The fact that my first 99-GB image (and a later 2.6-GB File & Folder backup) did achieve effective write speeds of 400+ MB/s proves that this is in fact possible with my hardware configuration. But I think I should be able to consistently get write speeds near 400 MB/s (the estimated real-life maximum for USB 3.1 Gen1). Yes, I don't expect to achieve this performance with my laptop, which only has USB 3.1 Gen1 ports. Independent benchmark tests (by Tom's Hardware) show that the drive I am using achieves sustained sequential write speeds near 1600 MB/s for up to 15 min, even after the buffer is saturated, with no thermal throttling (my exact drive model is the purple curve at the top of this graph): But if you have an enclosure that lacks those design features, then you will likely find that it slows down when it gets hot. ![]() That's why many external enclosures for NVMe SSDs have thermal pads on the inside that are meant to press against the storage chips on the SSD itself, and/or a heat sink design for the enclosure itself to help shed that heat efficiently. In that case, you can start off with fast write speeds that will then tank once you've saturated the cache.Īs for thermal throttling, NVMe SSDs in particular can generate quite a bit of heat to go with their impressive performance - and when they get too hot, they start throttling performance. On some cheaper SSDs, the "regular" storage has write speeds that are barely any better than traditional HDDs. But if you have sustained write activity such that you fill up the SLC cache before it can be flushed to the "regular" storage, then the SSD has to start writing directly to that "regular" storage, at a much slower speed. Essentially, when you write to an SSD, it gets stored in that SLC cache that can accept it at high speed, and then it gets committed to the "regular" storage chips in the background when activity dies off. All else being equal, denser cells mean slower write speeds because of how SSD writes work, so they compensate for that by having a relatively small quantity of fast, SLC-based storage to serve as a write buffer. The first SSDs were SLC (one bit per cell), then came MLC (two bits per cell), and now TLC (three bits per cell) even QLC (four bits per cell). For the former, SSDs have increased capacity without proportionally increasing price by moving to storing more bits per cell. The additional factors that may be coming into play here are exhaustion of the SSD's write buffer and thermal throttling. The apparent 10-fold reduction in write speed is disappointing, and doesn't allow me to take full advantage of the rapid backup algorithms offered by Reflect.Īn SSD with a rated performance of 2000 MB/s would have to be an NVMe SSD, but if you're connecting it over USB 3.1 Gen 2, which has a theoretical max of 5 Gbps (625 MB/s), that is going to bottleneck its performance. When I perform the backups, Reflect is the only application open. The computer is a Dell Latitude 7490 with USB 3.1 Gen1, for which Dell claims "real-world maximum rate of 400MB/s with overheads". My destination drive is a SanDisk Extreme PRO Portable SSD V2, which claims a sequential write speed of 2000 MB/s. When I clicked the "Test" button, the numbers changed to 37.6 MB/s and 38.2 MB/s, respectively.Īny ideas why the performance is so inconsistent (changing by a factor of 10!), and what I can do to achieve higher write speeds? ![]() Looking at Backup > Disk Write Performance, I found numbers of 352 MB/s (File System Cache) and 407 MB/s (Direct Disk I/O), but I don't know on what date that performance was measured. It doesn't seem to be just a deterioration with time (or with reduced drive capacity), because a 2.6-GB File & Folder full backup that was performed on 6/19 (with 1.32 TB free space remaining on the destination drive) achieved 410 MB/s write speed. For example, here are the results of three imaging backups of the system partitions: The write speeds during my backups have been very inconsistent.
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