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From: Chris Ball <cjb@laptop.org>
To: Ravi Pinjala <ravi@p-static.net>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: zero-length files in snapshots
Date: Fri, 12 Feb 2010 14:03:03 -0500	[thread overview]
Message-ID: <m3hbpmxons.fsf@pullcord.laptop.org> (raw)
In-Reply-To: <4B759C54.8050907@p-static.net> (Ravi Pinjala's message of "Fri, 12 Feb 2010 12:22:12 -0600")

Hi,

   > Is there a race in there? It seems like if a process starts
   > modifying a file between the sync and the snapshot, data could
   > still be lost. Is there something else going on here that I'm
   > missing that would prevent this race?

AIUI, you're correct that a writer process concurrent to a snapshot
leads to a race with data that doesn't make it in to the snapshot, but
I think you're wrong in thinking that there's much we could do about
it -- either you miss writes between sync and snapshot, as we do now,
or we do sync-and-snapshot atomically, and the concurrent writes are
missed because we decided to block further writes from that process
before we took the snapshot.  The only real answer is to quiesce the
writer process before you begin.  Does that make sense?

- Chris.
-- 
Chris Ball   <cjb@laptop.org>
One Laptop Per Child

  parent reply	other threads:[~2010-02-12 19:03 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-02-12  1:49 zero-length files in snapshots Nickolai Zeldovich
2010-02-12  3:11 ` Chris Ball
2010-02-12  4:50   ` Mike Fedyk
2010-02-12 15:19     ` Josef Bacik
2010-02-12 16:18       ` Mike Fedyk
2010-02-12 16:22         ` Josef Bacik
2010-02-12 16:27           ` Mike Fedyk
2010-02-12 16:32             ` Josef Bacik
2010-02-12 17:13               ` Mike Fedyk
2010-02-13 11:25                 ` Sander
2010-02-13 19:26                   ` Mike Fedyk
2010-02-19 22:22                     ` Sage Weil
2010-02-25 18:57                       ` Goffredo Baroncelli
2010-02-12 18:22       ` Ravi Pinjala
2010-02-12 18:45         ` Josef Bacik
2010-02-12 19:03         ` Chris Ball [this message]
2010-02-12 19:10       ` Christoph Hellwig

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