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From: Amir Goldstein <amir73il@gmail.com>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>,
	linux-xfs@vger.kernel.org, Brian Foster <bfoster@redhat.com>
Subject: Re: [PATCH, RFC] xfs: use per-AG reservations for the finobt
Date: Wed, 4 Jan 2017 12:21:26 +0200	[thread overview]
Message-ID: <CAOQ4uxj0wxLq+=RXEo6FxCoEyncduheV0RC4gTfpS-=BLyuULA@mail.gmail.com> (raw)
In-Reply-To: <20170103192426.GA14031@birch.djwong.org>

On Tue, Jan 3, 2017 at 9:24 PM, Darrick J. Wong <darrick.wong@oracle.com> wrote:
...
>
> There's also the unsolved problem of what happens if we mount and find
> agf_freeblks < (sum(ask) - sum(used)) -- right now we eat that state and
> hope that we don't later ENOSPC and crash.  For reflink and rmap we will
> have always had the AG reservation and therefore it should never happen
> that we have fewer free blocks than reserved blocks.  (Unless the user
> does something pathological like using CoW to create many billions of
> separate rmap records...)
>

Darrick,

Can you explain the "Unless the user..." part?

Is it not possible to actively limit the user from getting to the
pathologic case?
If AG reservation size is a function of the maximum block refcount, then
an operation that is about to increase the maximum block refcount to a size
that will increase the worst case reservation beyond a certain percentage
of the AG (or beyond available space for that matter) should be denied
by a conservative ENOSPC.

I imagine it would be much easier and also understandable from user
POV to get a preventative ENOSPC for over cloning, then to get it
some time in the far future for trying to delete or deduplicate blocks.

Amir.

  reply	other threads:[~2017-01-04 10:22 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-12-30 14:19 [PATCH, RFC] xfs: use per-AG reservations for the finobt Christoph Hellwig
2017-01-03 19:24 ` Darrick J. Wong
2017-01-04 10:21   ` Amir Goldstein [this message]
2017-01-04 21:23     ` Darrick J. Wong
2017-01-04 20:48   ` Brian Foster
2017-01-13 17:43   ` Christoph Hellwig
2017-01-14  6:22     ` Darrick J. Wong

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