From mboxrd@z Thu Jan 1 00:00:00 1970 From: Steven Haigh Subject: Re: If your using large Sata drives in raid 5/6 .... Date: Wed, 03 Feb 2010 20:27:40 +1100 Message-ID: <1831dfc0b72e282d0a17e24c959c8abe@localhost> References: <87f94c371002021440o3b30414bk3a7ccf9d2fa9b8af@mail.gmail.com> <87f94c371002021446y38dce6fds6acca2b4919ad773@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <87f94c371002021446y38dce6fds6acca2b4919ad773@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: Greg Freemyer Cc: Linux RAID List-Id: linux-raid.ids Numbers are a funny game. Sure, there is a 1000x increase in the chance of an error occurring *somewhere* on the drive - as we have 1000x more space. What you need t= o keep in perspective is that although building a 2Tb RAID5 from 3 x 1Tb drives may seem more likely to have problems than a 2Gb RAID5 from 3 x = 1Gb drives - however if you build a 2Tb RAID5 from 1Gb drives then you woul= d probably increase the likelihood of a failure to around the same levels= =2E =46rom what I read of the article, it seems like more of a sales pitch = and although it has what seem to be good points, they seem to ignore the fa= ct of scaling. ie which is more likely to fail horribly, 3 x 1Tb drives or 3000 x 1Gb drives or even 300 x 10Gb drives? Data storage always seems to be a minefield of misinformation and sales pitches masked as information - however the best practice is to use RAI= D to hopefully minimise downtime if a drive fails, but keep a copy of your d= ata elsewhere that you have easy access to if you need it. After this, it doesn't really matter if you have a single drive failure - or even a do= uble drive failure in the case of RAID6. If you have more failures and the array crashes horribly, then theres n= ot much you can do but rebuild and restore. On Tue, 2 Feb 2010 17:46:25 -0500, Greg Freemyer wrote: > All, >=20 > I think the below is accurate, but please cmiiw or misunderstand. >=20 > =3D=3D=3D > 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. =C2=A0The re= ason being > that the sector error rate for a bad sector has not changed with > increasing density. >=20 > 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. =C2=A0ie. 1000x= more > sectors with the same basic failure rate per sector. >=20 > 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. =C2=A0Thus the current recommendation is to use= raid > 6 with high density drives. >=20 > 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. >=20 > See >=20 > The whole article is good, but this paragraph is what really got my > attention: >=20 > "From a numbers perspective, Western Digital estimates that the use o= f > 4K sectors will give them an immediate 7%-11% increase in format > efficiency. ECC burst error correction stands to improve by 50%, and > the overall error rate capability improves by 2 orders of magnitude. > In theory these reliability benefits should immediately apply to all > 4K sector drives (making the Advanced Format drives more reliable tha= n > regular drives), but Western Digital is not pushing that idea at this > time." >=20 > So maybe raid-5 will once again be a reasonable choice again in the > future. >=20 > (I think these drives may already be available at least as > engineering samples. Basic linux kernel support went in summer 2009 = I > believe. I believe 2.6.33 will be the first kernel to have been teste= d > with these new class of drives.) >=20 > I don't know if there is a mdraid wiki, but if so and someone wants t= o > post the above there, please do. >=20 > Greg > -- > Greg Freemyer > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid"= in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html --=20 Steven Haigh =20 Email: netwiz@crc.id.au Web: http://www.crc.id.au Phone: (03) 9001 6090 - 0412 935 897 =46ax: (03) 8338 0299 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html