From mboxrd@z Thu Jan 1 00:00:00 1970 From: Max Waterman Subject: Re: RAID 1 vs RAID 0 Date: Wed, 18 Jan 2006 16:00:03 +0800 Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids Mark Hahn wrote: >> They seem to suggest RAID 0 is faster for reading than RAID 1, and I >> can't figure out why. > > with R0, streaming from two disks involves no seeks; > with R1, a single stream will have to read, say 0-64K from the first disk, > and 64-128K from the second. these could happen at the same time, and > would indeed match R0 bandwidth. but with R1, each disk has to seek past > the blocks being read from the other disk. seeking tends to be slow... Ah, a good way of putting it...I think I was pretty much there with my followup message. Still, it seems like it should be a solvable problem...if you order the data differently on each disk; for example, in the two disk case, putting odd and even numbered 'stripes' on different platters [or sides of platters]. > >> Clearly, the write performance is worse for RAID 1 than RAID 0 since >> with RAID 1 that data you are writing at the same time is the same for >> both drives; > > the cost for doing the double writes in R1 is not high, unless you've > already got a bottleneck somewhere that limits you to talking to one disk > at a time. for instance, R1 to a pair of disks at 50 MB/s apiece is > basically trivial for a decent server, since it's about 1% of memory > bandwidth, and a smallish fraction of even plain old 64x66 PCI. > >> array has more than two disks, that would make RAID 1 *faster* than RAID 0. > > R1 is not going to be faster than R0 on the same number of disks. Yeah, I think I see that now. Thanks. Max. > > regards, mark hahn. > > - > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >