Re: Testing devices for discard support properly

["Martin K. Petersen" <martin.petersen@oracle.com>]

Dave,

>> The answer is that it depends. It can return zeroes or a
>> device-specific initialization pattern (oh joy).
>
> So they ignore the "write zeroes" part of the command?

I'd have to look to see how ANCHOR and NDOB interact on a WRITE
SAME. That's the closest thing SCSI has to WRITE ZEROES.

You can check whether a device has a non-standard initialization
pattern. It's a bit convoluted given that devices can autonomously
transition blocks between different states based on the initialization
pattern. But again, I don't think anybody has actually implemented this
part of the spec.

>> We have:
>> 
>>    Allocate and zero:	FALLOC_FL_ZERO_RANGE
>>    Deallocate and zero:	FALLOC_FL_PUNCH_HOLE
>>    Deallocate:		FALLOC_FL_PUNCH_HOLE | FALLOC_FL_NO_HIDE_STALE
>> but are missing:
>> 
>>    Allocate:		FALLOC_FL_ZERO_RANGE | FALLOC_FL_NO_HIDE_STALE

Copy and paste error. "Allocate:" would be FALLOC_FL_NO_HIDE_STALE in
the ANCHOR case. It's really just a preallocation but the blocks could
contain something other than zeroes depending on the device.

> So we've defined the fallocate flags to have /completely/ different
> behaviour on block devices to filesystems.

Are you referring to the "Allocate" case or something else? From
fallocate(2):

"Specifying the FALLOC_FL_ZERO_RANGE flag [...] zeroes space [...].
Within the specified range, blocks are preallocated for the regions that
span the holes in the file.  After a successful call, subsequent reads
from this range will return zeroes."

"Specifying the FALLOC_FL_PUNCH_HOLE flag [...] deallocates space [...].
Within the specified range, partial filesystem blocks are zeroed, and
whole filesystem blocks are removed from the file.  After a successful
call, subsequent reads from this range will return zeroes."

That matches the block device behavior as far as I'm concerned.

-- 
Martin K. Petersen	Oracle Linux Engineering