Re: "hardware-assisted zeroing"

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

On 2022/1/3 19:08, 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.

This looks pretty much like ATA TRIM or SCSI UNMAP command.

If they are the same, then btrfs supports it by either fstrim command
(recommended) or discard mount option.

Thanks,
Qu

> 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.
>
> Does it describe a feature supported by Btrfs or Linux? Is it possible
> for a LUN manager to "know" that Btrfs has freed space on a volume, in
> a region that had previously been allocated?
>
>