From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from tartarus.angband.pl ([89.206.35.136]:58318 "EHLO tartarus.angband.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751485AbcIXXuW (ORCPT ); Sat, 24 Sep 2016 19:50:22 -0400 Received: from kilobyte by tartarus.angband.pl with local (Exim 4.84_2) (envelope-from ) id 1bnwhy-0000uN-Lf for linux-btrfs@vger.kernel.org; Sun, 25 Sep 2016 01:50:14 +0200 Date: Sun, 25 Sep 2016 01:50:14 +0200 From: Adam Borowski To: linux-btrfs@vger.kernel.org Subject: Re: Does data checksumming remain for files with No_COW file attribute? Message-ID: <20160924235014.GA2247@angband.pl> References: <767591474719974@web21o.yandex.ru> <20160924123749.GR7138@carfax.org.uk> <1498471474759532@web22g.yandex.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1498471474759532@web22g.yandex.ru> Sender: linux-btrfs-owner@vger.kernel.org List-ID: On Sun, Sep 25, 2016 at 02:25:32AM +0300, Alexander Tomokhov wrote: > Ok, so data checksumming does not remain for newly created empty files > with No_COW attribute. I think it's an important trait of Btrfs behavior > and should be added to wiki. So that users are informed that disabling > CoW on a per-file basis also loses checksum correctness of such file. Actually, it disables pretty much all btrfs features except for... CoW. You lose: * checksums * compression * safety against power loss (torn writes, etc) * transactions (not that anyone uses them...) * etc But, CoW is still there. Try it: make a subvolume, create a FS_NO_COW file (preferably one big enough), snapshot the subvolume, filefrag -v both copies. Write to one of them, changing only a part of file. Wait for writeout, filefrag -v them again. -- An imaginary friend squared is a real enemy.