When there is a disparity in reported I/O latency between the NFS server and Oracle database on the NFS client the question arises:
Where does the difference in latency come from between NFS Server and Oracle’s timing of pread?
One strategy is to take tcp traces on NFS server and NFS client at the same time, during a period of load that shows the disparity.
The two trace files can then be analyzed to show latency at each side and the delta of latency between packets found in both traces
Parsing script is ( thanks to Matt Amdur from Delphix for the core code)
Requires tethereal which comes with the wireshark install on LINUX:
yum install wireshark
On NFS server the latency of 8K block sends was measured with DTrace and showed:
avg ms 30.2 24.4 21.9 23.7 32.0 25.5
On the NFS client, the Oracle database is measured with oramon.sh and shows the 8k read latency as
avg ms 126.15 103.91 117.33 96.46 78.43 91.94
There is a substantial difference between the latency seen by NFS server and the latency seen by Oracle. Where is the latency coming from? Is it a problem on the NFS server, the NFS client or the network? How do we find out? One strategy is to collect TCP dumps on the NFS client and NFS server and compare them.
Collecting TCP traces
First, set up a situation where there is a load running on the database and there is a discrepancy between latency on NFS server and as seen by Oracle. During this period of latency discrepancy collect TCP traces. The traces will be large, so run them for 20 second for example, to limit the size.
TCP dumps can be collected on LINUX with tcpdump and on Solaris with snoop:
NFS server (Solaris NFS server example)
snoop -q -d aggr0 -s 512 -o nfs_server.cap 172.16.100.102
NFS client host is the IP at the end of the command line
Linux (linux client example)
tcpdump -s 512 host 172.16.100.87 -w nfs_client.cap
The trace files can be analyzed on a linux machine with wireshark installed and with the script parsetcp.pl on the two traces.
$ ./parsetcp.pl nfs_server.cap nfs_client.cap ==================== RAW DATA ============ Parsing NFS server trace: nfs_server.cap type avg ms count READ : 18.10, 12493 WRITE : 30.08, 4918 Parsing NFS client trace: nfs_client.cap type avg ms count READ : 18.74, 25974 WRITE : 30.05, 10197 ==================== MATCHED DATA ============ READ type avg ms nfs server: 18.12 nfs_client: 19.26 diff : 1.14 Processed 14010 packets (Matched: 11964 Missed: 2046) WRITE type avg ms server : 30.86 client : 31.83 diff : 0.96 Processed 5806 packets (Matched: 4391 Missed: 1415)
The script will find the average latency for reads and writes on both ends and then try to match the packets found both at the client and server and report the average latency for reads and writes on the packets that match.
In the above output the read time on on the NFS client receiving end is on par with what is seen on NFS server.
Thus if the receive time on the NFS client host is on par with the NFS server and Oracle latency is much greater than the receive time, then there looks to be some issue on the NFS client host machine such as heavy memory paging that would cause the latency seen by Oracle to be much higher than that seen by the tcp traces.
In the following 2 examples a swingbench load was run on the Oracle database on the NFS client. The database files were mounted via NFS from the NFS server. In one case the NFS client and database host was Open Solaris and in the other case it was LINUX. In both cases the Oracle database reported much slower latency than the NFS server threads.
test 1 ) Open Solaris is the NFS client
Open Solaris is the NFS Server as well
NFS Server latency ( from DTrace of nfs start/done )
avg_ms 1.4 1.8 1.1 1.1 1.5 0.9 0.7 1.1 1.0 0.9 1.1
NFS client: the Oracle database shows read latency much slower at 29-65ms from oramon.sh:
avg ms 29.92 32.62 50.38 56.38 65.45 46.88 45.13 44.93 39.48 55.8
Test 1 TCP trace analysis
What do the tcp traces say? Tracing TCP on the NFS server side, and NFS client side indicates that the slow latencies are coming from the NFS server
==================== Individual HOST DATA ============ Parsing nfs server trace: nfs_server.cap type avg ms count READ : 44.60, 7731 Parsing client trace: client.cap type avg ms count READ : 46.54, 15282 ==================== MATCHED DATA ============ READ type avg ms server : 48.39, client : 49.42, diff : 1.03, Processed 9647 packets (Matched: 5624 Missed: 4023)
TEST 1 summary:
The NFS Server says it is responding with 1ms latency (from DTrace) but the TCP traces show latency more on the order of 48ms which is in alignment with what Oracle sees on the NFS client, thus the NFS server must be introducing the large latency somehow and not the client.
