From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from pigeon.gentoo.org ([208.92.234.80] helo=lists.gentoo.org) by finch.gentoo.org with esmtp (Exim 4.60) (envelope-from ) id 1NeDSI-0005hi-Hm for garchives@archives.gentoo.org; Sun, 07 Feb 2010 20:10:06 +0000 Received: from pigeon.gentoo.org (localhost [127.0.0.1]) by pigeon.gentoo.org (Postfix) with SMTP id D7831E0D55 for ; Sun, 7 Feb 2010 20:10:05 +0000 (UTC) Received: from sentinel.math.Princeton.EDU (sentinel.math.Princeton.EDU [128.112.16.31]) by pigeon.gentoo.org (Postfix) with ESMTP id 45E51E0B7A for ; Sun, 7 Feb 2010 19:39:48 +0000 (UTC) Received: from math.princeton.edu ([128.112.18.16]) by sentinel.math.Princeton.EDU with esmtp (Exim 4.69) (envelope-from ) id 1NeCyx-0000FB-8k for gentoo-user@lists.gentoo.org; Sun, 07 Feb 2010 14:39:48 -0500 Received: from math.Princeton.EDU (localhost.localdomain [127.0.0.1]) by math.Princeton.EDU (8.13.8/8.13.8) with ESMTP id o17Jdlub032035 for ; Sun, 7 Feb 2010 14:39:47 -0500 Received: (from wwong@localhost) by math.Princeton.EDU (8.13.8/8.13.8/Submit) id o17JdlS8032032 for gentoo-user@lists.gentoo.org; Sun, 7 Feb 2010 14:39:47 -0500 Date: Sun, 7 Feb 2010 14:39:47 -0500 From: Willie Wong To: gentoo-user@lists.gentoo.org Subject: Re: [gentoo-user] 1-Terabyte drives - 4K sector sizes? -> bar performance so far Message-ID: <20100207193947.GB30196@math.princeton.edu> References: <5bdc1c8b1002070827i14f59047k39a695900ebe9889@mail.gmail.com> Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-Id: Gentoo Linux mail X-BeenThere: gentoo-user@lists.gentoo.org Reply-to: gentoo-user@lists.gentoo.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5bdc1c8b1002070827i14f59047k39a695900ebe9889@mail.gmail.com> User-Agent: Mutt/1.4.2.2i X-Archives-Salt: d2af43bb-78c5-48e7-b123-4a0544123a1a X-Archives-Hash: 9cf62284d762e290025e7e6117985107 On Sun, Feb 07, 2010 at 08:27:46AM -0800, Mark Knecht wrote: > > 4KB physical sectors: KNOW WHAT YOU'RE DOING! > > Pros: Quiet, cool-running, big cache > > Cons: The 4KB physical sectors are a problem waiting to happen. If you > misalign your partitions, disk performance can suffer. I ran > benchmarks in Linux using a number of filesystems, and I found that > with most filesystems, read performance and write performance with > large files didn't suffer with misaligned partitions, but writes of > many small files (unpacking a Linux kernel archive) could take several > times as long with misaligned partitions as with aligned partitions. > WD's advice about who needs to be concerned is overly simplistic, > IMHO, and it's flat-out wrong for Linux, although it's probably > accurate for 90% of buyers (those who run Windows or Mac OS and use > their standard partitioning tools). If you're not part of that 90%, > though, and if you don't fully understand this new technology and how > to handle it, buy a drive with conventional 512-byte sectors! > > > Now, I don't mind getting a bit dirty learning to use this > correctly but I'm wondering what that means in a practical sense. > Reading the mke2fs man page the word 'sector' doesn't come up. It's my > understanding the Linux 'blocks' are groups of sectors. True? If the > disk must use 4K sectors then what - the smallest block has to be 4K > and I'm using 1 sector per block? It seems that ext3 doesn't support > anything larger than 4K? The problem is not when you are making the filesystem with mke2fs, but when you partitioned the disk using fdisk. I'm sure I am making some small mistakes in the explanation below, but it goes something like this: a) The harddrive with 4K sectors allows the head to efficiently read/write 4K sized blocks at a time. b) However, to be compatible in hardware, the harddrive allows 512B sized blocks to be addressed. In reality, this means that you can individually address the 8 512B-sized chunks of the 4K sized blocks, but each will count as a separate operation. To illustrate: say the hardware has some sector X of size 4K. It has 8 addressable slots inside X1 ... X8 each of size 512B. If your OS clusters read/writes on the 512B level, it will send 8 commands to read the info in those 8 blocks separately. If your OS clusters in 4K, it will send one command. So in the stupid analysis I give here, it will take 8 times as long for the 512B addressing to read the same data, since it will take 8 passes, and each time inefficiently reading only 1/8 of the data required. Now in reality, drives are smarter than that: if all 8 of those are sent in sequence, sometimes the drives will cluster them together in one read. c) A problem occurs, however, when your OS deals with 4K clusters but when you make the partition, the partition is offset! Imagine the physical read sectors of your disk looking like AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDD but when you make your partitions, somehow you partitioned it ....YYYYYYYYZZZZZZZZWWWWWWWW.... This is possible because the drive allows addressing by 512K chunks. So for some reason one of your partitions starts halfway inside a physical sector. What is the problem with this? Now suppose your OS sends data to be written to the ZZZZZZZZ block. If it were completely aligned, the drive will just go kink-move the head to the block, and overwrite it with this information. But since half of the block is over the BBBB phsical sector, and half over CCCC, what the disk now needs to do is to pass 1) read BBBBBBBB pass 2) modify the second half of BBBB to match the first half of ZZZZ pass 3) write BBBBBBBB pass 4) read CCCCCCCC pass 5) modify the first half of CCCC to match the second half of ZZZZ pass 6) write CCCCCCCC Or what is known as a read-modify-write operation. Thus the disk becomes a lot less efficient. ---------- Now, I don't know if this is the actual problem is causing your performance problems. But this may be it. When you use fdisk, it defaults to aligning the partition to cylinder boundaries, and use the default (from ancient times) value of 63 x (512B sized) sectors per track. Since 63 is not evenly divisible by 8, you see that quite likely some of your partitions are not aligned to the physical sector boundaries. If you use cfdisk, you can try to change the geometry with the command g. Or you can use the command u to change the units used in the partitioning to either sectors or megabytes, and make sure your partition sizes are a multiple of 8 in the former, or an integer in the latter. Again, take what I wrote with a grain of salt: this information came from the research I did a little while back after reading the slashdot article on this 4K switch. So being my own understanding, it may not completely be correct. HTH, W -- Willie W. Wong wwong@math.princeton.edu Data aequatione quotcunque fluentes quantitae involvente fluxiones invenire et vice versa ~~~ I. Newton