Message-ID: <4D1B542F.9070506@ontolinux.com>
Date:	Wed, 29 Dec 2010 16:30:56 +0100
From:	Christian Stroetmann <stroetmann@...olinux.com>
To:	Olaf van der Spek <olafvdspek@...il.com>
CC:	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	Ric Wheeler <rwheeler@...hat.com>,
	Amir Goldstein <amir73il@...il.com>
Subject: Re: Atomic file data replace API

On the 29.12.2010 13:42, Olaf van der Spek wrote:
> On Wed, Dec 29, 2010 at 10:20 AM, Amir Goldstein<amir73il@...il.com>  wrote:
>> On Wed, Dec 29, 2010 at 12:58 AM, Olaf van der Spek
>> <olafvdspek@...il.com>  wrote:
>>> On Tue, Dec 28, 2010 at 11:36 PM, Ric Wheeler<rwheeler@...hat.com>  wrote:
>>>> I think that various developers have answered this for you several times.
>>> Not really, unfortunately. Haven't seen a single link to code that
>>> shows how to do it properly.

No, not this way. You were and still are asked for delivering the code. 
Don't pervert the threat of the discussion.

>>> Temp file, fsync, rename is often mentioned but that skips the
>>> preserving meta-data part and this part, which you also skipped:
>>> One use case would be updating a file in a safe way when you have
>>> write access to that file but not to anything else.
>>>
>> I think it is safe to say that the *only* option you have now is "temp
>> file, fsync, rename".
> I'm really looking for a concrete code snippet/function that does this.
> For example, file permissions should definitely be preserved.
>
>> There is no "generic atomic file data replace API in Linux", though it
>> is available via
>> private ioctl for XFS and EXT4.
>>
>> You have started a bit of a storm with your previous thread, which
>> doesn't help you
>> much in moving forward in the current thread (previous thread is still
>> more popular).
>> I suggest that you humbly swallow you need to know WHY is it hard to implement
>> non-durable atomic API and focus your attention on the very achievable
>> data replace API.
>>
>> IMHO, implementing atomic swap_inodes_data operation shouldn't be difficult
>> in most file systems (only implementation is simple, but testing and
>> maintaining
>> is not to be taken lightly).
>> Something along the lines of:
>> 1. aquire inodes write/truncate locks
>> 2. start transaction
>> 3. check/update quota limits
>> 4. swap inodes i_data content
>> 5. invalidate (or swap?) inodes page caches
>> 6. mark inodes dirty
>> 7. end transaction&  release locks
>>
>> The real challenge would be to get everyone to agree on a common API
>> and carve it in stone to the kernel's ABI (is it just swap_inodes_data?
>> maybe also swap_inode_data_ranges? what about some options?)
> Swapping data is an improvement but still not ideal. The API is also
> more complex than O_ATOMIC.
>
>> Also, as wacky and (some say) faulty the UNIX permissions models is,
>> current systems have grown old with it, and even 'improving' the behavior
>> of some applications, may wake up sleeping monsters, so it will not
>> be done until enough people have pointed out security or usability
>> issues, which could not be solved otherwise.
> Each app makes it's own decision about what API to use. Supporting
> atomic stuff doesn't change the behaviour of existing apps.

Wrong, we are talking here in the first place about general atomic FS 
operations. And to guarantee atomicity you have to change general FS 
functions in such a way that in the end all other applications are 
affected, or otherwise you have to implement an own (larger part of an) FS.
At this point there is no discussion anymore without code from you, 
because this subject is as well discussed to the maximum in information 
processing/informatics/computer science.

>> In other words, until you find an *application* that wants to allow other
>> user to modify the content of a file and preserve it's metadata and ownership.
>> And unless that application cannot find a better way to achieve what it wanted
>> to do in the first place, or unless that application already has a
>> large install base
>> which suffers from *a problem*, you will not have proven *the need*.
> Maybe I should ask devs of some large apps on their take of this issue.

Nonsense, because they are already using:
a) the functions available by an FS,
b) the functions available by a DBMS, or
c) a propritary special solution based on the available functions of the 
OS and additional functionality that they develope and maintain themselves
for their comparable use cases since decades due to the cost vs. benefit 
ratio.

>> Maybe preserving privileged extended attributes is *a need*.
>> I wouldn't know myself.
> Olaf

Christian Stroetmann
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