[DRBD-user] "no-disk-barrier" or "no-disk-flush"?

Joshua West jwest at brandeis.edu
Thu Jul 23 21:14:56 CEST 2009

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

Hey all,

I've got several DRBD resources, residing on top of LVM LV's:

[root at xen-ha-f1:~]# uname -a
Linux xen-ha-f1.unet.brandeis.edu #1 SMP Tue Jul
21 16:38:41 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux
[root at xen-ha-f1:~]# cat /etc/redhat-release
Red Hat Enterprise Linux Server release 5.3 (Tikanga)
[root at xen-ha-f1:~]# rpm -q --qf='%{Version}' xen
[root at xen-ha-g1:~]# vgs
  VG   #PV #LV #SN Attr   VSize   VFree 
  vg0    1   3   0 wz--n- 390.12G 283.12G
[root at xen-ha-g1:~]# lvs
  LV                    VG   Attr   LSize   Origin Snap%  Move Log
Copy%  Convert
  drbd-bigtest          vg0  -wi-ao
  drbd-vm_ha-test1      vg0  -wi-ao  
  drbd-vm_ha-test1_swap vg0  -wi-ao  
[root at xen-ha-g1:~]# drbd-overview
  0:vm_ha-test1       Connected Primary/Primary   UpToDate/UpToDate C r----
  1:vm_ha-test1_swap  Connected Primary/Primary   UpToDate/UpToDate C r----
  2:bigtest           Connected Secondary/Primary UpToDate/UpToDate C r----

There is only one physical volume (PV) for the above volume group (vg0):
/dev/md6... which is a RAID-1 set of two partitions on Seagate SAS
15krpm drives.  There is a SAS controller card; no hardware RAID or any
battery-backed cache.  This setup consists of two Dell PowerEdge 1950 III's.

Enough background information and onto the question...

The drbd.conf manpage states that DRBD will automatically choose the
first write-after-write method available supported by the backing
storage device, but it also states LVM does not support barriers. 
However... it looks like DRBD is indeed choosing the barriers method, as
I have "wo:b" in /proc/drbd for all three resources.

Should I be forcing DRBD to use the flush method instead?  By specifying
no-disk-barrier?  Or should I actually be using the barrier method,
instead of flush?  And what are the differences between the barriers and
flush methods?

If you need any further information just let me know.


Joshua West
Senior Systems Engineer
Brandeis University

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