[CentOS] NFS help

Larry Martell
Sun, 23 Oct 2016 13:02:37 -0000

Hi Matt-

Thank you for this very detailed and thoughtful reply.

On Fri, Oct 21, 2016 at 4:43 PM, Matt Garman <matthew.garman at gmail.com>
wrote:
> On Fri, Oct 21, 2016 at 4:14 AM, Larry Martell <larry.martell at
gmail.com> wrote:
>> We have 1 system ruining Centos7 that is the NFS server. There are 50
>> external machines that FTP files to this server fairly continuously.
>>
>> We have another system running Centos6 that mounts the partition the
files
>> are FTP-ed to using NFS.
>>
>> There is a python script running on the NFS client machine that is
reading
>> these files and moving them to a new dir on the same file system (a mv
not
>> a cp).
>
> To be clear: the python script is moving files on the same NFS file
> system?  E.g., something like
>
>     mv /mnt/nfs-server/dir1/file /mnt/nfs-server/dir2/file
>
> where /mnt/nfs-server is the mount point of the NFS server on the
> client machine?

Correct.

> Or are you moving files from the CentOS 7 NFS server to the CentOS 6 NFS
client?

No the files are FTP-ed to the CentOS 7 NFS server and then processed
and moved on the CentOS 6 NFS client.

> If the former, i.e., you are moving files to and from the same system,
> is it possible to completely eliminate the C6 client system, and just
> set up a local script on the C7 server that does the file moves?  That
> would cut out a lot of complexity, and also improve performance
> dramatically.

The problem doing that is the files are processed and loaded to MySQL
and then moved by a script that uses the Django ORM, and neither
django, nor any of the other python packages needed are installed on
the server. And since the server does not have an external internet
connection (as I mentioned in my reply to Mark) getting it set up
would require a large amount of effort.

Also, we have this exact same setup on over 10 other systems, and it
is only this one that is having a problem. The one difference with
this one is that the sever is CentOS7 - on all the other systems both
the NFS server and client are CentOS6.

> Also, what is the size range of these files?  Are they fairly small
> (e.g. 10s of MB or less), medium-ish (100s of MB) or large (>1GB)?

Small - They range in size from about 100K to 6M.

>> Almost daily this script hangs while reading a file - sometimes it
never
>> comes back and cannot be killed, even with -9. Other times it hangs for
1/2
>> hour then proceeds on.
>
> Timeouts relating to NFS are the worst.
>
>
>> Coinciding with the hanging I see this message on the NFS server host:
>>
>> nfsd: peername failed (error 107)
>>
>> And on the NFS client host I see this:
>>
>> nfs: V4 server returned a bad sequence-id
>> nfs state manager - check lease failed on NFSv4 server with error 5
>
> I've been wrangling with NFS for years, but unfortunately those
> particular messages don't ring a bell.
>
> The first thing that came to my mind is: how does the Python script
> running on the C6 client know that the FTP upload to the C7 server is
> complete?  In other words, if someone is uploading "fileA", and
the
> Python script starts to move "fileA" before the upload is
complete,
> then at best you're setting yourself up for all kinds of confusion,
> and at worst file truncation and/or corruption.

The python script checks the modification time of the file, and only
if it has not been modified in more then 2 minutes does it process it.
Otherwise it skips it and waits for the next run to potentially
process it. Also, the script can tell if the file is incomplete in a
few different ways. So if it has not been modified in more then 2
minutes, the script starts to process it, but if it finds that it's
incomplete it aborts the processing and leaves it for next time.

> Making a pure guess about those particular errors: is there any chance
> there is a network issue between the C7 server and the C6 client?
> What is the connection between those two servers?  Are they physically
> adjacent to each other and on the same subnet?  Or are they on
> opposite ends of the globe connected through the Internet?

Actually both the client and server are virtual machines running on
one physical machine. The physical machine is running CentOS7. There
is nothing else running on the physical machine other then the 2 VMs.

