[pve-devel] Default cache mode for VM hard drives

[Cesar Peschiera]

Hi to all

Only as comment:

1) While that the VM have write cache configured as none (that means write cache in VM guest), don't matter if the storage is DRBD, local, CEPH or Gluster, always the VM will have the cache of write in RAM, and obviously these data will not be in the disk or in the RAID controller of the storage selected.

2) The formula unique that i know for get that the data always are in the RAID controller or in the dishes of disks is with the combination of:
- OS of the guest with write cache disabled or with disk_barrires enabled.
- QEMU configured for the VM with directsync or writethrough.

3) About of swap disk in Linux, i guess that with directsync or writethrough will be sufficient, but i am not sure.

@Dietmar:
I believe that Mr. Stanislav say us that as a cache of write is in the RAM, and these data don't go to the hard disks or to the RAID controller, then these data aren't replicated with DRBD, and as a negative effect, if the host crashes or breaks down, there will be an irretrievable loss of data in the DRBD storage.

Best regards
Cesar Peschiera
  ----- Original Message ----- 
  From: Stanislav German-Evtushenko 
  To: Alexandre DERUMIER 
  Cc: pve-devel at pve.proxmox.com 
  Sent: Tuesday, November 18, 2014 2:25 PM
  Subject: Re: [pve-devel] Default cache mode for VM hard drives




  On Tue, Nov 18, 2014 at 8:17 PM, Alexandre DERUMIER <aderumier at odiso.com> wrote:

    >>What is your vm guest os ? kernel version ?
    >>
    >>Guest OS can be Windows (any) and Linux (any version).

    I ask the question because old windows virtio driver don't send flush correctly,

    and some linux kernel too (it's really safe since 2.6.32 - 2.6.33 if I remember, depend of filesystem, virtio driver -lvm-mdadm-dm version ..)
    http://monolight.cc/2011/06/barriers-caches-filesystems/

    >>2. It is often happen to Linux VM swap space.
    mmm,good point, I don't known how flush is working with swap 


  graylog2 was installed on Ubuntu 14.04 (virtio as I remember, not checked with ide)


  However:

  1) moving to IDE didn't help for Windows

  2) moving to IDE can't help for swap

  3) I don't remember if I tried ext4 in combination with ide because ext4 didn't produce any issues before I installed graylog2 recently. However there were many issues with ext3 (no barriers by default)


  Stanislav



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