linux-kernel - Re: Linux 2.6.29

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Date:	Wed, 1 Apr 2009 21:55:18 -0700 (PDT)
From:	david@...g.hm
To:	Bron Gondwana <brong@...tmail.fm>
cc:	"Andreas T.Auer" <andreas.t.auer_lkml_73537@...us.ath.cx>,
	Bill Davidsen <davidsen@....com>, linux-kernel@...r.kernel.org
Subject: Re: Linux 2.6.29

On Thu, 2 Apr 2009, Bron Gondwana wrote:

> On Wed, Apr 01, 2009 at 03:29:29PM -0700, david@...g.hm wrote:
>> On Wed, 1 Apr 2009, Andreas T.Auer wrote:
>>> On 01.04.2009 22:15 david@...g.hm wrote:
>>>> except if another file in the directory gets modified while it's
>>>> writing out the first two, that file now would need to get written out
>>>> as well, before the metadata for that directory can be written. if you
>>>> have a busy system (say a database or log server), where files are
>>>> getting modified pretty constantly, it can be a long time before all
>>>> the file data is written out and the system is idle enough to write
>>>> the metadata.
>>> Thank you, David, for this use case, but I think the problem could be
>>> solved quite easily:
>>>
>>> At any write-out time, e.g. after collecting enough data for delayed
>>> allocation or at fsync()
>>>
>>> 1) copy the metadata in memory, i.e. snapshot it
>>> 2) write out the data corresponding to the metadata-snapshot
>>> 3) write out the snapshot of the metadata
>>>
>>> In that way subsequent metadata changes should not interfere with the
>>> metadata-update on disk.
>>
>> the problem with this approach is that the dcache has no provision for
>> there being two (or more) copies of the disk block in it's cache, adding
>> this would significantly complicate things (it was mentioned briefly a
>> few days ago in this thread)
>
> It seems that it's obviously the "right way" to solve the problem
> though.  How much does the dcache need to know about this "in flight"
> block (ok, blocks - I can imagine a pathological case where there
> were a stack of them all slightly different in the queue)?

but if only one filesystem needs this caability is it really worth 
complicating the dcache for the entire system?

> You'd be basically reinventing MVCC-like database logic with
> transactional commits at that point - so each fs "barrier" call
> would COW all the affected pages and write them down to disk.

one aspect of mvcc systems is that they eat up space and require 'garbage 
collection' type functions. that could cause deadlocks if you aren't 
careful.

David Lang
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