zfs performance issues with iscsi (istgt)

Svein Skogen (Listmail account) svein-listmail at stillbilde.net
Mon Nov 8 08:37:57 UTC 2010

On 08.11.2010 09:13, DJ wrote:
> After scratching my head for a few weeks, I've decided to ask for some help.
> First, I've got two machines connected by gigabit ethernet, network performance is not a problem as I am able to substantially saturate the wire when not using iscsi [say iperf] or ftp. Both systems are 8.1-RELENG. They are both multi-core, 8G of RAM. 
> Symptoms: When doing writes (size relatively independent) from a client to a server via iSCSI I seem to be
>  hitting a wall between 18-26MB/s of write. This can be repeated continuously whether doing a newfs on a 2TB iscsi volume or doing a dd from /dev/zero to the iscsi target. I haven't compared read performance. What originally put me on to this was watching the newfs *fly* across the screen, and then hang for several seconds, and then *fly* again, and
>  then pause. 

I'll snip down this mail a little bit to ask some control questions
(having recently had quite a wrestle with iSCSI myself).

-Is jumbo frames involved? (and enabled on the initiator, all switches
in between, and the target)
-What's the number of PPS (some switches have PPS issues, which becomes
painfully relevant for small block IO)?  (I got rid of most of my
problems when I replaced the Netgear GS724Tv3 switch with a Cisco SG-300)
-Are you running digests?
-Do you have TSO/TSOv2 enabled at the endpoints?
-Does top -HSC reveal anything?
-Does systat -vmstat 1 reveal anything?
-What's the ICMP (ping) roundtrip times between the initiator and target


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