linux-kernel - Re: [RFC/PATCH 0/2] ext4: Transparent Decompression Support

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Date:	Thu, 25 Jul 2013 15:27:19 -0400
From:	Dhaval Giani <dgiani@...illa.com>
To:	Jörn Engel <joern@...fs.org>
CC:	Taras Glek <tglek@...illa.com>, linux-kernel@...r.kernel.org,
	tytso@....edu, vdjeric@...illa.com, glandium@...illa.com,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC/PATCH 0/2] ext4: Transparent Decompression Support

On 2013-07-25 1:53 PM, Jörn Engel wrote:
> On Thu, 25 July 2013 09:42:18 -0700, Taras Glek wrote:
>> Footprint wins are useful on android, but it's the
>> increased IO throughput on crappy storage devices that makes this
>> most attractive.
> All the world used to be a PC.  Seems to be Android these days.
>
> The biggest problem with compression support in the past was the
> physical properties of hard drives (the spinning type, if you can
> still remember those).  A random seek is surprisingly expensive, of a
> similar cost to 1MB or more of linear read.  So anything that
> introduces more random seeks will kill the preciously little
> performance you had to begin with.
>
> As long as files are write-once and read-only from that point on, you
> can just append a bunch of compressed chunks on the disk and nothing
> bad happens.  But if you have a read-write file with random overwrites
> somewhere in the middle, those overwrites will change the size of the
> compressed data.  You have to free the old physical blocks on disk and
> allocate new ones.  In effect, you have auto-fragmentation.
>
> So if you want any kind of support for your approach, I suspect you
> should either limit it to write-once files or prepare for a mob of
> gray-haired oldtimers with rainbow suspenders complaining about
> performance on their antiquated hardware.  And the mob may be larger
> than you think.

Yes, we plan to limit it to write-once. In order to write, you have to
replace the file.

Dhaval
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