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    Very slow Backup speed when using "Continuous Replication" to NFS target

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    • 0nelight0 Offline
      0nelight
      last edited by 0nelight

      Hi XCP Community,

      I use self-build Xen-Orchestra latest version; commit: 1961d; running as a VM
      xcp-ng 8.2.1 on an

      B450 AORUS ELITE Mainboard
      AMD Ryzen 5 3600XT 6-Core Processor

      XCP-NG ist installed on a single SSD with 256GB.

      Additonally there is an Adaptec-PCI-Raidcard connected with a Raid1 of 2x4TB HDDs.

      Local-Storage on the XCP-NG host ist EXT4! Dom0 Ram is 1,6 GB.

      I am connected to a RaspberryPi4 USB-HDD with NFS for Backups.

      For Test Purposes, every VM including Dom0 is on the single SSD only (you can Ignore the Raid1).

      Following things run as fast as expected:

      • Normal "Backup" of a random VM from Xen-Orchestra to the RaspberryPi4 - no matter if hosted on the SSD or on the Raid1.
      • Test-File-Transfer via Command Line into mounted NFS share on Dom0, Xen-Orchestra-VM or random VM.
      • iperf benchmark for network connection is fine between Dom0<->XO-VM; Dom0<->RapsberryPi4; XO-VM<->RaspberryPi4; RandomVM<->Dom0 and RandomVM<->XO-VM

      Networkspeed I get is 1 Gbit/s, HDD-Speed is far over 60MB/s.

      But I don't get this numbers when using "Continuous Replication".
      I only get 4-6 Mbyte/s speed when I want to Backup the "RandomVM" to the NFS-Share on the Raspberry. (No matter if the VM is "ON" or "OFF")

      I see via htop that there seems to be two processes involved "tapdisk" and "vhd-tool serve" when doing a "Continuous Replication". In the "Disk Write" Column of htop I only get approximatly 5.27 to 6.60 M/s for these two processes. Tapdisk is always a few Kbytes faster than the "vhd-tool serve".

      In the Logfiles I cannot find anything which would help me further.

      Any Ideas to find the bottleneck?

      Things tried:
      Used "http://" for connecting the server with XO. -> No change.

      Thanks!

      A 1 Reply Last reply Reply Quote 0
      • 0nelight0 Offline
        0nelight @Andrew
        last edited by

        @Andrew
        Yess!
        After specifying "async" on the NFS-Server I get 70 Mbytes/Second 🙂
        Is there any downside for using "async" in this context?
        Thank you!

        A 1 Reply Last reply Reply Quote 0
        • A Offline
          Andrew Top contributor @0nelight
          last edited by

          @0nelight NFS sync on the server?

          0nelight0 1 Reply Last reply Reply Quote 1
          • 0nelight0 Offline
            0nelight @Andrew
            last edited by

            @Andrew
            Yess!
            After specifying "async" on the NFS-Server I get 70 Mbytes/Second 🙂
            Is there any downside for using "async" in this context?
            Thank you!

            A 1 Reply Last reply Reply Quote 0
            • A Offline
              Andrew Top contributor @0nelight
              last edited by

              @0nelight The risk is data loss during an unplanned reboot/crash/power cycle, but for backup use that's not a big issue. 70MBytes/sec is a normal speed for backups.

              Using a SR on async NFS is a bigger risk as data that needs to be saved might not be saved if the system reboots. VMs are always writing data while they are running. It's a known performance vs. risk trade off.

              NFS sync is safe and slow (without hardware acceleration) and async is fast with some risk.

              0nelight0 1 Reply Last reply Reply Quote 0
              • 0nelight0 Offline
                0nelight @Andrew
                last edited by

                @Andrew Thanks! How to close this issue?

                1 Reply Last reply Reply Quote 0
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