[DRBD-user] cronjob for verify

Veit Wahlich cru.lists at zodia.de
Mon Oct 9 21:47:03 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 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.

// Veit

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