Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932257AbVIKCQY (ORCPT ); Sat, 10 Sep 2005 22:16:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932408AbVIKCQX (ORCPT ); Sat, 10 Sep 2005 22:16:23 -0400 Received: from omta03sl.mx.bigpond.com ([144.140.92.155]:35572 "EHLO omta03sl.mx.bigpond.com") by vger.kernel.org with ESMTP id S932257AbVIKCQX (ORCPT ); Sat, 10 Sep 2005 22:16:23 -0400 Message-ID: <43239374.8010604@eyal.emu.id.au> Date: Sun, 11 Sep 2005 12:16:20 +1000 From: Eyal Lebedinsky Organization: Eyal at Home User-Agent: Debian Thunderbird 1.0.2 (X11/20050817) X-Accept-Language: en-us, en MIME-Version: 1.0 To: Nuno Silva CC: list linux-kernel Subject: Re: RAID resync speed References: <432240E9.9010400@eyal.emu.id.au> <43224ABB.3030002@vgertech.com> <4322506A.1010303@eyal.emu.id.au> <43226701.1000606@vgertech.com> In-Reply-To: <43226701.1000606@vgertech.com> X-Enigmail-Version: 0.91.0.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1822 Lines: 53 Nuno Silva wrote: > Eyal Lebedinsky wrote: > >> Nuno Silva wrote: >> >>> Hi, >>> Eyal Lebedinsky wrote: >>> >>>> I noticed that my 3-disk RAID was syncing at about 40MB/s, now that I >>>> added a fourth disk it goes at only 20+MB/s. This is on an idle >>>> machine. >>> >>> 3*40=120 >>> 4*20=80 >> >> What does this mean? The raid is syncing at 20MB/s, not each disk, so >> I do >> not see what the multiplication is about. > > Yes, you're correct :-) Actually, I took another look at this matter and I now think that you had the correct approach. The rebuild speed is the speed at which the new disk is being built, not the total rebuild i/o. This means that it does not contain the read operations. So the PCI limit is a limiting factor. On a 32-bit 33MHz PCI controller (132MB/s theoretical bandwidth) a 2->3 rebuild cannot be faster 44MB/s and a 3->4 is limited to 33MB/s. I think this is true. The same limit will also apply to any raid i/o as we read/write to all the disks for any data. To use 5 60MB/s disks I will need 300MB/s bandwidth which a 64-bit 66MHz PCI can deliver. A 32-bit/66MHz will come close - what can PCIe do?. A proper RAID card will alleviate the PCI limitation as it will have dedicated channels for each disk (well, a good controller should) with full bandwidth and the PCI will only need to go at the one-disk speed (for raid-5). On-board SATA controllers will have better bandwidth if they sit on a better than PCI bus (or on more than one PCI bus). -- Eyal Lebedinsky (eyal@eyal.emu.id.au) attach .zip as .dat - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/