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Re: [dm-devel] [NOTES] thinp zeroing

On 02/22/12 15:14, Joe Thornber wrote:
* Requirements

   There are two distinct requirements for zeroing applicable to the
   thin-provisioning target:

   - Avoid data leaks (DATA_LEAK)

* Implementing DATA_LEAK

* Implementing ERASE

** Erase on deallocation

** Erasing from userland.

** Crash recovery

** Discards

thanks for all your hard work regarding thinp

I was thinking: why don't you implement a bitmap that takes care of emulating the discard functionality?

This would take care of all your issues above, and also be great for a lot of use cases even outside thinp (*).

Every read would first hit the bitmap; if the bitmap says that the region has been discarded, thinp would return zeroes to the requestor.

When a discard comes, you first set the bits in the discard-emulation-bitmap, and then also pass the discard to layers below. Passing the discard below has no user-visible effects (because discard is already implemented in thinp) however it is still advantageous to pass it to lower layers because there might be SSDs below thinp which can benefit from the discard.

I suggest a bitmap of 4kbytes / bit, and then if a discard comes that is not 4K aligned (that would be a mistake of the above layers, at least a "performance" mistake), you set the bitmaps only for the bits which are completely covered by the discard, and then you are left with at most two misaligned edges one at the beginning and one at the end of the discard region, and for those you will need to write zeroes to the layers below. So in the worst case you need to set a few bits and then perform two small writes of zeroes, but in most cases you just set a few bits.

(*) remember that most MD Raid levels do not pass discards below, so we -raid users- cannot really see zeroes where discard has been triggered. That's a problem when we want to backup a virtual machine disk image (DM volume) from the outside: non-zeroes don't compress well; it's like we backup deleted files everytime.

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