[CentOS] NFS help

[Larry Martell]

On Sun, Oct 23, 2016 at 9:02 AM, Larry Martell <larry.martell at gmail.com> wrote:
> 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.

I misspoke here - the CentOS7 NFS server is running on the physical
hardware, it's not a VM. The CentOS6 client is a VM.

>
>> 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