On 03/12/2013 11:30 PM, Mike Christie wrote:
On 03/12/2013 08:11 AM, Guillaume wrote:
Hello Mike
On 12 mar, 02:54, Mike Christie <micha...@cs.wisc.edu> wrote:
On 03/09/2013 05:58 AM, Guillaume wrote:
Hello,
I have a virtual tape library and a iscsi SAN. All have multiple
ethernet interfaces, This will ressult in multiples sessions to the
targets.So I wonder if I must use dm-multipath or not ? Does the current
Does the device show up as a tape device or a block device?
The VTL device emulates robotics, LTO cartridges and LTO5 tape drives.
The SAN is a block device.
Are you using SCST or TGT or LIO for this?
iscsi layer handle the multiple paths to an iqn or not ?
Another question about the output of "iscsiadm -m session" : the lines
of output begins by @IP:3260,n where n is an integer. Is this number a
priority level in some way, or does it only distinguish multiple
sessions to the same iqn ?
It is the iSCSI target portal group number. It is assigned by the
target. You can use it to group portals together. If the initiator and
target supports MC/s you could then do a iSCSI level multipathing called
MC/s over portals with the same target portal group number. open-iscsi
does not support MC/s
So what I understand is that I must use md-multipath to handle the
Use dm-multipath. md-multipath is older and does not have as many features.
Be _very_ cautious when using multipath on tapes.
Tapes rely on precise I/O sequencing, but multipath only guarantees
sequential I/O _submission_ at the initiator side.
If multipath decides to requeue I/O to a different path because one path
is blocked your tape is hosed.
I'm not saying it cannot be handled, I'm just saying we haven't tested
this properly. And certainly didn't implement any precautions to avoid this.
But this would be best discussed on the dm-devel list.
Cheers,
Hannes
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