linux-kernel - Re: [PATCH] ubifs: Add freeze support

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Message-ID: <20170527081822.GA32125@infradead.org>
Date:   Sat, 27 May 2017 01:18:22 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Richard Weinberger <richard@....at>
Cc:     Hyunchul Lee <hyc.lee@...il.com>,
        Artem Bityutskiy <dedekind1@...il.com>,
        adrian.hunter@...el.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mtd@...ts.infradead.org,
        kernel-team@....com
Subject: Re: [PATCH] ubifs: Add freeze support

On Fri, May 26, 2017 at 11:52:42AM +0200, Richard Weinberger wrote:
> Hyunchul,
> 
> Am 26.05.2017 um 01:30 schrieb Hyunchul Lee:
> > From: Hyunchul Lee <cheol.lee@....com>
> > 
> > for un/freeze support, implement freeze_super and un/freeze_fs
> > of super_operations.
> > ubifs_freeze_super just calls freeze_super. because freeze_super always
> > succeeds if file system is read-only,  UBIFS errors should be checked.
> > if there are errors, UBIFS is switched to read-only mode.
> > ubifs_freeze_fs runs commit if TNC/LPT isn't clean. though all writes
> > are blocked and sync_fs is called before, if commit alreay was started
> > before writes are blocked, TNC/LPT might have dirty COW nodes.
> 
> you explain how you implement that feature, but not why.
> What is the use-case?
> I always thought this interface is only being used by LVM.

It can also be used through the FIFREEZE ioctl, but without snapshots
underneath the fs it's not all that useful.