Bug 504059 - RHEL NFS client read performance
RHEL NFS client read performance
Product: Red Hat Enterprise Linux 4
Classification: Red Hat
Component: kernel (Show other bugs)
All Linux
high Severity high
: rc
: 4.8
Assigned To: Peter Staubach
Red Hat Kernel QE team
Depends On:
  Show dependency treegraph
Reported: 2009-06-03 21:24 EDT by Lachlan McIlroy
Modified: 2015-04-12 19:14 EDT (History)
9 users (show)

See Also:
Fixed In Version:
Doc Type: Bug Fix
Doc Text:
Story Points: ---
Clone Of:
Last Closed: 2009-08-13 11:36:50 EDT
Type: ---
Regression: ---
Mount Type: ---
Documentation: ---
Verified Versions:
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oVirt Team: ---
RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: ---

Attachments (Terms of Use)
log showing NFS operations every 10 seconds for a production server (16.40 KB, text/plain)
2009-06-04 16:16 EDT, Chris Evich
no flags Details

  None (edit)
Comment 1 Peter Staubach 2009-06-04 08:34:53 EDT
I think some more information will be required.

First, have they actually measured any sort of performance degradation
or are they just basing their opinions on "nfsstat -c" counts?

Is this a multi-user environment?  This would mean multiple users with
different uids.

These read-only files, do they change much?  How large are they?

What mount options are used on the NFS clients?

Let's start with this information.
Comment 3 Chris Evich 2009-06-04 08:57:50 EDT
Re: "are they just basing their opinions on "nfsstat -c" counts?"

Yes, and therefor conclusion ACCESS calls are responsible isn't 100% correct due to relative time/resource usage for the various calls.  I understand ACCESS is very fast and light-weight (please correct if not true).

Re: "Is this a multi-user environment?"

Unknown, though by the sound of it, it's single or very few unique UIDs.

Re: "do they change much?"

Unknown.  Customer states that they use stat() (or something similar) to monitor the files for changes.

Re: "How large are they?"

Customer states several terabytes between all files.  Exact number unknown.

Re: "What mount options are used on the NFS clients?"



Additional data requested from customer:
1) A 'sosreport -k rpm.rpmva=off' from the test system just prior to testing
2) A 128-byte binary network packet capture, of traffic between the test system and the filer.
3) A matching run of your workload under strace -ff -q -r -T -tt -v -o /tmp/filename [-p pid or command]
4) A run-time  providing representative data (e.g. 5-15 minutes).
5) A final 'sosreport -k rpm.rpmva=off' from the test system just after testing.
Comment 4 Chris Evich 2009-06-04 14:56:26 EDT
Customer's responses to your questions:

All data updates are written by a single user on the master filer.  The updates are then sent out to the child filers via netapp snapmirror.  No data is ever updated on the child filers except through snapmirror.  All mounts on the child filers are read only mounts, and are read by a single userid.  On the child filers it is 100% impossible to change the data.  The snapmirrors are never broken.  Unbroken snapmirrors are read-only replicas of the WAFL filesystem.

We receive data updates ~10 times a day, the data is updated via the snapmirror from the master filer.  Data files are not changed, but new data files are added to the data set.  Then we change the high water mark, and the system now "notices" the new updated files.  We work with the newest files to oldest.

All nodes in the compute cluster use the same files, I don't have a good count right now but it is in excess of 500k files.  Our total data set size is about 600GB. On the NFS server we get about 95% cache hit ratio out of 32gb read cache.
Comment 5 Issue Tracker 2009-06-04 14:58:16 EDT
Other data is being uploaded to dropbox.  I'll attach it privately once
it's here.

Internal Status set to 'Waiting on SEG'

This event sent from IssueTracker by cevich 
 issue 299688
Comment 6 Peter Staubach 2009-06-04 15:06:08 EDT
Thanx for the information.

Some more questions generated by the response in Comment #4 --

500,000+ files.  In how many directories?

On the NFS clients, how often is each file accessed?  Either via stat,
open, read, or whatever?

What is the hardware configuration of the NFS clients?
Comment 7 Chris Evich 2009-06-04 16:14:42 EDT
(In reply to comment #6)

Customer can't get exact breakdown numbers, but it sounds like the files are fairly spread out:

"there are a lot of directories for data organization.  When we can get some perf time I'll try to get a find fired off to make you a data file index."

I've asked customer for a rough estimation, i.e. tens, hundreds, thousands, or more files per directory.

Re: "On the NFS clients, how often is each file accessed?"

"How often the file is accessed depends on a number of factors.  Data that is most commonly used, is always kept in memory and accessed via the mmap().  Other data is read into mmaps and accessed from there.  Eventually we reach a triggered condition where we munmap() the files and start over.  We call this fill&flush, fill&flush is not used for all data.  Of the data eligible for fill/flush how often it is accessed will depend on how often we are using that data."

Re: "What is the hardware configuration of the NFS clients?"

"We are platformed on IBM X3650 servers with 16GB RAM and 8 cores total (2 sockets).  Network fabric is Cisco 6500 with fabic enabled blades and sup720's.  Each server has two gigE interfaces configured with the bonding driver in an active/passive configuration. 

