[dm-devel] [NOTES] thinp zeroing

[Zdenek Kabelac]

Dne 22.2.2012 17:30, Spelic napsal(a):
> 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
>>
>>
> 
> Hello
> 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.
> 

For backups there will be much better solution which will be able to get list
of provisioned blocks for a device (in case of snapshot - diffs).

IMHO Bitmaps are expensive - as you may observe with certain extX operations.

Zdenek