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It's not that good usually, e.g. PostgreSQL writes data in pages (8 KB by default), and changing 10K random rows in the 50M rows table can be quite close to the worst case of 1 changed page per changed row, so 8x of your estimate. Also need to multiply x2 to account for WAL writes. Also indexes. It's not hard to hit a throughput limit, especially with HDDs or networked storage. Although local SSDs are crazy fast indeed.

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Agreed: 80MB/s for the random 8K page updates. However, transaction logs in modern databases are committed to disk in batches, and each log entry is smaller than a page size. So a nice round number would be 100 MB/s for both.[1]

For comparison, that's about 1 gigabit per second in the era of 200 Gbps networking becoming common. That's a small fraction of SSD write throughput of any modern device, mobile or not. Nobody in their right mind would use HDD storage if scaling was in any way a concern.

[1] Indexes add some overhead to this, obviously, but tend to be smaller than the underlying tables.

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