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I wanted to experiment with combining bcache and a compressed block device on a hdd and seeing if this will speedup the boot time. Are there any means of compressing a block device at the block level? something like cloop but with read-write capabilies.

The write need not be very fast. I was thinking of something similar to how the zram deals with it's pages.

I know the future will bring us ubiquitous ssds and stable btrfs with compression, but that really doesn't scratch my itch now :P

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Are there any means of compressing a block device at the block level? something like cloop but with read-write capabilies

No, since you can't predict what compression ratio you are going to get, so you don't know how much real storage you need to hold the compressed data. The fecal matter would hit the rotary air impeller if you started filling such a device with very non compressible data.

zram gets around it by not allocating memory to hold the compressed data until it actually needs it, and assumes that you have plenty more ram than the maximum size of the zram device.

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