linux-kernel - Re: Linux 2.6.29

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Message-ID: <alpine.DEB.1.10.0903311813390.28893@asgard.lang.hm>
Date:	Tue, 31 Mar 2009 18:19:50 -0700 (PDT)
From:	david@...g.hm
To:	Bill Davidsen <davidsen@....com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: Linux 2.6.29

On Mon, 30 Mar 2009, Bill Davidsen wrote:

> Andreas T.Auer wrote:
>> On 30.03.2009 02:39 Theodore Tso wrote:
>>> All I can do is apologize to all other filesystem developers profusely
>>> for ext3's data=ordered semantics; at this point, I very much regret
>>> that we made data=ordered the default for ext3.  But the application
>>> writers vastly outnumber us, and realistically we're not going to be
>>> able to easily roll back eight years of application writers being
>>> trained that fsync() is not necessary, and actually is detrimental for
>>> ext3.
>
>> And still I don't know any reason, why it makes sense to write the
>> metadata to non-existing data immediately instead of delaying that, too.
>> 
> Here I have the same question, I don't expect or demand that anything be done 
> in a particular order unless I force it so, and I expect there to be some 
> corner case where the data is written and the metadata doesn't reflect that 
> in the event of a failure, but I can't see that it ever a good idea to have 
> the metadata reflect the future and describe what things will look like if 
> everything goes as planned. I have had enough of that BS from financial 
> planners and politicians, metadata shouldn't try to predict the future just 
> to save a ms here or there. It's also necessary to have the metadata match 
> reality after fsync(), of course, or even the well behaved applications 
> mentioned in this thread haven't a hope of staying consistent.
>
> Feel free to clarify why clairvoyant metadata is ever a good thing...

it's not that it's deliberatly pushing metadata out ahead of file data, 
but say you have the following sequence

write to file1
update metadata for file1
write to file2
update metadata for file2

if file1 and file2 are in the same directory your software can finish all 
four of these steps before _any_ of the data gets pushed to disk.

then when the system goes to write the metadata for file1 it is pushing 
the then-current copy of that sector to disk, which includes the metadata 
for file2, even though the data for file2 hasn't been written yet.

if you try to say 'flush all data blocks before metadata blocks' and have 
a lot of activity going on in a directory, and have to wait until it all 
stops before you write any of the metadata out, you could be blocked from 
writing the metadata for a _long_ time.

Also, if somone does a fsync on any of those files you can end up waiting 
a long time for all that other data to get written out (especially if the 
files are still being modified while you are trying to do the fsync). As I 
understand it, this is the fundamental cause of the slow fsync calls on 
ext3 with data=ordered.

David Lang
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