Date: Sat, 13 Feb 2021 11:54:03 -0500 From: Karl Denninger <karl@denninger.net> To: Stefan Esser <se@freebsd.org>, freebsd-fs@freebsd.org Subject: Re: Reading a corrupted file on ZFS Message-ID: <3aeb9d42-f69d-1f4c-b1f5-74dd4d28578c@denninger.net> In-Reply-To: <225e4da5-79ec-a57a-90e5-35989e6484d5@freebsd.org> References: <da892eeb-233f-551f-2faa-62f42c3c1d5b@artem.ru> <0ca45adf-8f60-a4c3-6264-6122444a3ffd@denninger.net> <899c6b4f-2368-7ec2-4dfe-fa09fab35447@artem.ru> <20210212165216.2f613482@fabiankeil.de> <10977ffc-f806-69dd-0cef-d4fd4fc5f649@artem.ru> <2f82f113-9ca1-99a9-a433-89e3ae5edcbe@denninger.net> <2bf4f69c-9d5d-5ff9-0daa-c87515437ca3@artem.ru> <5aa03138-1dc8-5a9c-1be6-d47ed22fc0cf@denninger.net> <225e4da5-79ec-a57a-90e5-35989e6484d5@freebsd.org>
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This is a cryptographically signed message in MIME format. --------------ms040701040105020602070909 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 2/13/2021 11:51, Stefan Esser wrote: > Am 12.02.21 um 20:49 schrieb Karl Denninger: >> On 2/12/2021 13:26, Artem Kuchin wrote: >>> 12.02.2021 19:37, Karl Denninger =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >>>> On 2/12/2021 11:22, Artem Kuchin wrote: >>>>> >>>>> This is frustrating. why..why.. >>>> >>>> You created a synthetic situation that in the real world=20 >>>> almost-never exists (ONE byte modified in all copies in the same=20 >>>> allocation block but all other data in that block is intact and=20 >>>> recoverable.) >>>> >>> I could be 1 GB file with ZFS wisth block size of 1MB and with=20 >>> rotten bits within the same 1MB of block on different disks. How i=20 >>> did it is not important, life is unpredictable, i'm not trying to=20 >>> avoid everything. The question is what to do when it happens. And=20 >>> currently the answer is - nothing. >>> >> The answer to a problem that does not present itself statistically=20 >> speaking in the real world is the last one you worry about.=C2=A0 Worr= y=20 >> about all the other ones and cover them first. > > True for some kinds of real world, false for others. > > I have a number of older 2 TB drives and use 2 of them to store > DVB transport stream data, for later processing. They are not > mirrored and the data format assumes that the transmission is not > perfect. I have noticed that some of these files have been lost > due to bad sectors, and while this would not be a problem on UFS, > ZFS thinks I should not trust the file at all, despite 99,9999% > of it being perfectly usable. (A fraction of the TS data is > extracted from these files and stored as PS data on a ZFS RAID, > where it will be safe and I never lost a file.) > > There are other applications that do not care for all data to be > perfectly conserved (I know of some) and where using a mirrored > setup configuration would just be a waste of money. > > Not everybody operates a data center with a large drive farm that > holds data worth magnitudes more than the hardware cost (in which > case the cost of redundancy is well justified). > > > A solution could be to use UFS for files that should be readable > (or recoverable with a tool) after single sector failures. > > But why should I have to mix UFS for such data with ZFS for the > rest of the system (and I know you very well that these two do > not mix too well if both are put under simultaneous high load). > > > Therefore: There are valid reasons to not have redundancy and > still not loose a large file of data for single bad block of a > non-redundant (e.g striped for performance) ZPOOL/ZFS. > > Regards, STefan > That's what the patch does -- it allows you to read the file but the=20 known-bad blocks will be "blanked" (marked that it's no good.) Perhaps that sysctl should be part of the system generally, but you CAN=20 do it the hard way with zdb even without it although it's a SERIOUS pain = in the neck. --=20 Karl Denninger karl@denninger.net <mailto:karl@denninger.net> /The Market Ticker/ /[S/MIME encrypted email preferred]/ --------------ms040701040105020602070909 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 MTUxNjQyMTdaMHsxCzAJBgNVBAYTAlVTMRAwDgYDVQQIDAdGbG9yaWRhMRkwFwYDVQQKDBBD 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