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New PCIe SSDs Load Games, Apps As Fast As Old SATA Drives

Posted by samzenpus on from the you-don't-need-a-race-car-if-you're-just-driving-to-work dept.

crookedvulture writes Slashdot has covered a bunch of new PCI Express SSDs over the past month, and for good reason. The latest crop offers much higher sequential and random I/O rates than predecessors based on old-school Serial ATA interfaces. They're also compatible with new protocols, like NVM Express, which reduce overhead and improve scaling under demanding loads. As one might expect, these new PCIe drives destroy the competition in targeted benchmarks, hitting top speeds several times faster than even the best SATA SSDs can muster. The thing is, PCIe SSDs don't load games or common application data any faster than current incumbents—or even consumer-grade SSDs from five years ago. That's very different from the initial transition from mechanical to solid-state storage, where load times improved noticeably for just about everything. Servers and workstations can no doubt take advantage of the extra oomph that PCIe SSDs provide, but desktop users may struggle to find scenarios where PCIe SSDs offer palpable performance improvements over even budget-oriented SATA drives.

2 of 162 comments (clear)

  1. Re:Latency vs bandwidth by m.dillon · · Score: 5, Interesting

    That's isn't correct. The queue depth for a normal AHCI controller is 31 (assuming 1 tag is reserved for error handling). It only takes a queue depth of 2 or 3 for maximum linear throughput.

    Also, most operating systems are doing read-ahead for the program. Even if a program is requesting data from a file in small 4K read() chunks, the OS itself is doing read-ahead with multiple tags and likely much larger 16K-64K chunks. That's assuming the data hasn't been cached in ram yet.

    For writing, the OS is buffering the data and issuing the writes asynchronously so writing is not usually a bottleneck unless a vast amount of data is being shoved out.

    -Matt

  2. That's because they're not much faster by Solandri · · Score: 5, Insightful

    Slashdot has covered a bunch of new PCI Express SSDs over the past month, and for good reason. The latest crop offers much higher sequential and random I/O rates than predecessors based on old-school Serial ATA interfaces.

    That's just it. Their speeds are not "much higher." They're only slightly faster. The speed increase is mostly an illusion created by measuring these things in MB/s. Our perception of disk speed is not MB/s, which is what you'd want to use if you only had x seconds of computing time and wanted to know how many MB of data you could read.

    Our perception of disk speed is wait time, or sec/MB. If I have y MB of data I need read, how many seconds will it take? This is the inverse of MB/s. Consequently, the bigger MB/s figures actually represent progressively smaller reductions in wait times. I posted the explanation a few months ago, the same one I post to multiple tech sites. And oddly enough Slashdot was the only site where it was ridiculed.

    If you measure these disks in terms of wait time to read 1 GB, and define the change in wait time from a 100 MB/s HDD to a 2 GB/s NVMe SSD as 100%, then:

    A 100 MB/s HDD has a 10 sec wait time.
    A 250 MB/s SATA2 SSD gives you 63% of the reduction in wait time (6 sec).
    A 500 MB/s SATA3 SSD gives you 84% of the reduction in wait time (8 sec).
    A 1 GB/s PCIe SSD gives you 95% of the reduction in wait time (9 sec).
    The 2 GB/s NVMe SSD gives you 100% of the reduction in wait time (9.5 sec).

    Or put another way:

    The first 150 MB/s speedup results in a 6 sec reduction in wait time.
    The next 250 MB/s speedup results in an extra 2 sec reduction in wait time.
    The next 500 MB/s speedup results in an extra 1 sec reduction in wait time.
    The next 1000 MB/s speedup results in an extra 0.5 sec reduction in wait time.

    Each doubling of MB/s results in half the reduction in wait time of the previous step. Manufacturers love waving around huge MB/s figures, but the bigger those numbers get the less difference it makes in terms of wait times.

    (The same problem crops up with car gas mileage. MPG is the inverse of fuel consumption. So those high MPG vehicles like the Prius actually make very little difference despite the impressively large MPG figures. Most of the rest of the world measures fuel economy in liters/100 km for this reason. If we weren't so misguidedly obsessed with achieving high MPG, we'd be correctly attempting to reduce fuel consumption by making changes where it matters the most - by first improving the efficiency of low-MPG vehicles like trucks and SUVs even though this results in tiny improvements in MPG.)