Samsung SSD XP941 Review: The PCIe Era Is Here
by Kristian Vättö on May 15, 2014 12:00 PM ESTBoot Support: Mac? Yes. PC? Mostly No.
Booting from PCIe devices has always been rather quirksome because the BIOSes in motherboards are designed to boot from SATA devices, as they usually do. Most PCIe SSDs and SATA adapters include boot support by loading special drivers before the BIOS, which makes the drive visible in the boot screen. Without the drivers the drive won't show up in the BIOS but once you boot into the OS it will be accessible like any other drive. The problem is, you can't boot from the drive unless it shows up in the BIOS' boot screen.
Unfortunately, the XP941 doesn't seem to include the drivers necessary to enable booting. At least no driver loading screen shows up during boot, which suggests that the XP941 doesn't even have such drivers (ASUS is also telling us that this is the case). I'm thinking that the XP941 is designed for UEFI booting because from what I have heard, you don't need the drivers for UEFI boot but there is still some sort of a UEFI key needed to make the drive bootable. I did try the UEFI boot method on my ASUS Z87 Deluxe board but even though I was able to install the OS to the drive just fine, it wouldn't show up in the boot order.
The good news is that 9-series chipsets bring some ease to the situation. Back when the 8-series was introduced, there weren't many PCIe SSDs on the market but this year we will see PCIe entering the mainstream segment. That obviously forces the motherboard manufacturers to work on PCIe boot support and we can confirm that at least AsRock's Z97 Extreme6, which has a PCIe 2.0 x4 M.2 slot, supports booting from the XP941 out of the box. Whether other 9-series motherboards support booting from the XP941 remains to be seen. Most manufacturers, however, seem to be limiting the M.2 slot to just two PCIe 2.0 lanes, so you wouldn't want to use the XP941 in those boards anyway (unless the XP941 is used in a standard PCIe slot with an adapter). Anyway, we'll be sure to investigate the bootability of the XP941 in our motherboard reviews and work with the OEMs in order to bring better support for PCIe booting.
Update 5/20: ASUS just sent me an email that all their Z97 based motherboards will get a BIOS update that enables booting from the XP941. The BIOS is currently in beta testing and ASUS is expecting public release in about two weeks.
Macs, on the other hand, can boot from the XP941 just fine. I confirmed this using an early 2009 Mac Pro and the volume in the drive shows up in the boot option screen just like any other volume does. I have to admit that I don't know why exactly this is the case, but I'm guessing it's a fundamental difference between how the EFI in Macs and the BIOS/UEFI in PCs handle device recognition.
Under OS X, the XP941 shows up like any other SATA device. Since it utilizes the AHCI command set, OS X thinks it's a SATA device even though it's not. However, it's also listed under PCI cards but the page doesn't provide any meaningful info.
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McTeags - Thursday, May 15, 2014 - link
I think there is a spelling mistake in the first sentence. Did you mean SATA instead of PATA? I don't know all of the tech lingo so maybe I'm mistaken.McTeags - Thursday, May 15, 2014 - link
Please disregard my comment. I googled it...BMNify - Thursday, May 15, 2014 - link
sata-e[serial], sata[serial], pata[parrallel] ,SCSI [several, and chainable to 15+ drives on one cable, we should have used that as generic] ,shugart these are all drive interfaces and there are more too going back in the day....metayoshi - Thursday, May 15, 2014 - link
"It's simply much faster to move electrons around a silicon chip than it is to rotate a heavy metal disk."While SSD performance blows HDDs out of the water, the quoted statement is technically not correct. If you take a single channel NAND part and put it up against today's mechanical HDDs, the HDD will probably blow the NAND part out of the water in everything except for random reads.
What really kills HDD performance isn't the just rotational speed as much as it is the track-to-track seek + rotational latency of a random workload. A sequential workload will reduce the seek and rotational latency so much that the areal density of today's 5 TB HDDs will give you pretty good numbers. In a random workload however, the next block of data you want to read is most likely on a different track, different platter, and different head. Now it has to seek the heads to the correct track, perform a head switch because only 1 head can be on at a time, and then wait for the rotation of the disk for that data block to be under the head.
A NAND part with a low number of channels will give you pretty crappy performance. Just look at the NAND in smartphones and tablets of today, and in the SD cards and USB thumb drives of yesteryear. What really makes SSDs shine is that they have multiple NAND parts on these things, and that they stripe the data across a huge number of channels. Just think RAID 0 with HDDs, except this time, it's done by the SSD controller itself, so the motherboard only needs 1 SATA (or other like PCIe) interface to the SSD. That really put SSDs on the map, and if a single NAND chip can do 40 MB/s writes, think about 16 of them doing it at the same time.
So yes, there's no question that the main advantage of SSDs vs HDDs is an electrical vs mechanical thing. It's just simply not true that reading the electrical signals off of a single NAND part is faster than reading the bits off of a sequential track in an HDD. It's a lot of different things working together.
iwod - Friday, May 16, 2014 - link
I skim read it. Few things i notice, No Power usage testing. But 0.05w idle is pretty amazing. Since the PCI-E supply the power as well i guess they could be much better fine grained? Although Active was 5.6W. So at the same time we want more performance == faster controller while using much lower power. it seems there could be more work to do.I wonder if the relative slow Random I/O were due to Samsung betting its use on NVMe instead of ACHI.
iwod - Friday, May 16, 2014 - link
It also prove my points about Random I/O. We see how Random I/O for xp941 being at the bottom of the chart while getting much better benchmarks results. Seq I/O matters! And It matters a lot. The PCI -E x4 interfaces will once again becomes bottleneck until we move to PCI-E 3.0 Which i hope we do in 2015.Although i have this suspicious feeling intel is delaying or slowing down our progression.
nevertell - Friday, May 16, 2014 - link
Can't you place the bootloader on a hard drive, yet have it load the OS up from the SSD ?rxzlmn - Friday, May 16, 2014 - link
'Boot Support: Mac? Yes. PC? Mostly No.'Uh, a Mac is a PC. On a serious tech site I don't expect lines like that.
Penti - Friday, May 16, 2014 - link
Firmware differences.Haravikk - Friday, May 16, 2014 - link
It still surprises me that PCs can have so many hurdles when it comes to booting from various devices; for years now Macs have been able to boot from just about anything you plug into them (that can store data of course). I have one machine already that uses an internal drive combined with an external USB drive as a Fusion Drive, and it not only boots just fine, but the Fusion setup really helps eliminate the USB performance issues.Anyway, it's good to see PCIe storage properly reaching general release; it's probably going to be a while before I adopt it on PCs, as I'm still finding regular SATA or M.2 flash storage runs just fine for my needs, but having tried Apple's new Mac Pros, the PCIe flash really is awesome. Hopefully the next generation of Mac Pros will have connectors for two, as combined in a RAID-0 or RAID-1 the read performance can be absolutely staggering.