Re: GlusterFS behaviour on stat syscall with relatime activated

Pranith Kumar Karampuri <pkarampu@xxxxxxxxxx> · Mon, 08 Feb 2016 10:55:40 +0530



    On 02/06/2016 12:19 AM, Simon
      Turcotte-Langevin wrote:

    
        Good
            day to you Pranith,
         
        Thank
            you for your answer, it was exactly this. However, we still
            have an issue with RELATIME on GlusterFS.
         
        Stating
            the file does not modify atime anymore, with quick-read
            disabled, however cat-ing the file does not replicate the
            atime.
      
    
    This is because of open-behind feature. Disable open-behind with:
    "gluster volume set <volname> open-behind off". I believe you
    will see the atime behavior you want to see with it. This will
    reduce the performance of small file reads (< 64KB). Instead of
    one lookup over the network now it will do, lookup + open(This will
    be sent to both the replica bricks which updates atime) + read (Only
    one of the bricks). Let me know if you want any more information.

    
    Pranith

    
        If
            I touch manually the file, the atime (or utimes) is
            replicated correctly.
         
        So
            to sum it up:
         
        ·        
              [node1] Touch –a file1
            
        
          o  
              à Access time is right on
            [node1] [node2] and [node3]
        ·        
              [node1] Cat file1
            
        
          o  
              à Access time is right on
            [node1]
        
          o  
              à Access time is wrong on
            [node2] and [node3]
         
        Would
            you have any idea what is going on behind the curtain, and
            if there is any way to fix that behavior?
         
        Thank
            you,
        Simon
         
        
            From: Pranith Kumar Karampuri
                [mailto:pkarampu@xxxxxxxxxx]
                

                Sent: 5 février 2016 00:55

                To: Simon Turcotte-Langevin
                <simon.turcotte-langevin@xxxxxxxxxxx>;
                gluster-users@xxxxxxxxxxx

                Cc: UPS_Development
                <UPS_Development@xxxxxxxxxxx>

                Subject: Re:  GlusterFS behaviour
                on stat syscall with relatime activated
          
        
          On 02/03/2016 10:12 PM, Simon
            Turcotte-Langevin wrote:
        
        
          Hi, we have multiple
              clusters of GlusterFS which are mostly alike. The typical
              setup is as such:
           
          -         
              Cluster of
              3 nodes
          -         
              Replication
              factor of 3
          -         
              Each node
              has 1 brick, mounted on XFS with RELATIME and NODIRATIME
          -         
              Each node
              has 8 disks in RAID 0 hardware
           
          The main problem we
              are facing is that observation of the access time of a
              file on the volume will update the access time.
           
          The steps to reproduce
              the problem are:
           
          -         
              Create a
              file (echo ‘some data’ > /mnt/gv0/file)
          -         
              Touch its
              mtime and atime to some past date (touch –d 19700101
              /mnt/gv0/file)
          -         
              Touch its
              mtime to the current timestamp (touch –m /mnt/gv0/file)
          -         
              Stat the
              file until atime is updated (stat /mnt/gv0/file)
          
            o  
                Sometimes
              it’s instant, sometimes it requires to execute the above
              command a couple of time
        
        atime changes
            on open call.

            
            Quick-read xlator opens the file and reads the content on
            'lookup' which gets triggered in stat. It does that to serve
            reads from memory to reduce number of network round trips
            for small files. Could you disable that xlator and try the
            experiment? On my machine the time didn't change after I
            disabled that feature using:

            
            "gluster volume set <volname> quick-read off"

            
            Pranith

            
          On the IRC channel, I spoke to a developer
              (nickname ndevos) who said that it might be a getxattr()
              syscall that could be called when stat() is called on a
              replicated volume.
           
          Anybody can reproduce this issue? Is it a
              bug, or is it working as intended? Is there any
              workaround?
           
          Thank you,
          Simon
          

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