From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 54DD2C433F5 for ; Mon, 3 Jan 2022 11:08:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232830AbiACLIt (ORCPT ); Mon, 3 Jan 2022 06:08:49 -0500 Received: from zmcc-3-mx.zmailcloud.com ([34.200.143.36]:59779 "EHLO zmcc-3-mx.zmailcloud.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229651AbiACLIt (ORCPT ); Mon, 3 Jan 2022 06:08:49 -0500 Received: from zmcc-3.zmailcloud.com (zmcc-3-mta-1.zmailcloud.com [104.154.87.183]) (using TLSv1.2 with cipher ADH-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by zmcc-3-mx.zmailcloud.com (Postfix) with ESMTPS id A7062405D4 for ; Mon, 3 Jan 2022 05:09:34 -0600 (CST) Received: from zmcc-3.zmailcloud.com (localhost [127.0.0.1]) by zmcc-3-mta-1.zmailcloud.com (Postfix) with ESMTPS id 783728034785 for ; Mon, 3 Jan 2022 05:08:48 -0600 (CST) Received: from localhost (localhost [127.0.0.1]) by zmcc-3-mta-1.zmailcloud.com (Postfix) with ESMTP id 6894780347A8 for ; Mon, 3 Jan 2022 05:08:48 -0600 (CST) Received: from zmcc-3.zmailcloud.com ([127.0.0.1]) by localhost (zmcc-3-mta-1.zmailcloud.com [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id gAQC1WnHY2x6 for ; Mon, 3 Jan 2022 05:08:48 -0600 (CST) Received: from epl-dy1-mint (unknown [154.21.21.52]) by zmcc-3-mta-1.zmailcloud.com (Postfix) with ESMTPSA id ECE398034785 for ; Mon, 3 Jan 2022 05:08:47 -0600 (CST) Message-ID: <2c80ca8507181b1e65a67bbd4dca459d24a47da2.camel@ericlevy.name> Subject: "hardware-assisted zeroing" From: Eric Levy To: "linux-btrfs@vger.kernel.org" Date: Mon, 03 Jan 2022 06:08:46 -0500 Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.5-0ubuntu1 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org 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. 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?