Message-ID: <3BCC2E22.4010400@antefacto.com>
Date: Tue, 16 Oct 2001 13:54:58 +0100
From: Padraig Brady <padraig@antefacto.com>
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To: "Eric W. Biederman" <ebiederm@xmission.com>
CC: Pierre PEIFFER <pierre.peiffer@sxb.bsf.alcatel.fr>,
        linux-kernel@vger.kernel.org
Subject: Re: e2compress in kernel 2.4
In-Reply-To: <3BBACF29.7BB980C4@sxb.bsf.alcatel.fr> <m1ofno3lnp.fsf@frodo.biederman.org>
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Would it not be better to use JFFS2 and the new blkmtd driver
(which makes any block device appear as an MTD device)?
http://lists.infradead.org/pipermail/linux-mtd/2001-June/002711.html

Padraig.

Eric W. Biederman wrote:

>Pierre PEIFFER <pierre.peiffer@sxb.bsf.alcatel.fr> writes:
>
>>Hi !
>>
>>    We are willing to port e2compress from 2.2 kernel series to 2.4 and
>>we are looking for the right way for porting the compression on the
>>write part.
>>
>>    For the read operation, we can adapt the original design: the 2.2
>>part of e2compress can be easily integrated in the 2.4 version; for the
>>write, it is a little bit more complicated...
>>
>
>I'm not certain you even want to reuse the read path, as is from 2.2
>
>>    So, here, we are a little bit confused because we don't know where
>>to introduce the compression, if we keep the same idea of the 2.2
>>design... In fact, on one hand, once the buffers will be compressed, the
>>pages will also become compressed, but on the other hand, we don't want
>>the pages to be compressed, because, the pages, once registered and
>>linked to the inode are supposed to be uncompressed...
>>
>>    So our idea was to introduce the notion of "cluster of pages", as
>>the notion of cluster of blocks, i.e. performs the write on several
>>pages at a time, then compress the buffers corresponding to these pages,
>>but here the data of the buffers should be splitted up from the data of
>>the pages and that's our problem... We don't know how to do this. Is
>>there a way to do this ?
>>
>
>You can't reuse the page cache buffers, so some amount of double buffering
>is needed.  The "cluster of pages" idea is already in the e2compr on-disk
>format so it is natural.  Doing the compression only at close (as is
>done in the 2.0 version ) may also be appropriate.  In either case
>what you need is an extra address_space per inode.  In the extra
>address space you can keep your compressed data.  
>
>The index on the compressed data should be something like
>(compressed_block * compressed_block_size) +
>index_into_compressed_block.  
>
>The problems you face are similiar to those faced by journaling and
>more so by delayed disk block allocation.  If you can get delayed
>allocation going then there is a good chance you can reduce
>fragmentation by only writing the data compressed, and reading
>and uncompressing the data on the fly.  
>
>Note: delayed allocation is a much easier problem than journalling
>as writes may be flushed anytime memory is low.  Though when you
>throw compression into the mix you might have another set of problems.
>
>2.4 should be able to handle logical disk blocks > PAGE_SIZE just
>fine if your write routine can handle gathering up a couple of them.
>
>>    And, from a more general point of view, do you think our approach
>>has a chance to succeed ?
>>
>
>I think you want to step back and understand the page cache in 2.4.
>It should be much easier to work with then going through the buffer
>cache was in 2.2 and earlier but it is going to require some
>noticeable algorithm changes, on how reads and writes are handled.
>
>Also please keep me in the loop.  I can't commit to anything but I'm
>just about interested enough to implement some of the needed changes.
>
>Eric
>


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