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List:       drbd-user
Subject:    Re: [DRBD-user] performance issue when writing to drbd devices
From:       Arnold Krille <arnold () arnoldarts ! de>
Date:       2013-09-27 22:25:47
Message-ID: 20130928002547.704d79a6 () xingu ! arnoldarts ! de
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Hi,

On Thu, 26 Sep 2013 17:23:31 +0200 Matthias Teege
<matthias-drbdu@mteege.de> wrote:
> TL;DR:
> - local logical volume -> local logical volume: FAST writing speeds
> - local LVM2 logical volume -> local drbd device: SLOW writing speeds
> Any ideas what could eat up our performance while writing to a local
> drbd device?

When you use internal meta-data, the meta-data is at the end of your
partition. That means that each write on your drbd device becomes
write-long_seek-write-long_seek_back on the backing device. When your
drbd volume is only 10GB, that effect doesn't hit you as hard as on a
1TB volume. But a seek is a seek and the penalty hits you on all disks
with rotating blatters. (You will not be hit by this effect on an
all-ssd setup;-)

My advice:
 - Go for external meta-data on another physical disk, preferable on a
   ssd.
 - When there is no other disk available, scratch the 1 of your 10-raid
   underneath as you currently have a 101 (or 011 or 110 depending on
   the exact setup).
 - Then if you are using sync-protocol C, your write is still limited
   by the network-bandwidth and -roundtrip-latency. If that is not
   enough, you might consider using protocol A. The docs and the
   experts tell you that you should only use that with
   battery-backed-controllers. But when your two machines have
   independant UPS and are either directly connected on the network or
   with a switch also on UPS, the chance of your second node not
   'getting' the write when the first node fails is very small...

Have fun,

Arnold

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