Re: [PATCH 15/26] Btrfs: add a new source file with device replace code

[Stefan Behrens <sbehrens@giantdisaster.de>]

On Fri, 9 Nov 2012 22:45:16 +0800, Liu Bo wrote:
> On Fri, Nov 09, 2012 at 11:19:17AM +0100, Stefan Behrens wrote:
>> On Fri, 9 Nov 2012 08:44:01 +0800, Liu Bo wrote:
>>> On Thu, Nov 08, 2012 at 06:24:36PM +0100, Stefan Behrens wrote:
>>>> On Thu, 8 Nov 2012 22:50:47 +0800, Liu Bo wrote:
>>>>> On Tue, Nov 06, 2012 at 05:38:33PM +0100, Stefan Behrens wrote:
>>>>>> +	trans = btrfs_start_transaction(root, 0);
>>>>>
>>>>> why a start_transaction here?  Any reasons?
>>>>> (same question also for some other places)
>>>>>
>>>>
>>>> Without this transaction, there is outstanding I/O which is not flushed.
>>>> Pending writes that go only to the old disk need to be flushed before
>>>> the mode is switched to write all live data to the source disk and to
>>>> the target disk as well. The copy operation that is part of the scrub
>>>> code works on the commit root for performance reasons. Every write
>>>> request that is performed after the commit root is established needs to
>>>> go to both disks. Those requests that already have the bdev assigned
>>>> (i.e., btrfs_map_bio() was already called) cannot be duplicated anymore
>>>> to write to the new disk as well.
>>>>
>>>> btrfs_dev_replace_finishing() looks similar and goes through a
>>>> transaction commit between the steps where the bdev in the mapping tree
>>>> is swapped and the step when the old bdev is freed. Otherwise the bdev
>>>> would be accessed after being freed.
>>>>
>>>
>>> I see, if you're only about to flush metadata, why not join a transaction?
>>
>> btrfs_join_transaction() would delay the current transaction and enforce
>> that the current transaction is used and not a new one.
>> btrfs_start_transaction() would use either the current transaction, or a
>> new one. It is less interfering.
> 
> hmm...btrfs_start_transaction() would not use the current transaction unless
> you're still in the same task, ie. current->journal_info remains unchanged,
> otherwise it will be blocked by the current transaction(wait_current_trans()).
> 
> If there are several btrfs_start_transaction() being blocked, after the current
> one's commit, one of them will allocate a new transaction, and the rest can join it.
> 
> But btrfs_join_transaction will join the current as much as possible.
> 
> And since here we don't do any reservation and seems to just update chunk/device
> tree(which will use global block rsv directly), I perfer btrfs_join_transaction().
> 

I am still not sure, which one is worse or better:
a) to delay a commit by calling btrfs_join_transaction() which joins and thereby delays a transaction, or
b) to go through one additional transaction.

Here is the log message of the commit that added btrfs_join_transaction(). For me, it sounds like one should use btrfs_join_transaction() only when it is _required_ to join a transaction, e.g. when a low level function is required to join the transaction that some higher level function has started:

commit f9295749388f82c8d2f485e99c72cd7c7876a99b
Author: Chris Mason <chris.mason@oracle.com>
Date:   Thu Jul 17 12:54:14 2008 -0400

    btrfs_start_transaction: wait for commits in progress to finish

    btrfs_commit_transaction has to loop waiting for any writers in the
    transaction to finish before it can proceed.  btrfs_start_transaction
    should be polite and not join a transaction that is in the process
    of being finished off.

    There are a few places that can't wait, basically the ones doing IO that
    might be needed to finish the transaction.  For them, btrfs_join_transaction
    is added.



>>
>> Since in dev-replace.c it is not required to enforce that a current
>> transaction is joined, btrfs_start_transaction() is the one to choose
>> here, as I understood it.
>>
>> But that's an interesting topic and I would appreciate to get a definite
>> rule which one to choose when.