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[107.215.223.229]) by smtp.gmail.com with ESMTPSA id m20sm3189975oiw.46.2021.08.30.15.35.45 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Mon, 30 Aug 2021 15:35:45 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 14.0 \(3654.120.0.1.13\)) From: Bakul Shah In-Reply-To: Date: Mon, 30 Aug 2021 15:35:44 -0700 Content-Transfer-Encoding: quoted-printable Message-Id: <7E649324-2690-4D9D-A175-B74089EFEE9A@iitbombay.org> References: <202108292212.17TMCGow1448973@darkstar.fourwinds.com> To: Theodore Ts'o X-Mailer: Apple Mail (2.3654.120.0.1.13) Subject: Re: [TUHS] Is it time to resurrect the original dsw (delete with switches)? X-BeenThere: tuhs@minnie.tuhs.org X-Mailman-Version: 2.1.26 Precedence: list List-Id: The Unix Heritage Society mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: The Unix Heretics Society mailing list Errors-To: tuhs-bounces@minnie.tuhs.org Sender: "TUHS" On Aug 30, 2021, at 4:56 AM, Theodore Ts'o wrote: >=20 > On Sun, Aug 29, 2021 at 08:36:37PM -0700, Bakul Shah wrote: >> Chances are your disk has a URE 1 in 10^14 bits ("enterprise" disks >> may have a URE of 1 in 10^15). 10^14 bit is about 12.5TB. For 16TB >> disks you should use at least mirroring, provided some day you'd want >> to fill up the disk. And a machine with ECC RAM (& trust but = verify!). >> I am no fan of btrfs but these are the things I'd consider for any = FS. >> Even if you have done all this, consider the fact that disk mortality >> has a bathtub curve. >=20 > You may find this article interesting: "The case of the 12TB URE: > Explained and debunked"[1], and the following commit on a reddit > post[2] discussiong this article: >=20 > "Lol of course it's a myth. >=20 > I don't know why or how anyone thought there would be a URE > anywhere close to every 12TB read. >=20 > Many of us have large pools that are dozens and sometimes hundreds = of TB. >=20 > I have 2 64TB pools and scrub them every month. I can go years > without a checksum error during a scrub, which means that all my > ~50TB of data was read correctly without any URE many times in a > row which means that I have sometimes read 1PB (50TB x 2 pools x 10 > months) worth from my disks without any URE. >=20 > Last I checked, the spec sheets say < 1 in 1x1014 which means less > than 1 in 12TB. 0 in 1PB is less than 1 in 12TB so it meets the > spec." >=20 > [1] = https://heremystuff.wordpress.com/2020/08/25/the-case-of-the-12tb-ure/ > [2] = https://www.reddit.com/r/DataHoarder/comments/igmab7/the_12tb_ure_myth_exp= lained_and_debunked/ It seems this guy doesn't understand statistics. He checked his 2 pools and is confident that a sample of 4 disks (likely) he knows that URE specs are crap. Even from an economic PoV it doen't make sense. Why wouldn't the disk companies tout an even lower error rate if they can get away with it? Presumably these rates are derived from reading many many disks and averaged. Here's what the author says on a serverfault thread: https://serverfault.com/questions/812891/what-is-exactly-an-ure @DavidBala=C5=BEic Evidently, your sample size of one invalidates the entirety of probability theory! I suggest you submit a paper to the Nobel Committee. =E2=80=93 Ian Kemp Apr 16 '19 at 5:37 @IanKemp If someone claims that all numbers are divisible by 7 and I find ONE that is not, then yes, a single find can invalidate an entire theory. BTW, still not a single person has confirmed the myth in practice (by experiment), did they? Why should they, when belief is more than knowledge...=E2=80=93 David Bala=C5=BEic Apr 16 '19 at = 12:22 Incidentally, it is hard to believe he scrubs his 2x64TB pools once a = month. Assuming 250MB/s sequential throughput and his scrubber can stream it at that rate, it will take him close to 6 days (3 days if reading them in parallel) to read every block. During this time these pools won't be useful for anything else. Unclear if he is using any RAID or a = filesystem that does checksums. Without that he would be unable to detect hidden data corruption. In contrast, ZFS will only scrub *live* data. As more of the disks are=20= filled up, scrub will take progressively more time. Similarly, replacing a zfs mirror won't read the source disk in its entirety, only the live data. > Of course, disks do die, and ECC and backups and checksums are good > things. But the whole "read 12TB get an error", saying really > misunderstands how hdd failures work. Losing an entire platter, or > maybe the entire 12TB disk die due to a head crash, adds a lot of > uncorrectable read errors to the numerator of the UER statistics. That is not how URE specs are derived. >=20 > It just goes to show that human intuition really sucks at statistics, Indeed :-)=