[DRBD-user] DRBD write after write (cache)

ionral daniele.nuzzo at email.it
Sat Feb 5 11:48:15 CET 2011

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

I have a doubt not just ... I'll explain better 

I have this situation, implemented with CentOS 5.5 x86_64: 

two nodes, each node has a raid controller with BBU, two network interface
bonding round-robin with dedicated cables 
I set up the raid array like this: 

sda1 / boot 
sda2 / 
sda3 swap 
sda5 - just partition (disk drbd) 

I configure drbd with dual primary and with the usual protocol C 
use other parameters to improve performance: 

common { 
  syncer {rate 100M;} 


disk { 
    on-io-error detach; 


net { 
    timeout 60; # 6 seconds (unit = 0.1 seconds) 
    connect-int 10, # 10 seconds (unit = 1 second) 
    ping-int 10, # 10 seconds (unit = 1 second) 
    ping-timeout 5; # 500 ms (unit = 0.1 seconds) 
    max-buffers 2048; 
    unplug-watermark 128; 
    max-epoch-size 2048; 
    ko-count 5; 
    after-sb-0pri disconnect; 
    after-sb-1pri disconnect; 
    after-sb-2pri disconnect; 
    rr-conflict disconnect; 


syncer { 
    rate 100M; 
    al-extents 257; 


on node1 { 
    device /dev/drbd0; 
    disk /dev/disk/by-id/scsi-3600605b0014416401257a2af36d89bfc-part5; 
    meta-disk internal; 

  on node2 { 
    device /dev/drbd0; 
    disk /dev/disk/by-id/scsi-3600605b0014403f0125c874d21c5521d-part5; 
    meta-disk internal; 

if I check the status of drbd the following response is: 

 0: cs: Connected ro: Primary/Primary ds: UpToDate/UpToDate C r ---- 
    ns: 640864680 nr: 135234784 dw: 776099464 dr: 1520599328 al: 836478 bm:
2185 lo: 0 pe: 0 ua: 0 ap: 0 ep: 1 wo: b OOS: 0 

I do not know if it is positive that the to write after write method is
Barrier .... reading the manuals of drbd I noticed that in these cases (BBU)
performance would be better for a wo:n 

What do you think? 

more to this I can tell you that another hardware configuration with a
simple SATA disks in software raid  0 configuration (sda and sdb) and
without drbd.conf options: no-disk-flushes -no-md-flushes, The drbd uses
instead the drain ... that seems to be dangerous without a BBU, but the drbd
has preferred this choice, and not clear to me ...

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