From: Josef Bacik <josef@redhat.com>
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 13:45:11 -0500 [thread overview]
Message-ID: <20100212184511.GD4191@localhost.localdomain> (raw)
In-Reply-To: <4B759C54.8050907@p-static.net>
On Fri, Feb 12, 2010 at 12:22:12PM -0600, Ravi Pinjala wrote:
> On 02/12/10 09:19, Josef Bacik wrote:
>> On Thu, Feb 11, 2010 at 08:50:48PM -0800, Mike Fedyk wrote:
>>> On Thu, Feb 11, 2010 at 7:11 PM, Chris Ball<cjb@laptop.org> wrote:
>>>> > echo x1> /mnt/x/d/foo.txt || exit 2
>>>> > btrfsctl -s /mnt/x/snap /mnt/x/d
>>>>
>>>> You're just missing a sync/fsync() between these two lines.
>>>>
>>>> We argued on IRC a while ago about whether this is a sensible default;
>>>> cmason wants the no-sync version of snapshot creation to be available,
>>>> but was amenable to the idea of changing the default to be sync before
>>>> snapshot, since it was pointed out that no-one other than him had
>>>> understood we were supposed to be running sync first.
>>>>
>>> You're saying that it only snapshots the on-disk data structures and
>>> not the in-memory versions? That can only lead to pain. What do you
>>> do if something else during this race condition? What would a sync do
>>> to solve this? Have the semantics of sync been changed in btrfs from
>>> "sync everything that hasn't been written yet" to "sync this
>>> subvolume"?
>>>
>>
>> Welcome to delalloc. You either get fast writes or you get all of your data on
>> the disk every 5 seconds. If you don't like delalloc, use ext3. The data
>> you've written to memory doesn't go down to disk unless explicitly told to, such
>> as
>>
>> 1) fsync - this is obvious
>> 2) vm - the vm has decided that this dirty page has been sitting around long
>> enough and should be written back to the disk, could happen now, could happen 10
>> years from now.
>> 3) sync - this is not as obvious. sync doesn't mean anything than "start
>> writing back dirty data to the fs", and returns before it's done. For btrfs
>> what that means is we run through _every_ inode that has delalloc pages
>> associated with them and start writeback on them. This will get most of your
>> data into the current transaction, which is when the snapshot happens.
>>
>> If you don't want empty files, do something like this
>>
>> btrfsctl -c /dir/to/volume
>> btrfsctl -s /dir/to/volume/snapshotname /dir/to/volume
>>
>> this is what we do with yum and its rollback plugin, and it works out quite
>> well. Thanks,
>>
>> Josef
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>>
>
> 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?
>
Data won't be lost, it just won't be there in the snapshot, and will be there in
the source. Thanks,
Josef
next prev parent reply other threads:[~2010-02-12 18:45 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 [this message]
2010-02-12 19:03 ` Chris Ball
2010-02-12 19:10 ` Christoph Hellwig
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