Date: Tue, 30 Apr 2019 08:12:22 -0600 From: Alan Somers <asomers@freebsd.org> To: Michelle Sullivan <michelle@sorbs.net> Cc: Karl Denninger <karl@denninger.net>, FreeBSD <freebsd-stable@freebsd.org> Subject: Re: ZFS... Message-ID: <CAOtMX2iB7xJszO8nT_KU%2BrFuSkTyiraMHddz1fVooe23bEZguA@mail.gmail.com> In-Reply-To: <34539589-162B-4891-A68F-88F879B59650@sorbs.net> References: <30506b3d-64fb-b327-94ae-d9da522f3a48@sorbs.net> <CAOtMX2gf3AZr1-QOX_6yYQoqE-H%2B8MjOWc=eK1tcwt5M3dCzdw@mail.gmail.com> <56833732-2945-4BD3-95A6-7AF55AB87674@sorbs.net> <3d0f6436-f3d7-6fee-ed81-a24d44223f2f@netfence.it> <17B373DA-4AFC-4D25-B776-0D0DED98B320@sorbs.net> <70fac2fe3f23f85dd442d93ffea368e1@ultra-secure.de> <70C87D93-D1F9-458E-9723-19F9777E6F12@sorbs.net> <CAGMYy3tYqvrKgk2c==WTwrH03uTN1xQifPRNxXccMsRE1spaRA@mail.gmail.com> <5ED8BADE-7B2C-4B73-93BC-70739911C5E3@sorbs.net> <d0118f7e-7cfc-8bf1-308c-823bce088039@denninger.net> <2e4941bf-999a-7f16-f4fe-1a520f2187c0@sorbs.net> <CAOtMX2gOwwZuGft2vPpR-LmTpMVRy6hM_dYy9cNiw%2Bg1kDYpXg@mail.gmail.com> <34539589-162B-4891-A68F-88F879B59650@sorbs.net>
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On Tue, Apr 30, 2019 at 8:05 AM Michelle Sullivan <michelle@sorbs.net> wrot= e: > > > > Michelle Sullivan > http://www.mhix.org/ > Sent from my iPad > > > On 01 May 2019, at 00:01, Alan Somers <asomers@freebsd.org> wrote: > > > >> On Tue, Apr 30, 2019 at 7:30 AM Michelle Sullivan <michelle@sorbs.net>= wrote: > >> > >> Karl Denninger wrote: > >>> On 4/30/2019 05:14, Michelle Sullivan wrote: > >>>>>> On 30 Apr 2019, at 19:50, Xin LI <delphij@gmail.com> wrote: > >>>>>> On Tue, Apr 30, 2019 at 5:08 PM Michelle Sullivan <michelle@sorbs.= net> wrote: > >>>>>> but in my recent experience 2 issues colliding at the same time re= sults in disaster > >>>>> Do we know exactly what kind of corruption happen to your pool? If= you see it twice in a row, it might suggest a software bug that should be = investigated. > >>>>> > >>>>> All I know is it=E2=80=99s a checksum error on a meta slab (122) an= d from what I can gather it=E2=80=99s the spacemap that is corrupt... but I= am no expert. I don=E2=80=99t believe it=E2=80=99s a software fault as su= ch, because this was cause by a hard outage (damaged UPSes) whilst resilver= ing a single (but completely failed) drive. ...and after the first outage = a second occurred (same as the first but more damaging to the power hardwar= e)... the host itself was not damaged nor were the drives or controller. > >>> ..... > >>>>> Note that ZFS stores multiple copies of its essential metadata, and= in my experience with my old, consumer grade crappy hardware (non-ECC RAM,= with several faulty, single hard drive pool: bad enough to crash almost mo= nthly and damages my data from time to time), > >>>> This was a top end consumer grade mb with non ecc ram that had been = running for 8+ years without fault (except for hard drive platter failures.= ). Uptime would have been years if it wasn=E2=80=99t for patching. > >>> Yuck. > >>> > >>> I'm sorry, but that may well be what nailed you. > >>> > >>> ECC is not just about the random cosmic ray. It also saves your baco= n > >>> when there are power glitches. > >> > >> No. Sorry no. If the data is only half to disk, ECC isn't going to sa= ve > >> you at all... it's all about power on the drives to complete the write= . > > > > ECC RAM isn't about saving the last few seconds' worth of data from > > before a power crash. It's about not corrupting the data that gets > > written long before a crash. If you have non-ECC RAM, then a cosmic > > ray/alpha ray/row hammer attack/bad luck can corrupt data after it's > > been checksummed but before it gets DMAed to disk. Then disk will > > contain corrupt data and you won't know it until you try to read it > > back. > > I know this... unless I misread Karl=E2=80=99s message he implied the ECC= would have saved the corruption in the crash... which is patently false...