----- Original Message -----
> From: "Lars Ellenberg" <[email protected]>
> To: [email protected]
> Sent: Thursday, 31 May, 2012 12:44:16 PM
> Subject: Re: [DRBD-user] Xen DomU on DRBD device: barrier errors
>
> I see. Then that workaround does not work (anymore?).
> Still, the problem is not in DRBD, but in the xen block driver.
>
> Just add "-DRBD" to your googling,
> should make it easier to find and not be distracted...
For the record, it seems to be caused by Linux 3.1's new default ext3 behavior
to enable barriers ([1] and [2]):
================
Hard disks have a memory buffer were they temporally store the instructions and
data issued from the OS while the disk processes it. The internal software of
modern disks changes the order of the instructions to improve performance,
which means that instructions may or may not be committed to the disk in the
same order the OS issued them. This breaks many of the assumptions that file
systems need to reliably implement things like journaling or COW, so disks
provide a "cache flush" instruction that the OS uses when it needs it. In the
Linux world, when a file system issues that instruction, it is called a
"barrier". File systems such as XFS, Btrfs and ext4 already use and enable
barriers by default; ext3 supports them but until this release it did not
enable them by default: while the data safety guarantees are higher, their
performance impact in Ext3 is noticeable in many common workloads, and it
considered that it was an unacceptable performance regression to enable them by
default. However, Linux distributions like Red Hat have enabled barriers by
default in Ext3 for a long time, and now the default for mainline has been
changed as well.
In other words: if you use Ext3 and you note performance regressions with this
release, try disabling barriers ("barrier=0" mount option).
================
If barriers weren't enabled before, I guess it doesn't hurt to disable them now.
I'll see about (status of) bug reports for xen/Ubuntu.
[1] http://lists.xen.org/archives/html/xen-devel/2011-10/msg02028.html
[2]
http://kernelnewbies.org/Linux_3.1#head-7536ebf800edb3e888431812b283b63ed2b2b7ea
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