Re: [RFC 0/9] block: add llseek(SEEK_HOLE/SEEK_DATA) support

Eric Blake
2024-04-02T13:31:23Z

On Tue, Apr 02, 2024 at 02:26:17PM +0200, Christoph Hellwig wrote:
> On Thu, Mar 28, 2024 at 04:39:01PM -0400, Stefan Hajnoczi wrote:
> > In the block device world there are similar concepts to holes:
> > - SCSI has Logical Block Provisioning where the "mapped" state would be
> > considered data and other states would be considered holes.
>
> But for SCSI (and ATA and NVMe) unmapped/delallocated/etc blocks do
> not have to return zeroes. They could also return some other
> initialization pattern pattern. So they are (unfortunately) not a 1:1
> mapping to holes in sparse files.

Yes, and Stefan already answered that:

https://lore.kernel.org/dm-devel/e2lcp3n5gpf7zmlpyn4nj7wsr36sffn23z5bmzlsghu6oapi5u@sdkcbpimi5is/t/#m58146a45951ec086966497e179a2b2715692712d

>> SCSI initiators can check the Logical Block Provisioning Read Zeroes
>> (LBPRZ) field to determine whether or not zeroes are guaranteed. The sd
>> driver would only rely on the device when LPBRZ indicates that zeroes
>> will be read. Otherwise the driver would report that the device is
>> filled with data.

As well as my question on whether the community would be open to
introducing new SEEK_* constants to allow orthogonality between
searching for zeroes (known to read as zero, whether or not it was
allocated) vs. sparseness (known to be unallocated, whether or not it
reads as zero), where the existing SEEK_HOLE seeks for both properties
at once.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization: qemu.org | libguestfs.org