From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: If your using large Sata drives in raid 5/6 .... Date: Wed, 03 Feb 2010 14:08:37 +0000 Message-ID: <4B698365.1040007@anonymous.org.uk> References: <87f94c371002021440o3b30414bk3a7ccf9d2fa9b8af@mail.gmail.com> <87f94c371002021446y38dce6fds6acca2b4919ad773@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <87f94c371002021446y38dce6fds6acca2b4919ad773@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: Linux RAID List-Id: linux-raid.ids On 02/02/2010 22:46, Greg Freemyer wrote: > All, > > I think the below is accurate, but please cmiiw or misunderstand. > > === > If your using normal big drives (1TB, etc.) in a raid-5 array, the > general consensus of this list is that it is a bad idea. The reason being > that the sector error rate for a bad sector has not changed with > increasing density. > > So in the days of 1GB drives, the likelihood of a undetected / > repaired bad sector was actually pretty low for the drive as whole. > But for today's 1TB drives, the odds are 1000x worse. ie. 1000x more > sectors with the same basic failure rate per sector. > > So a raid-5 composed of 1TB drives is 1000x more likely to be unable > to rebuild itself after a drive failure than a raid-5 built from 1 GB > drives of yesteryear. Thus the current recommendation is to use raid > 6 with high density drives. That sounds about right. One might still see RAID-5 as a way of pushing data loss through bad sectors back into a comfortable zone. After all, the likelihood of the same sector going bad on one of the other drives should be relatively small. Unfortunately it's too long since I studied probability for me to work it out properly. Then, to also protect yourself against dead drives, adding another drive a la RAID-6 sounds like the answer. But you can't think of RAID-6 protecting you from 2 drive failures any more. What is more, you need Linux md's implementation of single-sector recovery/rewriting for this to work. You cannot go around failing arrays because occasional single-sector reads fail. > The good news is that Western Digital is apparently introducing a new > series of drives with an error rate "2 orders of magnitude" better > than the current generation. It's not borne out in their figures; WD quote "less than 1 in 10^15 bits" which is the same as they quote for their older drives. What sums I've done, on the basis of a 1 in 10^15 bit unrecoverable error rate, suggest you've a 1 in 63 chance of getting an uncorrectable error while reading the whole surface of their 2TB disc. Read the whole disc 44 times and you've a 50/50 chance of hitting an uncorrectable error. You could read the whole drive in about 5 hours, according to the spec (at 110MB/s), so if you keep your drive busy you're going to reach this point in about 9 days. If you had a 5-drive array, you're going to get here inside 2 days. Bear in mind that this is on a drive working perfectly correctly as specified. We have to expect to be recovering from failed reads daily. ;-) Cheers, John. PS. Wish I'd written down my working for this. PPS. I'm not having a go at WD; other manufacturers' specs are similar.