[DRBD-user] cronjob for verify

Jan Bakuwel jan.bakuwel at gmail.com
Tue Oct 10 02:11:26 CEST 2017

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

Hi Veit,

On 10/10/17 08:47, Veit Wahlich wrote:
> Hi Jan,
> Am Dienstag, den 10.10.2017, 06:56 +1300 schrieb Jan Bakuwel:
>> I've seen OOS blocks in the past where storage stack appeared to be fine (hardware wise). What possible causes could there be? Hardware issues, bugs in storage stack including DRBD itself, network issues. In most (all?) cases it seems prudent to me to keep the resources in sync as much as possible and of course investigate once alerted by the monitoring system.
> a common configuration issue is using O_DIRECT for opening files or
> block devices. O_DIRECT is used by userspace processes to bypass parts
> of the kernel's I/O stack with the goal to reduce CPU cycles required
> for I/O operations and to eliminate/minimize caching effects.
> Unfortunately this also allows the content of buffers to be changed
> while they are still "in-flight", speaking simplified, e.g. while being
> read/mirrored by DRBD, software RAID, ...
> The general use for O_DIRECT is for applications that either want to
> bypass caching, such as benchmarks, or that implement caching by
> themselves, which is the case for e.g. some DBMS. But also qemu (as used
> by KVM and Xen) implements several kinds of caching and uses O_DIRECT
> for VM disks depending on the configured caching mode.

Thanks for that. Must say that possibility has escaped my attention so 
far. I'm using DRBD in combination with Xen and LVM for VMs so I assume 
O_DIRECT is in play here. Any suggestions where to go from here? A 
search for DRBD, LVM, Xen and O_DIRECT doesn't seem to bring up any 
results discussing this issue.

kind regards,

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