From: Dhaval Giani Subject: Re: [RFC/PATCH 0/2] ext4: Transparent Decompression Support Date: Thu, 25 Jul 2013 15:27:19 -0400 Message-ID: <51F17C17.2010702@mozilla.com> References: <1374699833.7083.2.camel@localhost> <20130724233628.GD3641@logfs.org> <51F14136.30409@mozilla.com> <51F1556A.20909@mozilla.com> <20130725175307.GA15590@logfs.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Taras Glek , linux-kernel@vger.kernel.org, tytso@mit.edu, vdjeric@mozilla.com, glandium@mozilla.com, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org To: =?UTF-8?B?SsO2cm4gRW5nZWw=?= Return-path: In-Reply-To: <20130725175307.GA15590@logfs.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 2013-07-25 1:53 PM, J=C3=B6rn 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 overwrite= s > somewhere in the middle, those overwrites will change the size of the > compressed data. You have to free the old physical blocks on disk an= d > 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