[DRBD-user] Dual primary and LVM

Igor Cicimov igorc at encompasscorporation.com
Thu Jul 27 13:09:02 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.

On Thu, Jul 27, 2017 at 9:04 PM, Igor Cicimov <
igorc at encompasscorporation.com> wrote:

> Hey Gionatan,
> On Thu, Jul 27, 2017 at 7:04 PM, Gionatan Danti <g.danti at assyoma.it>
> wrote:
>> Il 27-07-2017 10:23 Igor Cicimov ha scritto:
>>> When in cluster mode LVM will not use local cache that's part of the
>>> configuration you need to do during setup.
>> Hi Igor, I am not referring to LVM's metadata cache. I speak about the
>> kernel I/O buffers (ie: the one you can see from "free -m" under the buffer
>> column) which, in some case, work similarly to a "real" pagecache.
>> ​Well don't see how is this directly related to dual-primary setup since
> even with single primary what ever is not yet committed to disk is not
> replicated to the secondary as well. So in case you loose the primary what
> ever was in its buffers at the time is gone as well.
> ​But the rule-of-thumb lets say would be to have as less cache layers as
> possible without impact on the performance and retain the data consistency
> in the same time. With VMs you have additional cache layer in the guest as
> well as the one in the host. There are many documents discussing cache
> modes like these https://www.suse.com/documentation/sles11/book_kvm/
> data/sect1_1_chapter_book_kvm.html, https://www.ibm.com/support/
> knowledgecenter/en/linuxonibm/liaat/liaatbpkvmguestcache.htm,
> https://pve.proxmox.com/wiki/Performance_Tweaks for example.
> So which write cache mode you will use really depends on the specific
> hardware you use, the system amount of RAM, the OS sysctl settings (ie how
> often you flush to dis, params like vm.dirty_ratio,
> vm.dirty_background_ratio etc.), the disk types/speed, the HW RAID
> controller (for example with battery backed cache or not) ie DRBD has some
> tuning parameters like:
>     disk-flushes no;
>     md-flushes no;
>     disk-barrier no;
> which makes it possible to use the write-back caching on the *BBU-backed*
> RAID controller instead of flushing directly to disk. So many factors are
> in play but the main idea is to reduce the number of caches (or their
> caching time) between the data and the disk as much as possible without
> loosing data or performance.

​And in case of live migration I'm sure the tool you decide to use will
freeze the guest and make sync() call to flush the os cache *before*
stopping and starting the guest on the other node.​
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