On Sun, Feb 12, 2023 at 1:08 PM Chuck Lever III <chuck.lever@xxxxxxxxxx> wrote: > > > > > On Feb 12, 2023, at 1:01 AM, Wang Yugui <wangyugui@xxxxxxxxxxxx> wrote: > > > > Hi, > > > > question about the performance of sec=krb5. > > > > https://learn.microsoft.com/en-us/azure/azure-netapp-files/performance-impact-kerberos > > Performance impact of krb5: > > Average IOPS decreased by 53% > > Average throughput decreased by 53% > > Average latency increased by 3.2 ms > > Looking at the numbers in this article... they don't > seem quite right. Here are the others: > > > Performance impact of krb5i: > > • Average IOPS decreased by 55% > > • Average throughput decreased by 55% > > • Average latency increased by 0.6 ms > > Performance impact of krb5p: > > • Average IOPS decreased by 77% > > • Average throughput decreased by 77% > > • Average latency increased by 1.6 ms > > I would expect krb5p to be the worst in terms of > latency. And I would like to see round-trip numbers > reported: what part of the increase in latency is > due to server versus client processing? > > This is also remarkable: > > > When nconnect is used in Linux, the GSS security context is shared between all the nconnect connections to a particular server. TCP is a reliable transport that supports out-of-order packet delivery to deal with out-of-order packets in a GSS stream, using a sliding window of sequence numbers. When packets not in the sequence window are received, the security context is discarded, and a new security context is negotiated. All messages sent with in the now-discarded context are no longer valid, thus requiring the messages to be sent again. Larger number of packets in an nconnect setup cause frequent out-of-window packets, triggering the described behavior. No specific degradation percentages can be stated with this behavior. > > > So, does this mean that nconnect makes the GSS sequence > window problem worse, or that when a window underrun > occurs it has broader impact because multiple connections > are affected? Yes nconnect makes the GSS sequence window problem worse (very typical to generate more than gss window size number of rpcs and have no ability to control in what order they would be sent) and yes all connections are affected. ONTAP as linux uses 128 gss window size but we've experimented with increasing it to larger values and it would still cause issues. > Seems like maybe nconnect should set up a unique GSS > context for each xprt. It would be helpful to file a bug. At the time when I saw the issue and asked about it (though can't find a reference now) I got the impression that having multiple contexts for the same rpc client was not going to be acceptable. > > > > and then in 'man 5 nfs' > > sec=krb5 provides cryptographic proof of a user's identity in each RPC request. > > Kerberos has performance impacts due to the crypto- > graphic operations that are performed on even small > fixed-sized sections of each RPC message, when using > sec=krb5 (no 'i' or 'p'). > > > > Is there a option of better performance to check krb5 only when mount.nfs4, > > not when file acess? > > If you mount with NFSv4 and sec=sys from a Linux NFS > client that has a keytab, the client will attempt to > use krb5i for lease management operations (such as > EXCHANGE_ID) but it will continue to use sec=sys for > user authentication. That's not terribly secure. > > A better answer would be to make Kerberos faster. > I've done some recent work on improving the overhead > of using message digest algorithms with GSS-API, but > haven't done any specific measurement. I'm sure > there's more room for optimization. > > Even better would be to use a transport layer security > service. Amazon has EFS and Oracle Cloud has something > similar, but we're working on a standard approach that > uses TLSv1.3. > > > -- > Chuck Lever > > >
