Re: "hardware-assisted zeroing"

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2022/1/3 19:46, David Disseldorp wrote:
> On Mon, 03 Jan 2022 06:08:46 -0500, Eric Levy wrote:
>
>> I am operating a Btrfs file system on logical volumes provided through
>> an iSCSI target. The software managing the volumes shows that they are
>> configured for certain features, which include "hardware-assisted
>> zeroing" and "space reclamation". Presumably the meaning of these
>> features, at least the former, is that a file system, with support of
>> the kernel, may issue a SCSI command indicating that a region of a
>> block device would be cleared. For a file system, such an operation has
>> no direct value, because the contents of de-allocated space is
>> irrelevant, but for a logical volume, it creates an opportunity to free
>> space on the underlying file system on the back end.
>>
>> I have searched the term "hardware-assisted zeroing", without finding
>> any useful resources on the application of the term.
>
> "hardware-assisted zeroing" is often marketing speak for the WRITE SAME
> SCSI command, which is used by VMFS. I'm not aware of any Linux
> filesystems which make use of it.

Thanks for pointing this out, really not familiar with WRITE SAME command.

After some quick search, it looks like it's kinda of hole punching for
SCSI command set.

Then I guess that explains why Linux filesystems don't really make use
of it.

If we want a large zeroed file, both hole punching and fallocate will be
faster, and we don't need to issue any data IO.
All data read will be zeroed at read time. No IO is always faster than
any IO, even if it's "hardware-assisted".

Thanks,
Qu
>
> As Qu mentioned, "space reclamation" would refer to UNMAP / discard.
>
> Cheers, David