net_id);
 
+	if (test_bit(NFS_CS_NO_SHARE, &data->init_flags))
+		return NULL;
+
 	list_for_each_entry(clp, &nn->nfs_client_list, cl_share_link) {
 	        const struct sockaddr *clap = (struct sockaddr *)&clp->cl_addr;
+		if (test_bit(NFS_CS_NO_SHARE,&clp->cl_flags))
+			continue;
 		/* Don't match clients that failed to initialise properly */
 		if (clp->cl_cons_state < 0)
 			continue;

@@ -753,6 +758,8 @@ static int nfs_init_server(struct nfs_server *server,
 			data->timeo, data->retrans);
 	if (data->flags & NFS_MOUNT_NORESVPORT)
 		set_bit(NFS_CS_NORESVPORT, &cl_init.init_flags);
+	if (data->flags & NFS_MOUNT_NOSHARE_XPRT)
+		set_bit(NFS_CS_NO_SHARE, &cl_init.init_flags);
 	if (server->options & NFS_OPTION_MIGRATION)
 		set_bit(NFS_CS_MIGRATION, &cl_init.init_flags);
 

index 2d7525f..d9141d8 100644
@@ -88,6 +88,7 @@ enum {
 	Opt_acl, Opt_noacl,
 	Opt_rdirplus, Opt_nordirplus,
 	Opt_sharecache, Opt_nosharecache,
+	Opt_sharetransport, Opt_nosharetransport,
 	Opt_resvport, Opt_noresvport,
 	Opt_fscache, Opt_nofscache,
 	Opt_migration, Opt_nomigration,

@@ -146,6 +147,8 @@ static const match_table_t nfs_mount_option_tokens = {
 	{ Opt_nordirplus, "nordirplus" },
 	{ Opt_sharecache, "sharecache" },
 	{ Opt_nosharecache, "nosharecache" },
+	{ Opt_sharetransport, "sharetransport"},
+	{ Opt_nosharetransport, "nosharetransport"},
 	{ Opt_resvport, "resvport" },
 	{ Opt_noresvport, "noresvport" },
 	{ Opt_fscache, "fsc" },

@@ -634,6 +637,7 @@ static void nfs_show_mount_options(struct seq_file *m, struct nfs_server *nfss,
 		{ NFS_MOUNT_NOACL, ",noacl", "" },
 		{ NFS_MOUNT_NORDIRPLUS, ",nordirplus", "" },
 		{ NFS_MOUNT_UNSHARED, ",nosharecache", "" },
+		{ NFS_MOUNT_NOSHARE_XPRT, ",nosharetransport", ""},
 		{ NFS_MOUNT_NORESVPORT, ",noresvport", "" },
 		{ 0, NULL, NULL }
 	};

@@ -1239,6 +1243,12 @@ static int nfs_parse_mount_options(char *raw,
 		case Opt_nosharecache:
 			mnt->flags |= NFS_MOUNT_UNSHARED;
 			break;
+		case Opt_sharetransport:
+			mnt->flags &= ~NFS_MOUNT_NOSHARE_XPRT;
+			break;
+		case Opt_nosharetransport:
+			mnt->flags |= NFS_MOUNT_NOSHARE_XPRT;
+			break;
 		case Opt_resvport:
 			mnt->flags &= ~NFS_MOUNT_NORESVPORT;
 			break;

index 3b7fa2a..9e9d7d3 100644
@@ -41,6 +41,7 @@ struct nfs_client {
 #define NFS_CS_DISCRTRY		1		/* - disconnect on RPC retry */
 #define NFS_CS_MIGRATION	2		/* - transparent state migr */
 #define NFS_CS_INFINITE_SLOTS	3		/* - don't limit TCP slots */
+#define NFS_CS_NO_SHARE		4		/* - don't share across mounts */
 	struct sockaddr_storage	cl_addr;	/* server identifier */
 	size_t			cl_addrlen;
 	char *			cl_hostname;	/* hostname of server */

index 576bddd..81c49ff 100644
@@ -73,5 +73,6 @@ struct nfs_mount_data {
 
 #define NFS_MOUNT_LOCAL_FLOCK	0x100000
 #define NFS_MOUNT_LOCAL_FCNTL	0x200000
+#define NFS_MOUNT_NOSHARE_XPRT	0x400000
 
 #endif

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Will it? I googled around a bit but couldn't find anything that tells me what trunking really was in this context. Then I found commit 05f4c350ee02 which makes it quite clear (thanks Chuck!).