= I think you=E2=80=99ll agree.. I don't think that's what Karl meant. I think he meant that the non-ECC RAM could've caused latent corruption that was only detected when the crash forced a reboot and resilver. > > Michelle > > > > > > -Alan > > > >>> > >>> Unfortunately however there is also cache memory on most modern hard > >>> drives, most of the time (unless you explicitly shut it off) it's on = for > >>> write caching, and it'll nail you too. Oh, and it's never, in my > >>> experience, ECC. > > > > Fortunately, ZFS never sends non-checksummed data to the hard drive. > > So an error in the hard drive's cache ram will usually get detected by > > the ZFS checksum. > > > >> > >> No comment on that - you're right in the first part, I can't comment i= f > >> there are drives with ECC. > >> > >>> > >>> In addition, however, and this is something I learned a LONG time ago > >>> (think Z-80 processors!) is that as in so many very important things > >>> "two is one and one is none." > >>> > >>> In other words without a backup you WILL lose data eventually, and it > >>> WILL be important. > >>> > >>> Raidz2 is very nice, but as the name implies it you have two > >>> redundancies. If you take three errors, or if, God forbid, you *writ= e* > >>> a block that has a bad checksum in it because it got scrambled while = in > >>> RAM, you're dead if that happens in the wrong place. > >> > >> Or in my case you write part data therefore invalidating the checksum.= .. > >>> > >>>> Yeah.. unlike UFS that has to get really really hosed to restore fro= m backup with nothing recoverable it seems ZFS can get hosed where issues o= ccur in just the wrong bit... but mostly it is recoverable (and my experien= ce has been some nasty shit that always ended up being recoverable.) > >>>> > >>>> Michelle > >>> Oh that is definitely NOT true.... again, from hard experience, > >>> including (but not limited to) on FreeBSD. > >>> > >>> My experience is that ZFS is materially more-resilient but there is n= o > >>> such thing as "can never be corrupted by any set of events." > >> > >> The latter part is true - and my blog and my current situation is not > >> limited to or aimed at FreeBSD specifically, FreeBSD is my experience= . > >> The former part... it has been very resilient, but I think (based on > >> this certain set of events) it is easily corruptible and I have just > >> been lucky. You just have to hit a certain write to activate the issu= e, > >> and whilst that write and issue might be very very difficult (read: hi= t > >> and miss) to hit in normal every day scenarios it can and will > >> eventually happen. > >> > >>> Backup > >>> strategies for moderately large (e.g. many Terabytes) to very large > >>> (e.g. Petabytes and beyond) get quite complex but they're also very > >>> necessary. > >>> > >> and there in lies the problem. If you don't have a many 10's of > >> thousands of dollars backup solutions, you're either: > >> > >> 1/ down for a looooong time. > >> 2/ losing all data and starting again... > >> > >> ..and that's the problem... ufs you can recover most (in most > >> situations) and providing the *data* is there uncorrupted by the fault > >> you can get it all off with various tools even if it is a complete > >> mess.... here I am with the data that is apparently ok, but the > >> metadata is corrupt (and note: as I had stopped writing to the drive > >> when it started resilvering the data - all of it - should be intact... > >> even if a mess.) > >> > >> Michelle > >> > >> -- > >> Michelle Sullivan > >> http://www.mhix.org/ > >> > >> _______________________________________________ > >> freebsd-stable@freebsd.org mailing list > >> https://lists.freebsd.org/mailman/listinfo/freebsd-stable > >> To unsubscribe, send any mail to "freebsd-stable-unsubscribe@freebsd.o= rg"
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