From mboxrd@z Thu Jan 1 00:00:00 1970 From: Steven Dake Subject: Re: A modern RAID solution? Date: Mon, 28 Oct 2002 11:17:40 -0700 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <3DBD7F44.3000804@mvista.com> References: <20021027153048.GA5642@angle.setup.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: List-Id: linux-scsi@vger.kernel.org To: Alexy Khrabrov Cc: linux-scsi@vger.kernel.org Alexy, As a previous RAID card engineer, I can definately say if your system is fast you won't see much improvement from an I/O card based RAID adaptor for either RAID 0 or RAID 1. Keep in mind that the OS must still send messages to each adaptor over the PCI bus so the data transferred is nearly the same and with SCSI command qeueing each disk can accept multiple commands at once. RAID 5 will see improvement because the I/O processors have special hardware to do 8 or 16 parallel XOR calculations. Also generally I/O processors on the RAID cards run at much slower speeds then the 2 ghz standard PC these days. One advantage of a hardware raid card is that is can accept more commands in an I/O queue (reducing read/write response time but improving performance). There aren't alot of applications where this is useful unless you do alot of writing and not much reading or the raid card implements a special i/o channel for reads (which none do that I am aware of). The adaptec "host raid 0/1" means that the host machine (your pc) is responsible for the striping or mirroring. The adaptec adaptor just provides "bios mapping" to the disk layout specified by the design so the device can be booted. This is equivalent to the MD driver except the MD driver is probably more optimized then Adaptec's driver if they have one for linux. This is also how the "IDE RAID" cards work just providing BIOS mapping so int13s i/os match the layout of the disks. Alexy Khrabrov wrote: >Now that I got all those drives spinning, I'm eager to try out >a RAID array. I reckon it's a better way to hedge against disk >failures than backups on tape -- even 20/40 GB dds4 or DLT is >not enough those days, and loading/rotating is tedious. > >I heard an opinion that software RAID on Linux with SCSI is >"almost" as good as a hardware controller. What is the experience >here? Also, since I'm running LVM throughout, including the root >partition, does RAID coexist well with LVM? > >But if I go the way of the hardware controller, is it better >to get a separate one, or one of those new cards from Adaptec >which say they have a Host RAID 0/1 or some such? > >I'm looking at a 3 drives RAID to begin with, perhaps 4. > > >