Test 2) LINUX NFS client
Open Solaris is the NFS server
NFS Server 8K block latency (from DTrace)
avg_ms 0.1 0.1 0.1 0.1 0.1 0.1
Oracle latency on LINUX from oramon.sh
single block ms 35.73 58.90 44.88 54.43 54.57 63.92
Again we see the same stats as the first test case, ie the NFS Server says it’s latency is low and the Oracle database on the NFS client says the latency is slow
TCP trace analysis
==================== Individual HOST DATA ============ Parsing NFS server trace: nfs_server.cap type avg ms count READ : 1.17, 9042 Parsing client trace: client.cap type avg ms count READ : 1.49, 21984 ==================== MATCHED DATA ============ READ type avg ms count server : 1.03 client : 1.49 diff : 0.46
Processed 13815 packets (Matched: 8169 Missed: 5646)
summary of test 2:
In this case the NFS Server TCP traces show fast latency as well as the NFS client TCP traces, thus the long latency times must be introduced on the client machine somehow and not the NFS Server.
Tests are run with swingbench benchmark.
The Oracle databases are set with the parameter “filesystemio_options=SETALL” which enables direct I/O, so that all I/O requests from Oracle should be become NFS requests to NFS server.
Roughly the latency stats for 8k reads originating from the Oracle are
|Oracle on Linux||Oracle on Solaris||latency data source|
|NFS Server||.1 ms||2 ms||dtrace nfs:::op-read-start/op-read-done|
|TCP trace NFS Server||1 ms||44 ms||snoop|
|TCP trace NFS Client||1.5||45 ms||tcpdump on LINUX and snoop on Open Solaris|
|Oracle||58 ms||47 ms||“db file sequential read” wait (which is basically a timing of “pread” for 8k random reads specifically|
In the above examples there was nothing outstanding in the vmstat results on the NFS client and the NFS server.
In both cases the problem arose with high level of swingbench sessions. The latency disparity appeared above a user load of 16. The latency disparity increased as user load increased.
It turned out that in the case where the NFS server was introducing the disparity that the maximum NFS threads on the server was 16. The solution was to increase the NFS server thread max: sharectl set -p servers=512 nfs.
In the case where the LINUX client introduce the disparity it turns out that the maximum outstanding NFS requests was 16. The solution was to raise sunrpc.tcp_slot_table_entries = 128.
The tcpdump.pl script didn’t find the answers but it did identify which side of the connection was at fault and it would identify if the network itself was the problem.
One thing to keep in mind is that snoop and tcpdump can be resource intensive and may introduce a major amount of latency. In the above cases with and without tcp tracing enabled showed no more than 1ms of difference and the latency being investigated was an order of magnitude greater. In other cases where there has been more I/O between the NFS server and client and/or the transfer sizes were higher (like 1M) then the impact of TCP tracing has eclipsed the latency disparity being investigated and tcp tracing would not have been appropriate investigation path.
$ sudo snoop -D Using device aggr0 (promiscuous mode) kyle -> 172.16.100.92 drops: 0 TCP D=50461 S=22 Push Ack=2424998673 Seq=3990657488 Len=84 Win=49640 172.16.100.92 -> kyle drops: 0 TCP D=22 S=50461 Ack=3990657572 Seq=2424998673 Len=0 Win=254 172.16.100.147 -> (broadcast) drops: 0 ARP C Who is 172.16.100.115, 172.16.100.115 ? 172.16.100.68 -> (broadcast) drops: 0 ARP C Who is 172.16.100.115, 172.16.100.115 ? 172.16.100.68 -> (broadcast) drops: 0 ARP C Who is 172.16.100.184, 172.16.100.184 ? 172.16.100.199 -> (broadcast) drops: 0 ARP C Who is 172.16.100.200, 172.16.100.200 ?