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My hard drive is a SanDisk SSD. The drive is new. I've booted into ArchLinux Live System, with the intention of preparing the hard drive for full disk encryption. I've noticed a significant drop of write speed over time, see image:

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The same issue can also be observed when dd'ing directly from /dev/zero to /dev/sda.

Is this a usual behavior when dd'ing SSDs? Is there a wrong setting in the configuration/BIOS? Or is the SSD broken?

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While some performance loss is to be expected when filling an SSD entirely with random data, I don't know what's going on with your particular model.

The blocksize you chose for dd is far too large; but I'd be suprised if it was the cause of performance drops as severe as you're seeing.

Will you be using TRIM (cryptsetup --allow-discards)? If so, it's pointless to overwrite it like this, and the only preparation you need is blkdiscard. In most encryption setups, the real security issues are elsewhere. Here's what a encrypted&trimmed SSD looks like.

  • I doubt that 16MB would be far from optimal for the block size. Anyway a block size that's too high only really hurts when both input and output are doing I/O (or when the block size gets to be a sizable fraction of RAM), which isn't the case here. – Gilles Mar 4 '15 at 23:29

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