Date: Mon, 15 Nov 1999 21:06:08 +0100 From: Bernd Walter <ticso@cicely.de> To: Greg Lehey <grog@lemis.com> Cc: Bernd Walter <ticso@cicely.de>, Mattias Pantzare <pantzer@ludd.luth.se>, freebsd-fs@FreeBSD.ORG Subject: Re: RAID-5 and failure Message-ID: <19991115210607.A6252@cicely7.cicely.de> In-Reply-To: <19991115145200.09633@mojave.sitaranetworks.com> References: <ticso@cicely.de> <199911061716.SAA20783@zed.ludd.luth.se> <19991106183316.A9420@cicely7.cicely.de> <19991113213325.57908@mojave.sitaranetworks.com> <19991115203828.B5417@cicely7.cicely.de> <19991115145200.09633@mojave.sitaranetworks.com>
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On Mon, Nov 15, 1999 at 02:52:00PM -0500, Greg Lehey wrote: > On Monday, 15 November 1999 at 20:38:28 +0100, Bernd Walter wrote: > > On Sat, Nov 13, 1999 at 09:33:25PM -0500, Greg Lehey wrote: > >> > >> 4. The system crashes after writing the first data block for a RAID-5 > >> stripe and before writing the last data block. > >> > >> When the system comes up, both data and parity are inconsistent. > >> > >> 5. The system crashes after writing the last data block for a RAID-5 > >> stripe and before writing the last parity block. > >> > >> When the system comes up, data is consistent, and parity is > >> inconsistent. > >> > >> There are a number of ways of dealing with situations 4 and 5. The > >> real problem is that they only occur when the system crashes, so > >> whatever recovery information is required must be stored in > >> non-volatile storage. Some systems do include a NOVRAM for this kind > >> of information, but in general purpose systems the only possibility is > >> to write the information to disk, which would make the inherently slow > >> RAID-5 write even slower. My attitude here is that RAID-5 writes are > >> comparatively infrequent, and so are crashes. In the case of (5), you > >> could rebuild parity after a crash. In the case of (4), I have no > >> good answer. Suggestions welcome. > > > > Case 4 is not that different from case 5 as any differences should be > > handled by the FS using the volume. > > The problem is that in case 4 you don't have anything to go by. You > don't know which data are inconsistent unless you keep a log. The FS > using the volume has followed the kernel into the eternal bit bucket. > Of course - but that may happen with R0 too and even it may be possible with a single disk. The FS should realy be able to handle this case as it knows that there is an outstanding write operation. -- B.Walter COSMO-Project http://www.cosmo-project.de ticso@cicely.de Usergroup info@cosmo-project.de To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
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