From mboxrd@z Thu Jan 1 00:00:00 1970 From: torn5 Subject: Ext4 allocation strategy Date: Fri, 12 Aug 2011 15:47:00 +0200 Message-ID: <4E452ED4.9000307@shiftmail.org> Mime-Version: 1.0 Content-Type: text/plain; format=flowed; charset=UTF-8 Content-Transfer-Encoding: 7bit To: linux-ext4@vger.kernel.org Return-path: Received: from blade3.isti.cnr.it ([194.119.192.19]:50722 "EHLO BLADE3.ISTI.CNR.IT" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752517Ab1HLNzH (ORCPT ); Fri, 12 Aug 2011 09:55:07 -0400 Received: from [192.168.1.112] (firewall.itb.cnr.it [155.253.6.254]) by mx.isti.cnr.it (PMDF V6.5-x5 #31826) with ESMTPSA id <01O4RNUWZG0MWT79LN@mx.isti.cnr.it> for linux-ext4@vger.kernel.org; Fri, 12 Aug 2011 15:47:00 +0200 (MEST) Sender: linux-ext4-owner@vger.kernel.org List-ID: Hi all, I can't find info on the ext4 allocation strategy, while this would be useful to me to understand ext4 performance implications when I put it above complex layers of RAID and LVM. Just to be clear, i'm NOT talking about the delayed allocation thing. XFS has the "Allocation Groups" which are relatively well described, for ext4 I don't know what to look for. And I also can't find an application which gives me a visual map of the ext4 allocated blocks in my filesystem so I cannot easily "inspect" this by myself. So can you tell me something? In particular, does ext4 start from the beginning of the partition and goes upward filling the earliest free spaces like it was for the old FAT32, or does it try to spread allocations evenly on all the LBAs available to that partition, e.g. by bisecting the largest free space every time? Suppose some time passes from one file creation to another. And how does it behave if it has to create zillions of small files simultaneously (multithreaded application): where does it place them? Thank you