From: Theodore Tso Subject: Re: bigalloc and max file size Date: Thu, 27 Oct 2011 02:35:05 -0400 Message-ID: <5846CEDC-A1ED-4BB4-8A3E-E726E696D3E9@mit.edu> References: <51BECC2B-2EBC-4FCB-B708-8431F7CB6E0D@dilger.ca> Mime-Version: 1.0 (Apple Message framework v1251.1) Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Theodore Tso , linux-ext4 development , Alex Zhuravlev To: Andreas Dilger Return-path: Received: from DMZ-MAILSEC-SCANNER-4.MIT.EDU ([18.9.25.15]:52309 "EHLO dmz-mailsec-scanner-4.mit.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750846Ab1J0GkN convert rfc822-to-8bit (ORCPT ); Thu, 27 Oct 2011 02:40:13 -0400 In-Reply-To: <51BECC2B-2EBC-4FCB-B708-8431F7CB6E0D@dilger.ca> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Oct 26, 2011, at 7:36 PM, Andreas Dilger wrote: > Ted, > we were having a discussion about bigalloc and the maximum file size > (as limited by the 2^32 logical block number in struct ext4_extent). >=20 > Currently the maximum file size is blocksize * 2^32, 16TB for 4kB blo= cks. >=20 > Since it is not possible to allocate sub-blocks in the bigalloc code, > what about storing the "chunk number" in the extent logical block? This adds all sorts of complexity, and it's why we can't simply increas= e the block size above a page size. Basically, avoiding all of the com= plex changes needed was the whole point of the bigalloc feature. Effectively, if we stored the "chunk number" in the extent tree blocks,= it's equivalent of setting the block size to the larger chunk size. A= mong other things, it means that we have to support setting or clearing= the uninitialized bit in chunks larger than a page, and how dirtying a= 4k page in a sparse file would require clearing 64k or 1 megabyte (wha= tever the chunk size might be) either on disk or in memory. In any case, it's not a simple change that we can make before the merge= window. If someone wants to work on that, we can support that simply= by supporting block size > page size, with all of the complexity and p= ain which has prevented us from supporting that for the last ten years=85= =2E -- Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html