Date: Tue, 16 Jan 2007 13:01:15 +0300 From: "Andrew Pantyukhin" <infofarmer@FreeBSD.org> To: "Stefan Lambrev" <stefan.lambrev@sun-fish.com> Cc: Josef Karthauser <joe@tao.org.uk>, stable@freebsd.org, fs@freebsd.org Subject: Re: Dell hardware raid 0 (sas5ir) or gmirror? Message-ID: <cb5206420701160201m4765d262gf7edd059e8e908e@mail.gmail.com> In-Reply-To: <45AC9418.60802@sun-fish.com> References: <20070115112106.GA2304@genius.tao.org.uk> <cb5206420701150600k57212896s3ef664beb4107ae0@mail.gmail.com> <45AC9418.60802@sun-fish.com>
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On 1/16/07, Stefan Lambrev <stefan.lambrev@sun-fish.com> wrote: > Andrew Pantyukhin wrote: > > On 1/15/07, Josef Karthauser <joe@tao.org.uk> wrote: > >> I'm purchasing a new server, and was wondering what anyone thought about > >> whether to pay extra for the SAS5IR card so I can RAID0 the two drives, > >> or whether to just rely on gmirror. My worry about the former is that I > >> can't seem to find management tools for controlling the hardware > >> controller. What if one of the drives fails? How would I know? > > > > By all means I would go the gmirror way, and I always do > > even when a hardware raid controller is already present. > > I really do not understand this. :) > When you say something like this it will be good to explain why you > think so. > I have few servers with good hw raid controllers and I'm very happy with > them, I also use > gmirror on my desktop pc, but it is not as good as hw raid on servers > for sure. > Also it is harder to support it (during OS updates and etc). > Also it (gmirror) will put some load on the CPU and hw raid have it's > own CPU/memory for this. > LSI have a nice tool to monitor/config RAID arrays that just works under > fbsd in my case so I'm happy with it. > There are a lot of reasons to use hw raids on mission critical servers... As a matter of fact Jonathan was also surprised by my answer. Here's a part of my response to him: ===================================================== raid3, raid5 and other computation-hungry configu- rations are a cpu hog, that's why people prefer hardware controllers for that. I'm quite sure at some point FreeBSD will gain an ability to use crypto/XOR hardware for the benefit of software raid performance and maybe then software raid5 will become a popular solution. As for raid0/raid1 - there's no cpu penalty at all. gmirror/gstripe in FreeBSD might need further tweaks and optimizations, but benchmarks show that with 2-4 drives performance almost equals the theoretical limits. Reliability of OS-integrated software raid is expected to be even higher than that of hardware one, because there's no hardware to fail and software bugs might be found in all solutions. What I really like about software raid is very high flexibility and manageability. There's no issue of having the right driver or the right userland tool, it just works. And it's a snap to setup. And you are free to experiment with virtual (file-based, for one) file systems before you implement a solution. As always, there are more than one correct opinions. I just expressed my own and I hope my explanation answers some of your interest. ===================================================== What I would add to answer some of your claims, Stefan, is that there's no single correct solution here. I would argue that money spent on main CPU/RAM are a better investment compared to a hardware raid 0/1 solution, OS buffers are there anyway, so why not make them larger/faster if you need that. As for CPU load, I'd argue it's negligible, at least in 2 SATA HDD situations (which are popular in all markets). For larger configurations, consisting of 5 drives and more I would advise against raid 0/1 and therefore against software raid. If you do continue to use gmirror/gstripe, I would expect some tweaks to be needed, but in general such systems should scale very well, especially on SMP systems, as FreeBSD 6.x brought mpsafe file system access to the table. All in all there are reasons to use hw raids and there are some not to use them. For some reasons I hold our homegrown (FreeBSD) solutions closer to my heart and choose them in favor of 3d-party ones. Cheers!
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