Each child filer is a IBM re-badged NetApp 6080c with 32gb of read cache and 28 14k FCAL drives.  95% of disk operations complete from read cache.  Each filer head is connected to the network fabric with a 6 gigE member port channel."
Comment 8 Chris Evich 2009-06-04 16:16:05 EDT
Created attachment 346583 [details]
log showing NFS operations every 10 seconds for a production server

to illustrating the I/O rates
Comment 17 Bikash 2009-06-09 14:46:29 EDT
Whay are athey using 8k as the rsize and wsize? What is the Data Ontap version?
Comment 18 Issue Tracker 2009-06-09 15:32:45 EDT
Event posted on 06-09-2009 03:32pm EDT by cevich


Most IO's are less than 4k, 8k figure comes from Netapp best practices
guide.  Ontap version is but planning on migrating to 7.3.1.

This event sent from IssueTracker by cevich 
 issue 299688
Comment 54 Peter Staubach 2009-06-18 15:17:18 EDT
You can't just concatenate those capture files together like that.  They
have a header at the beginning which identifies them.

It might easier to just count directories and then assume that everything
else is a regular file.  Then, the "df -i" statistics could be useful.

I might suggest stopping the find and trying to get the information from
some other avenue.

The more interesting information is that the working set size of the files
being accessed or scanned.  Are all 89 million looked at?  Some subset?
At what rate, ie. how many files per second?

Perhaps the strace would be useful in this regard.

I will look at the capture files, but I am not terribly hopeful of
getting much useful out of them.  The best capture file for me would be
that 6G file as 1 continuous capture and from the NFS client's
perspective.  That way, I could look to see which files were being
looked at.  By starting and stopping the network capture, packets are
missed and the information starts to become less and less reliable.
It is not possible to determine the time duration that the captures
were done for, so I can't infer how many files per second were being

So, additionally, what is the hardware configuration for a typical NFS
client in this deployment?
Comment 62 Peter Staubach 2009-06-19 17:04:45 EDT
My current analysis --

In the Big capture file, I see somewhere around 7,395,500 NFS RPC calls.
Of these, somewhere around 3,175,300 are NFS ACCESS calls.  These ACCESS
calls were made for 750,950 unique file handles.  Of these file handles,
somewhere around 455,200 were used more than once.  The peak number of
ACCESS calls per file handle was 170 and it was a directory, probably a
(the?) mount point.

I also noticed a lot of FSSTAT calls.  Is df(1) being run frequently?

A question -- is the equivalent of "noatime" and "nodiratime" being
used on the NFS server file system?

A question -- the NFS client described by the sosreport in the IT
appears to have 16G memory and 8 3.16GHz cpus.  Have any tunings been

Some back of the envelope scratching -- 3271 seconds of capture time.
2/10,000 seconds per ACCESS call or about 650 seconds doing ACCESS
calls.  Divide that by the number of processors indicates that each
processor spent about 80 seconds out of 3271 seconds doing ACCESS
calls.  That's somewhere around 2.5% of the time.  Of course, all
of this is fuzzy math.

Looking at just absolute numbers, the number of ACCESS calls appears
to be very high.  It is almost 43%.  However, when some context is
considered, then this doesn't seem so high any more.  The number of
files is 750,000+, which reduces to an average of about 4 ACCESS calls
per file over the period of more than 50 minutes.  Given that each
ACCESS call takes on the order of 0.2 milliseconds, about 1
millisecond per file over the 50+ minutes was spent checking ACCESS.

I think that I would recommend looking elsewhere for increased
performance if a real need exists.

Perhaps with this sheer number of files, a different architecture,
utilizing multiple clients, each working on a subset of the files,
may help?
Comment 63 Bikash 2009-06-22 12:43:33 EDT
I am not sure any of our NetApp BPG suggests to use 8k in TCP environment. It used to be the case while using UDP. If it does then that needs to corrected. Now we strongly recommend to use 64k for rsize and wsize. We are in the process of updating the documents to suggest that way.
Comment 64 Peter Staubach 2009-06-22 13:13:31 EDT
64k is good, but definitely not for UDP.  For UDP, 32k would be a better
bigger value than 8k.
Comment 65 Bikash 2009-06-22 13:19:40 EDT
We highly recommend to use TCP in our NFS environments. UDP is no longer a best practice. While TCP is used we also recemmend to use 64k in the new RH kernels like RHEL5 and later.
Comment 66 Peter Staubach 2009-06-23 11:08:13 EDT
Any comments or feedback on Comment #62?
Comment 67 Chris Evich 2009-06-29 15:21:28 EDT
(I've made your analysis public since it may provide helpful info. to others facing a similar question)

Thanks for the detailed analysis.  I agree, we probably need to look elsewhere for bigger knobs to tweak.   We've provided all this (and your questions) back to the customer and are waiting on feedback.
Comment 70 Chris Evich 2009-08-13 11:36:50 EDT
Customer states client-side NFS acl processing is driving a significant load on their NetApp filers.  Customer is engaged with Red Hat Consulting to provide client-side "fix".  Not sure why server-side ACL processing cannot simply be disabled (i.e. in RHEL, simply specify no_acl as an export option).  Either way, there isn't a client-side bug here.

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