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Comment by flogic | Hacker News Reader

The first hit for "SSD write speeds" says ~500Mb/s (I hope I got the right b). I didn't bother clicking the links that just the blurb under it. He's dumping 128Mb in ~200 ms. I'm not sure there is much room for improvement.

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MB. Bytes. nobody quotes disk speeds in bits (or if they do I typically ignore them).

I've frequently observed sustained 500MB/sec writes and reads on my cheap ($250) 250GB SSDs. One of my favorite instances was running out of RAM while assembling a gigapan in Hugin. I added a swap file on my SSD and continued- it ran over night with nearly 500MB/sec reads and writes more or less continuously, but the job finished fine.

I weep for the memory sectors that got re-written continuously for an entire night.

If the controller on the SSD was working properly those writes were distributed evenly over the flash.

I used to, but given not a single one of my SSD drives (I have 4 deployed in my house) has so much as balked once in a year of continuous deployment, I am cautiously optimistic.

I've had very good reliability from my SSD drives as well. Some have been running almost continuously since 2009.

I ran a 60GB SSD as my Windows machine's system drive (with pagefile) for four years before it started showing problems, and that machine saw a few hours use almost every day. It was >90-95% full for most of that time.

SSD controllers do write-leveling, the blocks a filesystem writes to is virtual and remapped (think VMEM)

wouldn't an ~8GB page/swap file being continuously rewritten on a 250GB drive still consume a non-negligible number of write cycles over several days / weeks at most?

It depends on how much free space you have on the SSD. But yes, especially because the swap file isn't ssd-aware, you get a high degree of write amplification which wears the disk more than necessary. That being said, newish SSDs can take a beating, even under these kinds of workloads.

I think swap files are written page-wise, so as long as the start of the page file is aligned, all the writes should be aligned. (assuming memory page size and SSD page size are the same)

Does writes being aligned on page boundaries have anything to do with them being 13 base 2 orders of magnitude smaller than flash erase blocks? An unaligned page means you erase (mostly) one or (very rarely) two flash blocks. An aligned page means you erase one flash block. The biggest amplification is the 1<<13 4k -> 32MB amplification...

BTW I already have 16GB RAM on the machine. The swap file was 32GB.

Depends on the SSD. The PCIe SSD in a 2013 Retina MBP can approach 1GB/sec, and of course high-end PCIe server stuff can do better again; you may also have a striped RAID setup.

> The first hit for "SSD write speeds" says ~500Mb/s (I hope I got the right b).

Nope, it's MB not Mb.

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