PCI-E lanes and versions question

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phroenips

New Member
Jul 14, 2013
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So, I know the different versions of PCI-E (1.0, 1.0a, 2.0, 3.0) are all interoperable (in theory anyway), and that a lot of server/workstation motherboards have physical x8 slots, but only x4 lanes available to it. What happens if you combine those scenarios?

For instance, my motherboard has an x4 PCI-E 2.0 in a physical x8 slot. What would happen if a put an x8 PCI-E 1.0a HBA in there?

Would the end result be x4 of 1.0a bandwidth available to me? Or since it's 2.0, will I somehow have x4 of 2.0 bandwidth (which the card is spreading across 8 lanes of 1.0a)? The former makes the most logical sense, but I'd love to be wrong :)
 

PigLover

Moderator
Jan 26, 2011
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So, I know the different versions of PCI-E (1.0, 1.0a, 2.0, 3.0) are all interoperable (in theory anyway), and that a lot of server/workstation motherboards have physical x8 slots, but only x4 lanes available to it. What happens if you combine those scenarios?

For instance, my motherboard has an x4 PCI-E 2.0 in a physical x8 slot. What would happen if a put an x8 PCI-E 1.0a HBA in there?

Would the end result be x4 of 1.0a bandwidth available to me? Or since it's 2.0, will I somehow have x4 of 2.0 bandwidth (which the card is spreading across 8 lanes of 1.0a)? The former makes the most logical sense, but I'd love to be wrong :)
Nice thought, but no. You get the number of lanes that are wired. If the slot only has x4 worth of lanes wired then that's it...you get x4 max. This does not change even if it is physically x8 or x16.

For speeds you get the lowest common denominator. Put a 1.0 card into a 2.0 slot and you get 1.0 speeds. Put a 2.0 card into a 1.0 slot and you get 1.0 speeds, etc. The lowest rev-level dominates the speeds that are negotiated.

So, in your example, if you put a 1.0 x4 card into a 2.0 slot that has x4 wired then you get 1.0 x4 speed.
 

mrkrad

Well-Known Member
Oct 13, 2012
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The QLE8152 iscsi/fcoe/nic operates at x4 pci-e 2.0 - its about $50-90 on ebay. I would only suggest using thermal epoxy to mount a heatsink to the core chip so it doesn't get to 212f operating temps under load.

That is a lot of speed - works great with esxi 5.1 !

Then you can afford to have that fancy nfs or iscsi server feeding in!