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Date:      Thu, 21 Apr 2011 15:51:42 +0200
From:      Lars Wilke <lw@lwilke.de>
To:        freebsd-questions@freebsd.org
Subject:   Re: ZFS performance strangeness
Message-ID:  <dcp588-gf8.ln1@it-betrieb.de>
References:  <espd78-2at.ln1@it-betrieb.de> <4DA710AA.2030501@my.gd>

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Hi,

thanks to all who answered.

* Damien Fleuriot wrote:
>  I refer you to this post by Jeremy Chadwick with tuning values *AND*
>  their actual explanation.
>
>  http://lists.freebsd.org/pipermail/freebsd-stable/2011-February/061642.html

Good post indeed, but i think i found my problem elsewhere, though i have to admit
that i don't really understand what is happening.

The Citrix XenServers 5.6 boxes are connected with 2x1GB, using bonding with
balance-slb, to an HP ProCurve 2910al-24G Switch.

It seems this balance-slb mode is patch from Citrix and i could not come
up with some documentation for it. Only thing i know it is a modified
form of balance-alb.

The FreeBSD box is also connected to this switch with two NICs,
one em and one igb driven NIC. If i do not use lagg and assign just
a single address to one of the NICs on FBSD, everything works well.

I do only set vm.kmem_size_scale and vfs.zfs.arc_max and get r/w speed
of a little bit above 100MB/s when doing: dd bs=1M count=4096 ...
and remounting before reading or writting.

But if i do use lagg in loadbalance or in lacp mode (here also
enabled lacp on the necessary switch ports) i see via a tcpdump, that
ip packets seem to get lost!? On top of that this only happens when
reading data via NFS, writting is fine.

Then again lagg in failover mode seems to work ok.

The NICs themself look fine and are automatically set to 1000baseT full
duplex. There are also no errors reported via netstat.

I am not sure if the switch does not like playing with the arp
addresses on its ports or if it has something to do with the slb
implementation from citrix. When i have a downtime for the xenserver
i will reconfigure it to use lacp, then lets see if that works.

cheers
   --lars




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