What is your target?
One thing I'm wondering about the multi-path ... how does it understand that different I_T nexuses are on different network interfaces at both ends?
Eddy
----- Original Message ----- From: "Dmitry Yusupov" <dmitry_yus@xxxxxxxxx>
To: <open-iscsi@xxxxxxxxxxxxxxxx>
Cc: "James Bottomley" <James.Bottomley@xxxxxxxxxxxx>; "Mike Christie" <michaelc@xxxxxxxxxxx>; "'SCSI Mailing List'" <linux-scsi@xxxxxxxxxxxxxxx>
Sent: Wednesday, May 25, 2005 1:22 AM
Subject: Re: [PATCH RFC 2/2] implement transport scan callout for iscsi
On Tue, 2005-05-24 at 21:28 -0400, open_iscsi wrote:But it is not multi-pathing. Multi-pathing belongs at a higher layer.
Yes, you could make multi-pathing perform a similar action but being at a higher layer, it means more operations to achieve the same thing. Also, multi-pathing is better suited for failover than multi-connections.
There is another point here ... an HBA will probably use multi-connections
irrespective of what higher layers want.
Regarding the numbers, we get 400,000 IOPS with our hardware solution using
multiple connections and multiple micro-engines.
This number is impressive. I can not believe it is on Initiator side since quite a bit of code are involved besides TCP/IP: userspace app, VFS, SCSI-ML and LLDD (even though iSCSI HBA can do zero-copy on receive).
With open-iscsi/linux-iscsi-5.x on very fast hardware the best we could get is 75,000 IOPS. And we believe it is a world record among other iSCSI software initiators.
I also did comparison between multipath-like and MC/S-like setups and found that multipath-like setup scales much better, especially for WRITE's we found that scale factor is ~1.75. I.e. with single session we've got ~500MB/sec throughput and with two sessions we've got ~800MB/sec.
I have not tried
multi-pathing but I can tell you that I had to count clocks to get that
number and found that even a few extra clocks could mean a lot. So since
multi-pathing takes a lot of extra clocks, then I think there is a benefit.
However with a software solution the extra clocks for the multi-pathing may
not be significant.
I would think that you would want to let the lower layers do their best to
get the best thruput and leave the failover logic to the upper layers.
Eddy
----- Original Message ----- From: "James Bottomley" <James.Bottomley@xxxxxxxxxxxx>
To: "open_iscsi" <ESQuicksall_open_iscsi@xxxxxxxxxxx>
Cc: <open-iscsi@xxxxxxxxxxxxxxxx>; "Mike Christie" <michaelc@xxxxxxxxxxx>;
"'SCSI Mailing List'" <linux-scsi@xxxxxxxxxxxxxxx>
Sent: Tuesday, May 24, 2005 9:00 PM
Subject: Re: [PATCH RFC 2/2] implement transport scan callout for iscsi
> On Tue, 2005-05-24 at 20:25 -0400, open_iscsi wrote:
>> The MC/S feature of iSCSI is not multi-pathing. Multi-pathing would be
>> the
>> use of multiple sessions to reach the same target. Generally the two
>> sessions would use the same InitiatorName+ISID but use different >> Target
>> Portal Groups at the target. In SCSI terms, it is the same initiator
>> accessing different SCSI ports.
>
> Well, yes, every driver vendor with a multi-path solution in-driver > that
> made a single presentation to the mid-layer has argued that one...
>
> The bottom line is that implementation must be in-driver. So every
> driver doing it this way has to have their own separate multi-path
> implementation. Whether you call it FC/AL or MC/S (or any of the other
> buzz acronyms) it's still a driver implementation of pathing.
>
>> MC/S can be used to improve band width of a session without using
>> multi-pathing and it belongs in the driver because it is hidden from >> the
>> upper layers. Think of it like parallel wires, each carrying separate
>> (but
>> sequenced) commands in parallel.
>
> So far, no-one has been able to produce any figures to show that MC/S > is
> significantly better than symmetric active dm-multipath to an iSCSI
> target, but if you have them, please publish them.
>
> Hiding something from the upper layers which the upper layers could do
> equally well themselves is what's considered wrong: it adds code bloat
> with no tangible benefit.
>
> James
>
>
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