> On Tue, 4 Mar 2008, Ian G Batten wrote: > >> software RAID5 is a performance >> disaster area at the best of times unless it can take advantage of >> intimate knowledge of the intent log in the filesystem (RAID-Z does >> this), > > actually, unless you have top-notch hardware raid controllers, software > raid 5 > may be better then hardware raid 5. many controllers only do a decent job > doing > raid 0 or raid 1. this is something to measure with your particular > hardware. > I've seen many cases where the cards do a horrible job with raid 5 > compared to > software. I can only second that. I'm still wondering what "top-notch hardware raid controllers" are. From my experience the only decent "controllers" you can get are those in the heavy priced SAN equipments with gigs of cache on the SAN controllers and tens or hundreds of spindles behind it. > >> For a terabyte, 3x500GB SATA drives in a RAID5 group will be blown >> out of the water by 4x500GB SATA drives in a RAID 0+1 configuration >> in terms of performance and (especially) latency, especially if it >> can do the Solaris trick of not faulting an entire RAID 0 sub-group >> if one spindle fails. Rebuild still isn't pretty, mind you. You can partly implement the trick of not faulting an entire drive by splitting the drive into smaller parts. With a 500G drive you can split it into 10 segments of 50G and create 10 RAID(1 or 5) devices on it. Just put thos 10 RAID segments into the same volume group. In case of some single sector defects it will only degrade 10% of your entire RAID and you could easily move those PV's to a spare RAID1 disk before touching any disk. Simon ---- Cyrus Home Page: http://cyrusimap.web.cmu.edu/ Cyrus Wiki/FAQ: http://cyrusimap.web.cmu.edu/twiki List Archives/Info: http://asg.web.cmu.edu/cyrus/mailing-list.html
