On 02/12/2017 11:49 PM, Konstantin Olchanski wrote: > On Sun, Feb 12, 2017 at 04:52:59PM -0800, ToddAndMargo wrote: >> I am asking as several motherboards I have looked at only >> have one M.2 slot and raid would need two. >> >> I may be misunderstanding here, but I think that if >> you need two drives, you need a PCIe carrier to >> put them into instead of the M.2 slot. >> >> Maybe I misunderstand for the M.2 slot works. >> > > The best I can tell, on-mobo socket M.2 M-key can be configured > as either SATA (stealing the connection to one of the sata connectors) > or as x1, x2 or x4 PCIe. On the mobos that I have this is controlled > by a BIOS setting (SATA vs PCIe). > > In other words, in SATA mode, the M.2 socket is a funny looking SATA > port (combined data and power), while in PCIe mode, it is just a funny looking > PCIe x4 slot. > > The "x4" part means the connector has provisions for 4 PCIe "lanes" (in and out signal "lvds pairs"). > > Same as with normal PCIe slots, not all PCIe lanes are necessary connected > to anything - quite common are x16 slots wired for 8 lanes, x8 slots wired for 4 lanes > and x4 slots wired for 1 lane. (sometimes you can see how only half of the connector > has metal pins). > > The Intel CPU and chipset ("north bridge") have only so many PCIe lanes available. > For each motherboard their routing to external devices (network interfaces, > additional SATA interfaces, etc) and to the PCIe slots (and M.2 slots) can > be documented or not. Some mobos allow flexible routing, i.e. "x16 and x16 > to 2 PCIe slots, nothing to other slots" or "x8 to every slots", etc. > > The actual connectivity can be see in the very small print of "lspci -vvvv" (requires > root permission) - shown is the device capability (number of PCIe lanes implemented > on the device) and actual connection (how many PCIe lanes are active). > > All this seems to be very poorly understood: > > I was recently amused to open a few decomissioned professionaly built servers > that had x8 PCIe RAID cards installed in "wired for x4" PCIe slots, > while slots "wired for x8" were empty. Neither the people who build the machines noticed > this not the people who used them for 10 years ever noticed that the RAID cards > were in the wrong slots. > > So I wonder how many people buy super expensive fancy PCIe x4 SSDs for ultimate performance > only to have them run at x1 speed because the mobo is not setup or wired correctly. > > The bottom line, with your x16 quad M.2 PCIe riser card, you better check that all 4 M.2 SSDs > are actually connected at x4 speed. (each SSD should show up in "lspci"). > > Some mobos have better description of PCIe routing than others - read the mobo manual > very carefully. (read *before* buying, to avoid nasty surprises, like the USB-connected > network on the RaspberryPi machines). > > Thank you for the education. Now I understand! -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~