Integrated LSI 3008 :: Number Of Hdd Support

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Hi guys! Unfortunately there is no offtopic list but the subject is somehow related to CentOS as the OS is/will be CentOS :)
So, under this thin cover i ask :
Is it possible that for a SAS controler like LSI 3008 that in specs says that : “This high-performance I/O controller supports T-10
data protection model and optical support, PCIe hot plugging, and up to 1,000 connected devices” in a vendor implementation
(motherboard integrated) to support only 8 (or 16) devices?
The technical support from the OEM told me that “the onboard SAS controller maximum amount of supported harddrives is 16pcs”
and “if you are planning on using more than 16 drives then you have to use a PCI-E card based SAS controller with external ports
(which Supermicro does not sell)”
and both statements sound insane to me!
First, because the specs for 3008 says something else and i dont know how one can artificially reduce the number of supported hdds
(beside the firmware – but why would one do that?) and the second statement is just hogwash as the externl/internal status of the ports have nothing to do with the sas cascading and the number of devices supported!! (and of course is really cheap to convert an internal port to an external port with a bracket)
So, i ask you guys that have more knowledge and expertise: was this Senior Application Engineer that answered me a total incompetent?

Thank you!
Adrian

4 thoughts on - Integrated LSI 3008 :: Number Of Hdd Support

  • In my experience the mass market HBAs and RAID cards typically do support only 8 or 16 drives. For the internal variety in a standard rack-mount server you’ll usually see either 2 or 4 iPass cables (each of which support
    4 drives) connected to the backplane. The marketing material you’ve referenced has a white lie in it: supporting more than 16 drives on a single card is very likely only possible with an additional SAS expander board. I believe Supermicro does sell some pre-configured systems with such hardware, but expect the throughput to fall through the floor if you use such hardware.

    Bottom line: the Supermicro application engineer knows what he’s talking about.

  • ok, then i should give a little detail : the purpose was to have an 1U
    server as a head of a JBOD chassis that have 2 SAS backplanes. The connection would be a simple SAS cascade to the backplanes.

    why? what is the difference between the silicon from a HBA card and the same silicon on motherboard?

    The reason of my post is also to understand why is/is not possible..

    Thank you, Adrian

  • I’m sure he’s referring to what is essentially lane sharing. A SAS
    expander in many ways is like an ethernet switch. You have 8 lanes coming off your SAS3008, 4 each in the SFF8087 connector or 8 individual SATA like sockets on the motherboard. You can plug any number of these into a host port on a SAS expander and you then have n*6Gbit of bandwidth to the expander from the host. Then you plug targets and/or additional expanders into the downstream ports. Everything on the downstream ports has to share the bandwidth so you can run into a wall if you try to push to much bandwidth to to many devices at once. In practice though it is not usually a problem with a 2-3x over-subscription of lanes with HDD’s. You will see it though if you are really pushing allot of SSD’s.

    For example I have 32 1 and 2 TB SATA disks in 2 separate external enclosures. The enclosures are daisy chained to a single 4 lane 3Gbit port so I have a theoretical max of 12Gbit to use. What I do to test is simply use dd to write zeros simultaneously to each one of the drives.
    DD is able to write at the full speed of the drives until I get enough of them going that the total throughput hits around 900MB/s. So there is some overhead of the switching and whatnot but it is not really bad in practice. I would just go in not expecting to be able to exceed
    80-85% of your upstream link speed.

    The above example is using a pcie card but I have done the same thing using a built in. What support may be alluding to is that the SFF8087 –
    SFF8088 slot adapters are normally built with the SFF8088 as a host port and the SFF8087’s as a target meaning they are meant to be used in a JBOD. They do sell the reverse like what you are looking for but they are generally more expensive and harder to find.

    There is no reason you can’t do what it is your talking about as long as you buy the proper hardware. I would suggest though if you are looking at using an external slot adapter it may just be cheaper and easier to buy a SAS2008 based pcie card with external ports. Or if your JBOD does not have a 6G expander grab a 3G card for $30-40.

  • yeah, but i am not (6 gbps for 24 hdds is ok for me) .. so i would to get back to my original problem/question :
    why would the mb integrated lsi 3008 not support sas cascaded (through sas backplanes) more than 8/16 devices (as the OEM technical support said) when
    3008 specifications says that it could?

    Did anyone tried to use sas cascaded storage with some integrated in the motherboard sas chip?

    Well, in the end i will take a chance and buy 2 such servers and it will end in 2 possible ways : either bad mouthing OEM for the implementation or their technical support :)

    Thank you, Adrian

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