Probably the code I wrote could interfere.

Why would it attempt to attach to an existing session? I would hope there the two different mounts with separate TCP connections would look completely separate - different transport, different cache, different session. ??

I wasn't aware of this, nor were our support people. I've passed it on so maybe they can bug Netapp....

A customer upgraded from SLES10 (2.6.16 based) to SLES11 (3.0 based) and saw a slowdown on some large DB jobs of between 1.5 and 2 times (i.e. total time 150% to 200% of what is was before).
After some analysis they created multiple virtual IPs on the server and mounted the several filesystem each from different IPs and got the performance back (they see this as a work-around rather than a genuine solution).
Numbers are like "500MB/s on a single connection, 850MB/sec peaking to 1000MB/sec on multiple connections".

If I can get something more concrete I'll let you know.

As this worked well in 2.6.16 (which doesn't try to share connections) this is seen as a regression.

On links that are easy to saturate, congestion windows are important and having a single connection is probably a good idea - so the current default is certainly correct.
On a 10G ethernet or infiniband connection (where the issue has been measured) congestion just doesn't seem to be an issue.

Thanks,
NeilBrown

Slightly more concrete:

4 mounts from the one server, 10 threads of fio on each mount.
All over a 10G Ethernet.

1 IP address without "nosharetransport": ~700MB/s 4 IP addresses without "nosharetransport": ~1100MB/s 1 IP address with "nosharetransport": ~1100MB/s

This is all NFSv3. NFSv4 is much slower with nosharecache (32MB/s!), but I might have botched the backport to 3.0 (or didn't address the v4 specific issues you raised). I haven't looked into that yet.

NeilBrown

We've toyed with the idea of using multiple TCP connections per mount for years. The choice was made to stick with one connection (and one session on NFSv4.1) for each server.

The main limitation has been having a single RPC slot table for the transport, allowing only 16 concurrent RPC requests per server at a time. Andy and Trond did some good work making the slot table widen itself dynamically as the TCP window opens.

A secondary concern is head-of-queue blocking. The server end can certainly stall a client by not taking the top request off the socket queue, and thereby delay any requests that are behind that one in the queue. I think the preferred solution there is to build out support for RPC over SCTP, and use SCTP's multi-stream feature. Alternately we might choose to try out M-TCP. Server implementations can also be made sensitive to this issue to help prevent delays.

A tertiary issue is contention for the transport on multi-socket systems. For a long while I've suspected it may occur, but I've never measured it in practice.

Re: the problem at hand: You've definitely measured a performance regression. However, I don't think those numbers explain _why_ it is occurring.

The first thing to check is whether SuSE11 has the dynamic RPC slot table logic I mentioned above. I think it starts with upstream commit d9ba131d, but someone should correct me if I'm wrong.

Chuck Lever
chuck[dot]lever[at]oracle[dot]com

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Since the current client only requests SP4_NONE state protection, the BIND_CONN_TO_SESSION is implicit:

If, when the client ID was created, the client opted for SP4_NONE state protection, the client is not required to use BIND_CONN_TO_SESSION to associate the connection with the session, unless the client wishes to associate the connection with the backchannel. When SP4_NONE protection is used, simply sending a COMPOUND request with a SEQUENCE operation is sufficient to associate the connection with the session specified in SEQUENCE.

But Neil would need to make sure he's using all the state associated with the existing client.

Sounds to me sharing these things shouldn't be a problem in your case, but I don't know.

--b.