[DRBD-user] Effectiveness of barriers on disks with volatile write cache

Lars Ellenberg lars.ellenberg at linbit.com
Fri Sep 24 20:53:39 CEST 2010

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

On Fri, Sep 24, 2010 at 11:12:35AM +0200, Nicolae Mihalache wrote:
> Hello,
> I've been reading about the barriers (no-disk-barrier option) in drbd.
> I understand that when the primary gets a IO completion notification,
> it will issue a barrier request (actually start a new epoch) to the
> secondary.
> However, if the disk of the primary has a write cache, it will
> immediately issue an IO completion notification, without actually
> writing the data to the disk. So what happens is that the secondary
> will use lots of barriers to guarantee the write order of the primary
> while in fact the primary itself has no guarantee about the order.
> My conclusion is in contradiction with what is written in the user
> guide http://www.drbd.org/users-guide/re-drbdconf.html:
> "When selecting the method you should not only base your decision on
> the measurable performance. In case your backing storage device has a
> volatile write cache (plain disks, RAID of plain disks) you should use
> one of the first two (i.e. barrier or flush)."
> Can someone point the fault in my reasoning?

Look for this message, or read the whole thread.
 Date: Thu, 5 Aug 2010 14:40:56 +0200                                                                                                
 From: Lars Ellenberg <lars.ellenberg at linbit.com>                                                                                    
 Subject: Re: [DRBD-user] barrier mode on LVM containers                                                                             

DRBD's usage of barriers/cache flushes is to make sure
we won't "forget" to resync parts that need to be synced
after a node crash.

: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

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