Date: Mon, 03 Aug 2020 18:33:49 -0500 From: "Josh Paetzel" <josh@tcbug.org> To: "Paul Pathiakis via freebsd-fs" <freebsd-fs@freebsd.org> Subject: Re: zfs scrub enable by default Message-ID: <e2979bd2-6dad-4c28-b76b-9e29fcf7e5ab@www.fastmail.com> In-Reply-To: <0d1d14c9-64ba-368c-b2f4-56efa741cc5e@inbox.lv> References: <cca34d1a-1892-41ec-ce45-84865100c6e1@FreeBSD.org> <24edb075-155c-439d-1ef5-541893036429@freebsd.org> <0d1d14c9-64ba-368c-b2f4-56efa741cc5e@inbox.lv>
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On Mon, Aug 3, 2020, at 4:41 PM, John Long via freebsd-fs wrote: > On 03/08/2020 20:25, Allan Jude wrote: > > On 2020-08-03 12:10, Steve Wills wrote: > >> Hi, > >> > >> I wonder why we don't enable zfs periodic scrub by default? > >> > >> https://svnweb.freebsd.org/base/head/usr.sbin/periodic/periodic.conf?view=markup#l162 > >> > >> > >> Anyone happen to know? > >> > >> Thanks, > >> Steve > >> > >> _______________________________________________ > >> freebsd-fs@freebsd.org mailing list > >> https://lists.freebsd.org/mailman/listinfo/freebsd-fs > >> To unsubscribe, send any mail to "freebsd-fs-unsubscribe@freebsd.org" > > > > I think switching this to on-by-default is a good thing. > > > > To be clear, which the check is part of 'periodic daily', it only > > triggers a scrub if it has been more than 35 days since the last scrub. > > > > FreeNAS already has does this, and that accounts for a large number of > > FreeBSD ZFS deployments. > > There is a difference. FreeNAS is an appliance. It should minimize the > need for management. > > FreeBSD is the base OS. It should do as little as possible so people can > set it up the way we want rather than spend days or weeks unbreaking > surprising, non-standard behavior and hundreds of tweaks for everybody > who requested them. > > > There is tuning you can do in ZFS to try to lessen the impact of a scrub > > on your production workloads. > > That's up to the guy running the box to do or not to do. > > > The periodic script lets you select which pools to include (defaults to > > all), so you can tune it to only scrub your root pool every 35 days, and > > not the large pool that might take too long to scrub or whatever. It > > also lets you set a different threshold for each pool. > > A NAS appliance could benefit from something like this. A generic OS cannot. > > > So I don't see any reason not to enable it by default, and just document > > how to adjust it if people really need to disable it. Honestly, I think > > those who are disabling it are doing themselves a disservice. > > It's offensive for you to presume to think you know better what the > other guy needs than he does himself. Thank you for clarifying it > though, I think it's valuable to understand the thinking of people who > want to coopt the OS into their personal playground. > > /jl > _______________________________________________ > freebsd-fs@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-fs > To unsubscribe, send any mail to "freebsd-fs-unsubscribe@freebsd.org" > ZFS needs scrub in the same way that UFS needs fsck. (in that i you don't scrub a ZFS volume for long enough and you don't fsck a UFS volume for long enough they will stop being useful filesystems) If you've ever had the joy of running UFS on a volume so large you couldn't fsck it or run say XFS on a volume so large you couldn't run xfs_repair you'll know that you can get away with that for a while, but eventually the chickens come home to roost and you'll be restoring from tape or failing over to your DR site, or complaining about lost data. (Note that XFS is journaled and is far more resilient than UFS to this sort of thing but it's still an issue) (Thankfully UFS added background fsck years ago but there was a time when a FreeBSD system wouldn't go multiuser until fsck had completed and really large volumes (say greater than 2TB, (and remember this was 15+ years ago before you start talking about your laptop has a 2TB SSD) weren't feasible unless you disabled fsck or had 9+ hours to spare every time you rebooted. When FreeBSD grew ZFS support the periodic scrub script didn't exist. Thankfully we don't have a "Allow perfect to be the enemy of the good" stance and in spite of that shortcoming (and many others) ZFS went in. Over time the FreeBSD ZFS support was iterated on, a scrub periodic script was added...but it was left disabled by default: POLA violation and all that. The reality is having scrub off is (in the vast majority of cases) the wrong thing to do. I say that as someone who has nearly an exabyte in ZFS right now, and lead the team that brought ZFS based FreeNAS to the world. So the suggestion to "fix the glitch" and enable periodic scrubs makes perfect sense to me. Those who don't know any better will just magically receive the benefit of something they should've been doing all along. Those who do know better have a lot of time on their hands (at least judging by this thread!) and with way less typing than the length of these emails can disable the periodic script. Sounds win-win to me. As far as the personal playground comment....we're all just trying to do the right thing for the biggest group of people while knowing all along that perfection for everyone is an unachievable goal (however it is possible that while chasing perfection we may achieve greatness). As an interesting data point, no version of FreeBSD ever shipped with fsck disabled even if it was an unusable feature for some of the users.... (Just my ten cents, my two cents is free) -- Thanks, Josh Paetzel
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