> Clearly two machines on the same subnet, separated only by one switch
> is the simplest case (i.e. the kind of simple LAN one might have in
> his home).  But once you start crossing subnets, then routing configs
> come into play.  And maybe you're using hostnames rather than IP
> addresses directly, so then name resolution comes into play (DNS or
> /etc/hosts).  And each switch hop you add requires that not only your
> server network config needs to be correct, but also your switch config
> needs to be correct as well.  And if you're going over the Internet,
> well... I'd probably try really hard to not use NFS in that case!  :)
>
> Do you know if your NFS mount is using TCP or UDP?  On the client you
> can do something like this:
>
>     grep nfs /proc/mounts | less -S
>
> And then look at what the "proto=XXX" says.  I expect it will be
> either "tcp" or "udp".  If it's UDP, modify your
/etc/fstab so that
> the options for that mountpoint include "proto=tcp".  I *think*
the
> default is now TCP, so this may be a non-starter.  But the point is,
> based purely on the conjecture that you might have an unreliable
> network, TCP would be a better fit.

I assume TCP, but I will check tomorrow when I am on site.

> I hate to simply say "RTFM", but NFS is complex, and I still go
back
> and re-read the NFS man page ("man nfs").  This document is long
and
> very dense, but it's worth at least being familiar with its content.

Yes, I agree. I skimmed it last week, but I will look at it in detail tomorrow.

>> The first client message is always at the same time as the hanging
starts.
>> The second client message comes 20 minutes later.
>> The server message comes 4 minutes after that.
>> Then 3 minutes later the script un-hangs (if it's going to).
>
> In my experience, delays that happen on consistent time intervals that
> are on the order of minutes tend to smell of some kind of timeout
> scenario.  So the question is, what triggers the timeout state?
>
>> Can anyone shed any light on to what could be happening here and/or
what I
>> could do to alleviate these issues and stop the script from hanging?
>> Perhaps some NFS config settings? We do not have any, so we are using
the
>> defaults.
>
> My general rule of thumb is "defaults are generally good enough; make
> changes only if you understand their implications and you know you
> need them (or temporarily as a diagnostic tool)".

I would like to try increasing the timeout.

> But anyway, my hunch is that there might be a network issue.  So I'd
> actually start with basic network troubleshooting.  Do an
"ifconfig"
> on both machines: do you see any drops or interface errors?  Do
> "ethtool <interface>" on both machines to make sure both
are linked up
> at the correct speed and duplex.  Use a tool like netperf to check
> bandwidth between both hosts.  Look at the actual detailed stats, do
> you see huge outliers or timeouts?  Do the test with both TCP and UDP,
> performance should be similar with a (typically) slight gain with UDP.
> Do you see drops with UDP?
>
> What's the state of the hardware?  Are they ancient machines cobbled
> together from spare parts, or reasonable decent machines?  Do they
> have adequate cooling and power?  Is there any chance they are
> overheating (even briefly) or possibly being fed unclean power (e.g.
> small voltage aberrations)?

The hardware is new, and is in a rack in a server room with adequate
and monitored cooling and power. But I just found out from someone on
site that there is a disk failure, which happened back on Sept 3. The
system uses RAID, but I don't know what level. I was told it can
tolerate 3 disk failures and still keep working, but personally, I
think all bets are off until the disk has been replaced. That should
happen in the next day or 2, so we shall see.

> Oh, also, look at the load on the two machines... are these
> purpose-built servers, or are they used for other numerous tasks?
> Perhaps one or both is overloaded.  top is the tool we use
> instinctively, but also take a look at vmstat and iostat.  Oh, also
> check "free", make sure neither machine is swapping (thrashing).

I've been watching and monitoring the machines for 2 days and neither
one has had a large CPU load, not has been using much memory.

> If
> you're not already doing this, I would recommend setting up
"sar"
> (from the package "sysstat") and setting up more granular logging
than
> the default.  sar is kind of like a continuous
> iostat+free+top+vmstat+other system load tools rolled into one that
> continually writes this information to a database.  So for example,
> next time this thing happens, you can look at the sar logs to see if
> any particular metric went significantly out-of-whack.

That is a good idea, I will check those logs, and set up better logging.

>
> That's all I can think of for now.  Best of luck.  You have my
> sympathy... I've been administering Linux both as a hobby and
> professionally for longer than I care to admit, and NFS still scares
> me.  Just be thankful you're not using Kerberized NFS.  ;)

Thanks!
Larry