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- Feb 15, 2014
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I've looked around a bunch with very little success, so I'm going to crowdsource this question:
Does a cable exist that can provide SATA (and/or SAS) plus PCIe to a single SFF-8639 (U.2 backplane side connector), so that a compact backplane could provide full SATA (with optional SAS) and PCIe connectivity?
This is a weird one at first glance, so here are the details:
Background
U.2 (SFF-8639) extends the legacy SAS connector (SFF-8482) by adding all the extra pins necessary for PCIe x4 signalling, independently of the existing SAS/SATA pins. (As a note, SFF-8482 was itself an extension of the SATA connector that added a second channel, to allow for multipath connections to disks.) This effectively means that U.2 backplanes can be analyzed as a legacy SATA/SAS backplane plus a PCIe backplane squished together into one PCB and connector, with independent uplinks for PCIe and for SATA/SAS. This allows for a backplane that is wired for both SATA/SAS and PCIe to take SATA/SAS drives or U.2 (and U.3) drives.
U.3 is a scam heavily pushed by Microchip and Broadcom. It allows for simpler backplanes by reusing some of the connections used for SAS as PCIe. This reduces the number of differential pairs that need to be routed across the backplane PCB and across cabling back to the respective hosts. However, it requires a tri-mode controller and (for most practical scenarios) a tri-mode expander. These are absurdly expensive, even by the standards of legacy SAS devices. To make things worse, implementations are often less than optimal - Broadcom, for instance, presents NVMe drives as SCSI devices, negating much of the advantage enjoyed by NVMe drives through lower protocol overhead. This is in addition to the - typically - limited bandwidth available to the tri-mode SAS controller (Both the LSI/Broadcom SAS 9500 and SAS 9600 have a PCIe 4.0 x8 connection to the host, meaning that any setup over two PCIe 4.0 SSDs or four PCIe 3.0 SSDs is immediately oversubscribed. This is a worse oversubscription scenario than was typically seen in plain SAS setups, despite the fact that many NVMe devices do make use of most of their available bandwidth.) In fairness, HPE is trying something interesting, with U.3 backplanes wired only for PCIe x1. This is interesting more than useful, but it allows PCIe connectivity without needing additional differential pairs when compared to SAS/SATA, at the cost of much reduced bandwidth (The promise of higher-speed, lower lane count SSDs has not really turned into a popular option for anybody. Maybe PCIe 6.0 will bring single-lane SSDs with performance equivalent to x4 PCIe 4.0 drives.).
Although most U.2 server chassis support SATA/SAS through dedicated uplink connectors, separate from the PCIe connector, this is not a popular configuration outside rackmount servers, as far as I can tell. However, there are some retrofit hot-swap bays for ATX-style chassis that present a U.2 drive-side connector (example unit from Icy Dock) and can be used with either SATA/SAS or PCIe. Thing is, they could actually operate be simultaneously wired up for both with an appropriate cable.
What am I looking for:
My goal is to have one or more hot swap bays in a standard ATX-style chassis (in 5.25" or 3.5" external bays), which will support SATA, SAS (single-lane) and PCIe without needing to be rewired, as is typical in a server chassis (see for instance the Dell R6515 10-bay backplane or the Supermicro BPN-SAS3-826EL1-N4 backplane). To make this work in a backplane that presents a disk-side SFF-8639, I would need a cable like the following crude diagram:
Similar cables that omit the SATA/SAS part are plentiful, if absurdly expensive, but I have not been able to find a "fully-featured" cable.
Are you completely insane? Do you not have anything better to do?
I mean, yes I am, and yes I do. I'd like to satisfy my OCD on this one without sticking a Dell R630 under my desk.
Does a cable exist that can provide SATA (and/or SAS) plus PCIe to a single SFF-8639 (U.2 backplane side connector), so that a compact backplane could provide full SATA (with optional SAS) and PCIe connectivity?
This is a weird one at first glance, so here are the details:
Background
U.2 (SFF-8639) extends the legacy SAS connector (SFF-8482) by adding all the extra pins necessary for PCIe x4 signalling, independently of the existing SAS/SATA pins. (As a note, SFF-8482 was itself an extension of the SATA connector that added a second channel, to allow for multipath connections to disks.) This effectively means that U.2 backplanes can be analyzed as a legacy SATA/SAS backplane plus a PCIe backplane squished together into one PCB and connector, with independent uplinks for PCIe and for SATA/SAS. This allows for a backplane that is wired for both SATA/SAS and PCIe to take SATA/SAS drives or U.2 (and U.3) drives.
U.3 is a scam heavily pushed by Microchip and Broadcom. It allows for simpler backplanes by reusing some of the connections used for SAS as PCIe. This reduces the number of differential pairs that need to be routed across the backplane PCB and across cabling back to the respective hosts. However, it requires a tri-mode controller and (for most practical scenarios) a tri-mode expander. These are absurdly expensive, even by the standards of legacy SAS devices. To make things worse, implementations are often less than optimal - Broadcom, for instance, presents NVMe drives as SCSI devices, negating much of the advantage enjoyed by NVMe drives through lower protocol overhead. This is in addition to the - typically - limited bandwidth available to the tri-mode SAS controller (Both the LSI/Broadcom SAS 9500 and SAS 9600 have a PCIe 4.0 x8 connection to the host, meaning that any setup over two PCIe 4.0 SSDs or four PCIe 3.0 SSDs is immediately oversubscribed. This is a worse oversubscription scenario than was typically seen in plain SAS setups, despite the fact that many NVMe devices do make use of most of their available bandwidth.) In fairness, HPE is trying something interesting, with U.3 backplanes wired only for PCIe x1. This is interesting more than useful, but it allows PCIe connectivity without needing additional differential pairs when compared to SAS/SATA, at the cost of much reduced bandwidth (The promise of higher-speed, lower lane count SSDs has not really turned into a popular option for anybody. Maybe PCIe 6.0 will bring single-lane SSDs with performance equivalent to x4 PCIe 4.0 drives.).
Although most U.2 server chassis support SATA/SAS through dedicated uplink connectors, separate from the PCIe connector, this is not a popular configuration outside rackmount servers, as far as I can tell. However, there are some retrofit hot-swap bays for ATX-style chassis that present a U.2 drive-side connector (example unit from Icy Dock) and can be used with either SATA/SAS or PCIe. Thing is, they could actually operate be simultaneously wired up for both with an appropriate cable.
What am I looking for:
My goal is to have one or more hot swap bays in a standard ATX-style chassis (in 5.25" or 3.5" external bays), which will support SATA, SAS (single-lane) and PCIe without needing to be rewired, as is typical in a server chassis (see for instance the Dell R6515 10-bay backplane or the Supermicro BPN-SAS3-826EL1-N4 backplane). To make this work in a backplane that presents a disk-side SFF-8639, I would need a cable like the following crude diagram:
Code:
SFF-8643 (PCIe 3.0/4.0 x4)------\ SATA data connector (SATA/SAS)-------U.2/SFF-8639 SATA power connector (power)----/
Similar cables that omit the SATA/SAS part are plentiful, if absurdly expensive, but I have not been able to find a "fully-featured" cable.
Are you completely insane? Do you not have anything better to do?
I mean, yes I am, and yes I do. I'd like to satisfy my OCD on this one without sticking a Dell R630 under my desk.