Date:	Fri, 15 Jun 2012 16:25:45 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Ted Ts'o <tytso@....edu>
Cc:	Arnd Bergmann <arnd.bergmann@...aro.org>,
	Alex Lemberg <Alex.Lemberg@...disk.com>,
	HYOJIN JEONG <syr.jeong@...sung.com>,
	Saugata Das <saugata.das@...aro.org>,
	Artem Bityutskiy <dedekind1@...il.com>,
	Saugata Das <saugata.das@...ricsson.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mmc@...r.kernel.org, patches@...aro.org, venkat@...aro.org,
	"Luca Porzio (lporzio)" <lporzio@...ron.com>
Subject: Re: [PATCH 2/3] ext4: Context support

On 2012-06-15, at 4:04 PM, Ted Ts'o wrote:
> On Thu, Jun 14, 2012 at 09:55:31PM +0000, Arnd Bergmann wrote:
>> There is one more option we have to give the best possible performance,
>> although that would be a huge amount of work to implement:
>> 
>> Any large file gets put into its own context, and we mark that
>> context "write-only" "unreliable" and "large-unit". This means the
>> file system has to write the file sequentially, filling one erase
>> block at a time, writing only "superpage" units (e.g. 16KB) or
>> multiples of that at once. We can neither overwrite nor read back
>> any of the data in that context until it is closed, and there is
>> no guarantee that any of the data has made it to the physical medium
>> before the context is closed. We are allowed to do read and write
>> accesses to any other context between superpage writes though.
>> After closing the context, the data will be just like any other
>> block again.
> 
> Oh, that's cool.  And I don't think that's hard to do.  We could just
> keep a flag in the in-core inode indicating whether it is in "large
> unit" mode.  If it is in large unit mode, we can make the fs writeback
> function make sure that we adhere to the restrictions of the large
> unit mode, and if at any point we need to do something that might
> violate the constraints, the file system would simply close the
> context.

This is very similar to what was implemented in mballoc preallocation.
Large files will get their own preallocation context, while small files
would share a context (i.e. an 8MB extent) and be packed densely into
this extent to avoid seeking.  It wouldn't be unreasonable to just give
each mballoc context a different eMMC context.

> The only reason I can think of why this might be problematic is if
> there is a substantial performance cost involved with opening and
> closing contexts on eMMC devices.  Is that an issue we need to be
> worried about?
> 
>> Right now, there is no support for large-unit context and also not for
>> read-only or write-only contexts, which means we don't have to
>> enforce strict policies and can basically treat the context ID
>> as a hint. Using the advanced features would require that we
>> keep track of the context IDs across partitions and have to flush
>> write-only contexts before reading the data again. If we want to
>> do that, we can probably discard the patch series and start over.
> 
> Well, I'm interested in getting something upstream, which is useful
> not just for the consumer-grade eMMC devices in handsets, but which
> might also be extensible to SSD's, and all the way up to PCIe-attached
> flash devices that might be used in large data centers.
> 
> I think if we do things right, it should be possible to do something
> which would accomodate a large range of devices (which is why I
> brought up the concept of exposing virtualized contexts to the file
> system layer).
> 
> Regards,
> 
> 						- Ted
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Cheers, Andreas





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