Date: Tue, 9 Apr 2019 15:27:57 -0500 From: Karl Denninger <karl@denninger.net> To: freebsd-stable@freebsd.org Subject: Re: Concern: ZFS Mirror issues (12.STABLE and firmware 19 .v. 20) Message-ID: <9a96b1b5-9337-fcae-1a2a-69d7bb24a5b3@denninger.net> In-Reply-To: <c96e31ad-6731-332e-5d2d-7be4889716e1@FreeBSD.org> References: <f87f32f2-b8c5-75d3-4105-856d9f4752ef@denninger.net> <c96e31ad-6731-332e-5d2d-7be4889716e1@FreeBSD.org>
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This is a cryptographically signed message in MIME format. --------------ms080500070903000806050602 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On 4/9/2019 15:04, Andriy Gapon wrote: > On 09/04/2019 22:01, Karl Denninger wrote: >> the resilver JUST COMPLETED with no errors which means the ENTIRE DISK= 'S >> IN USE AREA was examined, compared, and blocks not on the "new member"= >> or changed copied over. > I think that that's not entirely correct. > ZFS maintains something called DTL, a dirty-time log, for a missing / o= fflined / > removed device. When the device re-appears and gets resilvered, ZFS wa= lks only > those blocks that were born within the TXG range(s) when the device was= missing. > > In any case, I do not have an explanation for what you are seeing. That implies something much more-serious could be wrong such as given enough time -- a week, say -- that the DTL marker is incorrect and some TXGs that were in fact changed since the OFFLINE are not walked through and synchronized.=C2=A0 That would explain why it gets caught by a scrub = -- the resilver is in fact not actually copying all the blocks that got changed and so when you scrub the blocks are not identical.=C2=A0 Assumin= g the detached disk is consistent that's not catastrophically bad IF CAUGHT; where you'd get screwed HARD is in the situation where (for example) you had a 2-unit mirror, detached one, re-attached it, resilver says all is well, there is no scrub performed and then the *non-detached* disk fails before there is a scrub.=C2=A0 In that case you= will have permanently destroyed or corrupted data since the other disk is allegedly consistent but there are blocks *missing* that were never copied over. Again this just showed up on 12.x; it definitely was *not* at issue in 11.1 at all.=C2=A0 I never ran 11.2 in production for a material amount o= f time (I went from 11.1 to 12.0 STABLE after the IPv6 fixes were posted to 12.x) so I don't know if it is in play on 11.2 or not. I'll see if it shows up again with 20.00.07.00 card firmware. Of note I cannot reproduce this on my test box with EITHER 19.00.00.00 or 20.00.07.00 firmware when I set up a 3-unit mirror, offline one, make a crap-ton of changes, offline the second and reattach the third (in effect mirroring the "take one to the vault" thing) with a couple of hours elapsed time and a synthetic (e.g. "dd if=3D/dev/random of=3Doutfil= e bs=3D1m" sort of thing) "make me some new data that has to be resilvered"= workload.=C2=A0 I don't know if that's because I need more entropy in the= filesystem than I can reasonably generate this way (e.g. more fragmentation of files, etc) or whether it's a time-based issue (e.g. something's wrong with the DTL/TXG thing as you note above in terms of how it functions and it only happens if the time elapsed causes something to be subject to a rollover or similar problem.)=C2=A0 I spent quite a lot of time trying to make reproduce the issue on my "sandbox" machine and was unable -- and of note it is never a large quantity of data that is impacted, it's usually only a couple of dozen checksums that show as bad and fixed.=C2=A0 Of note it's also never just = one; if there was a single random hit on a data block due to ordinary bitrot sort of issues I'd expect only one checksum to be bad.=C2=A0 But generati= ng a realistic synthetic workload over the amount of time involved on a sandbox is not trivial at all; the system on which this is now happening handles a lot of email and routine processing of various sorts including a fair bit of database activity associated with network monitoring and statistical analysis. I'm assuming that using "offline" as a means to do this hasn't become "invalid" as something that's considered "ok" as a means of doing this sort of thing.... it certainly has worked perfectly well for a very long time! --=20 Karl Denninger karl@denninger.net <mailto:karl@denninger.net> /The Market Ticker/ /[S/MIME encrypted email preferred]/ --------------ms080500070903000806050602 Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="smime.p7s" Content-Description: S/MIME Cryptographic Signature MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgMFADCABgkqhkiG9w0BBwEAAKCC DdgwggagMIIEiKADAgECAhMA5EiKghDOXrvfxYxjITXYDdhIMA0GCSqGSIb3DQEBCwUAMIGL MQswCQYDVQQGEwJVUzEQMA4GA1UECAwHRmxvcmlkYTESMBAGA1UEBwwJTmljZXZpbGxlMRkw FwYDVQQKDBBDdWRhIFN5c3RlbXMgTExDMRgwFgYDVQQLDA9DdWRhIFN5c3RlbXMgQ0ExITAf BgNVBAMMGEN1ZGEgU3lzdGVtcyBMTEMgMjAxNyBDQTAeFw0xNzA4MTcxNjQyMTdaFw0yNzA4 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