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It was wrong back then and it's wrong now

It was wrong back then and it's wrong now

Posted Nov 9, 2008 0:37 UTC (Sun) by giraffedata (subscriber, #1954)
In reply to: It was wrong back then and it's wrong now by khim
Parent article: Linux and object storage devices

For rotating storage it hides bad block remapping.
Bad block remapping? It hides all block mapping. In a truly low level disk interface, Linux would address the disk by cylinder/head/sector. Indeed, there are things Linux could do more effectively if it controlled the storage at that level. And that's nowhere near the lowest conceivable abstraction, either.

I don't think checksum calculation for blocks on HDD belongs to OS kernel (it can be calculated in HDD more or less for free, but general-purpose CPU will spend significant power doing it)

I see no reason for it to be a cheaper computation in the HDD than in the main computer. If there are special purpose processors in the HDD to do it cheaply, it's because that's where we've decided to do it; not vice versa.

the search is for the abstraction which best mediates between the needs of the computer and the needs of the storage.

I'd like to put it differently, because it's not what I can do for the computer and the storage, but what they can do for me. So: Which abstraction best leverages the abilities of the computer and those of the storage, to provide the most efficient storage service?

There was a time when the best dividing line was such that the main computer watched the bits stream off the head until it detected the start of a record, etc. That let us consolidate expensive CPUs. Today, we can squeeze more storage service out cheaper by moving a great deal of that function to the other end of the cable. More recent technological changes might mean it's most efficient for file layout to move out there too.

I can think of a few reasons to stick function inside the storage product and the storage box instead of the main computer products and box:

  • The implementation must change more when the storage layer below it changes than when the application layer above it does.
  • Multiple main computers share the storage box.
  • It's expensive to move information over the cable.

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