From: Zan Lynx Subject: Re: [patch] ext2/3: document conditions when reliable operation is possible Date: Mon, 24 Aug 2009 16:22:22 -0600 Message-ID: <4A93129E.6080704@acm.org> References: <200903161426.24904.rob@landley.net> <20090323104525.GA17969@elf.ucw.cz> <87ljqn82zc.fsf@frosties.localdomain> <20090824093143.GD25591@elf.ucw.cz> <82k50tjw7u.fsf@mid.bfk.de> <20090824130125.GG23677@mit.edu> <20090824195159.GD29763@elf.ucw.cz> <4A92F6FC.4060907@redhat.com> <20090824205209.GE29763@elf.ucw.cz> <4A930160.8060508@redhat.com> <20090824212518.GF29763@elf.ucw.cz> <4A930EB9.8030903@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Pavel Machek , Theodore Tso , Florian Weimer , Goswin von Brederlow , Rob Landley , kernel list , Andrew Morton , mtk.manpages@gmail.com, rdunlap@xenotime.net, linux-doc@vger.kernel.org, linux-ext4@vger.kernel.org To: Ric Wheeler Return-path: In-Reply-To: <4A930EB9.8030903@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org Ric Wheeler wrote: > Pavel Machek wrote: >> Degraded MD RAID5 does not work by design; whole stripe will be >> damaged on powerfail or reset or kernel bug, and ext3 can not cope >> with that kind of damage. [I don't see why statistics should be >> neccessary for that; the same way we don't need statistics to see that >> ext2 needs fsck after powerfail.] >> Pavel >> > What you are describing is a double failure and RAID5 is not double > failure tolerant regardless of the file system type.... Are you sure he isn't talking about how RAID must write all the data chunks to make a complete stripe and if there is a power-loss, some of the chunks may be written and some may not? As I read Pavel's point he is saying that the incomplete write can be detected by the incorrect parity chunk, but degraded RAID-5 has no working parity chunk so the incomplete write would go undetected. I know this is a RAID failure mode. However, I actually thought this was a problem even for a intact RAID-5. AFAIK, RAID-5 does not generally read the complete stripe and perform verification unless that is requested, because doing so would hurt performance and lose the entire point of the RAID-5 rotating parity blocks. -- Zan Lynx zlynx@acm.org "Knowledge is Power. Power Corrupts. Study Hard. Be Evil."