From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keld =?iso-8859-1?Q?J=F8rn?= Simonsen Subject: Re: Distributed spares Date: Tue, 14 Oct 2008 15:06:44 +0200 Message-ID: <20081014130644.GC3176@rap.rap.dk> References: <48F3C2A1.3080607@tmr.com> <20081013232921.GB22265@rap.rap.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: "Martin K. Petersen" Cc: Billy Crook , Justin Piszcz , Bill Davidsen , Neil Brown , Linux RAID List-Id: linux-raid.ids On Tue, Oct 14, 2008 at 06:12:29AM -0400, Martin K. Petersen wrote: > >>>>> "Keld" =3D=3D Keld J=F8rn Simonsen writes: >=20 > Keld> I have also been thinking a little on this. My idea is that if > Keld> bit errors develop on disks, then there is first maybe one bit > Keld> error, and the crc check on the disk sectors then finds and > Keld> corrects these. >=20 > Keld> If you rewrite such bit errors, then that bit error will be > Keld> corrected, and you prevent the one-bit error from developing to > Keld> a two-bit error that is not correctable by the CRC. >=20 > I think you are assuming that disks are much simpler than they > actually are. >=20 > A modern disk drive protects a 512-byte sector with a pretty strong > ECC that's capable of correcting errors up to ~50 bytes. Yes, that's > bytes. >=20 > Also, many drive firmwares will internally keep track of problematic > media areas and rewrite or reallocate affected blocks. That includes > stuff like rewriting sectors that are susceptible to bleed due to > being adjacent to write hot spots. Good to know. Could yo tell me if this is actually true for normal state-of-the art SATA disks, or only true for more expensive disks? Do you have a good reference for it. best regards keld -- 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