From: Andreas Dilger Subject: Re: Extent Depth Histogram Fsck Date: Tue, 26 Jun 2012 14:07:51 -0600 Message-ID: References: <0408C81F72528E40A0D3235A1F67FFC804A37F@SN2PRD0202MB144.namprd02.prod.outlook.com> <55654AE5-17FE-44EC-B7FE-A8D5FAAB3560@dilger.ca> <0408C81F72528E40A0D3235A1F67FFC804A3B3@SN2PRD0202MB144.namprd02.prod.outlook.com> Mime-Version: 1.0 (1.0) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8BIT Cc: Andreas Dilger , "linux-ext4@vger.kernel.org" To: "Nelson, John R" Return-path: Received: from mail-pb0-f46.google.com ([209.85.160.46]:60198 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751960Ab2FZUHh convert rfc822-to-8bit (ORCPT ); Tue, 26 Jun 2012 16:07:37 -0400 Received: by pbbrp8 with SMTP id rp8so529537pbb.19 for ; Tue, 26 Jun 2012 13:07:36 -0700 (PDT) In-Reply-To: <0408C81F72528E40A0D3235A1F67FFC804A3B3@SN2PRD0202MB144.namprd02.prod.outlook.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On 2012-06-26, at 12:07, "Nelson, John R" wrote: > ok i see! > So when there are like > 3/3/4 that means double index blocks?? How many extents can a single extent index hold in a block? The header takes 12 bytes, and each extent or index pointer takes 12 bytes, so for 4kB blocksize there can be (4096 / 12) - 1 = 340 extents per block. The maximum extent size for 4kB blocks is 2^15*4kB = 128MB, so each index block can map up to ~42GB, so a two-level tree can map just over 14TB under ideal conditions. Cheers, Andreas > ________________________________________ > From: Andreas Dilger [adilger@dilger.ca] > Sent: Tuesday, June 26, 2012 11:40 AM > To: Nelson, John R > Cc: linux-ext4@vger.kernel.org > Subject: Re: Extent Depth Histogram Fsck > > On 2012-06-26, at 8:34 AM, Nelson, John R wrote: >> What does the extent depth histogram mean? Is it a measure of something? >> >> like mine is >> >> >> Extent depth histogram: 36010/81 > > This means that of all the extent-mapped files in the filesystem, > 36010 files have an extent tree of depth 0 (i.e. they fit inside the inode) > 81 files have an extent tree of depth 1 (i.e. there is a single index block) > > Typically, files larger than 4 * 128MB = 512MB need an index block, but if > the maximum-sized extents cannot be allocated then an index block will be needed for smaller files. Only if you have very large files (> 40GB), > or a very fragmented free space would you need more than a single level > of index blocks. > > Cheers, Andreas > > > > > > >