From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=5Bfi=RA=vger.kernel.org=linux-btrfs-owner@kernel.org>
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
	aws-us-west-2-korg-lkml-1.web.codeaurora.org
X-Spam-Level: 
X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,
	MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no
	version=3.4.0
Received: from mail.kernel.org (mail.kernel.org [198.145.29.99])
	by smtp.lore.kernel.org (Postfix) with ESMTP id C0081C10F00
	for <linux-btrfs@archiver.kernel.org>; Mon, 25 Feb 2019 17:20:02 +0000 (UTC)
Received: from vger.kernel.org (vger.kernel.org [209.132.180.67])
	by mail.kernel.org (Postfix) with ESMTP id 9B665213A2
	for <linux-btrfs@archiver.kernel.org>; Mon, 25 Feb 2019 17:20:02 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1728563AbfBYRUB (ORCPT <rfc822;linux-btrfs@archiver.kernel.org>);
        Mon, 25 Feb 2019 12:20:01 -0500
Received: from mx2.suse.de ([195.135.220.15]:57346 "EHLO mx1.suse.de"
        rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP
        id S1728368AbfBYRUB (ORCPT <rfc822;linux-btrfs@vger.kernel.org>);
        Mon, 25 Feb 2019 12:20:01 -0500
X-Virus-Scanned: by amavisd-new at test-mx.suse.de
Received: from relay2.suse.de (unknown [195.135.220.254])
        by mx1.suse.de (Postfix) with ESMTP id 14561AE82
        for <linux-btrfs@vger.kernel.org>; Mon, 25 Feb 2019 17:20:00 +0000 (UTC)
Received: by ds.suse.cz (Postfix, from userid 10065)
        id 5FA97DA83C; Mon, 25 Feb 2019 18:21:19 +0100 (CET)
Date:   Mon, 25 Feb 2019 18:21:19 +0100
From:   David Sterba <dsterba@suse.cz>
To:     Qu Wenruo <wqu@suse.com>
Cc:     linux-btrfs@vger.kernel.org
Subject: Re: [PATCH] btrfs-progs: balance: Sync the fs before balancing
 metadata chunks
Message-ID: <20190225172118.GA24609@twin.jikos.cz>
Reply-To: dsterba@suse.cz
Mail-Followup-To: dsterba@suse.cz, Qu Wenruo <wqu@suse.com>,
        linux-btrfs@vger.kernel.org
References: <20190129065739.31707-1-wqu@suse.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190129065739.31707-1-wqu@suse.com>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-btrfs-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-btrfs.vger.kernel.org>
X-Mailing-List: linux-btrfs@vger.kernel.org

On Tue, Jan 29, 2019 at 02:57:39PM +0800, Qu Wenruo wrote:
> The most straightforward is to sync the fs before balancing metadata
> chunks.
> 
> We could enhance the kernel bytes_may_use calculation, but I doubt about
> the complexity.
> So I take the easy fix to reduce the false ENOSPC reports.

Agreed.

> +	/*
> +	 * There may be many over-reserved space for metadata block groups,
> +	 * especially for inlined file extents.
> +	 *
> +	 * Do a sync here will free those over-reserved space and hugely
> +	 * reduce the possibility of some false ENOSPC
> +	 */
> +	if (args->flags & BTRFS_BALANCE_METADATA) {
> +		ret = btrfs_util_sync(path);

As the fd is already open, we should use the _fd version,

> +		if (ret) {
> +			error("failed to sync the fs before balance: %m");
> +			ret = -errno;
> +			goto out;

and possibly only warn if there's an error returned as the sync failure
is not a critical condition.