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. > and three-disk RAID5 assemblages are a performance disaster > area irrespective of hardware in a failure scenario. The rebuild > will involve taking 50% of the IO bandwidth of the two remaining > disks in order to saturate the new target; rebuild performance --- > contrary to intuition --- improves with larger assemblages as you can > saturate the replacement disk with less and less of the bandwidth of > the surviving spindles. this is true, up to the point where the bus gets saturated with the re-sync info, after that more disks will not improve the rebuild time. > 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. either of these cases will survive a single drive failure, what I would look at is either 3x1TB drives in raid 1, or 4x500G drives in raid 6 to get the ability to survive 2 drives failing. it takes long enough to rebuild an array with large drives that the chances of a second drive failing during the rebuild become noticable. David Lang ---- 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
