Date: Wed, 5 Aug 2020 09:47:40 -0400 From: Karl Denninger <karl@denninger.net> To: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> Cc: freebsd-fs@freebsd.org Subject: Re: zfs scrub enable by default Message-ID: <180eeb29-3cd8-efbd-41fe-97df8fd78ccc@denninger.net> In-Reply-To: <alpine.GSO.2.20.2008050808330.10299@scrappy.simplesystems.org> References: <cca34d1a-1892-41ec-ce45-84865100c6e1@FreeBSD.org> <CAJjvXiEXEdAFXpXkGvt4fymA17kNdp6XkZV5taGKLoP2GvMHbw@mail.gmail.com> <d1b580da-1539-5fc9-f7a3-3f013bba4ef3@FreeBSD.org> <CANCZdfq2PneFvB4rnz2iGu5srFFFjs8N=7FwRO3DYjosESWXtQ@mail.gmail.com> <CAGuotKD0mCS3KmMA-EGL1uH_fByYOhMKbPVDoTdB8dg5kC-u9g@mail.gmail.com> <105090343.294898.1596586694925.JavaMail.zimbra@gray.id.au> <alpine.GSO.2.20.2008042010300.10299@scrappy.simplesystems.org> <e5e7a916-4da2-6467-1616-1b1a75f32509@denninger.net> <alpine.GSO.2.20.2008050808330.10299@scrappy.simplesystems.org>
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This is a cryptographically signed message in MIME format. --------------ms000209070104010805030705 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 8/5/2020 09:15, Bob Friesenhahn wrote: > On Tue, 4 Aug 2020, Karl Denninger wrote: > >> Let me give you two allegedly "degenerate" cases that are actually=20 >> not degenerate at all. >> >> 1. A laptop or workstation.=C2=A0 It is backed up.=C2=A0 It uses ZFS b= ecause=20 >> it's faster, and I can establish a filesystem for some project very=20 >> easily and quickly, it's segregated, I can work on it and destroy it=20 >> trivially when done.=C2=A0 I can set quotas on that, etc.=C2=A0 If I w= ant to=20 >> move its mountpoint, I can trivially do so. And so on.=C2=A0 Note that= =20 >> here there is no redundancy at all; no raidZx, no mirroring, etc.=C2=A0= =20 >> I'm merely using it for convenience. > > Did you remember to set copies=3D2 or copies=3D3 for zfs filesystems wh= ere=20 > you hope not to experience data loss?=C2=A0 It needs to be set as soon = as=20 > possible since it only applies to new files.=C2=A0 This is a way to get= =20 > more media redundancy, although the whole drive may fail. > > Zfs scrub is not going to protect your precious data from loss given=20 > just one drive unless you increase the copies setting.=C2=A0 Zfs itself= =20 > already uses redundant copies for its own data structures. > copies=3D2 does nothing if the drive itself fails (as opposed to a handfu= l=20 of sectors failing on said drive.) SSDs rarely lose a block undetected by their own controller.=20 More-commonly when they fail they fail HARD and the entire volume=20 becomes inaccessible immediately.=C2=A0 The same is frequently (but less = frequently than with an SSD) true for spinning rust.=C2=A0 In my experien= ce=20 with various "non-removable" spinning rust going back to the original=20 5Mb winchester devices in a decent percentage of the instances the=20 device becomes either completely inaccessible all at once or it may as=20 well be since the unreadable sectors are numerous enough and the=20 timeouts on attempted reads long enough that recovery of the=20 non-impacted data will frequently take days (or worse.) Don't even get me started on controller/adapter failures.=C2=A0 I've had = two=20 in my career that scribbled on everything connected to them at once.=C2=A0= =20 Even ZFS in a RaidZx or mirrored configuration won't protect you from=20 that.=C2=A0 You either have backups or you have nothing. As I said, a workstation or laptop is, in such a configuration, backed=20 up to some other storage (external, either plugged in or on a network)=20 and if the drive fails it is replaced and restored. ZFS makes this easy=20 to do and also quite economical in terms of time and either storage or=20 network bandwidth with its snapshot capability. The latter is also a reason to use ZFS standing alone, irrespective of=20 redundancy of data storage. I've had scrub catch incipient errors (but you have to look at the stats = to know it found and fixed the blocks) before the volume failed and was=20 able to replace the bad volume before it puked entirely.=C2=A0 I apprecia= te=20 that when it happens but the point here is that turning on automated=20 scrubs is like automated patrol reads on an old-style RAID adapter; it=20 sounds great right up until it can cause trouble without benefit.=C2=A0 T= o=20 have it as an option in the installer is a good thing, but to silently=20 turn it on, especially if it's done during an upgrade where it wasn't on = before, is IMHO a bad thing as it has at least a decent probability of=20 producing unpleasant surprises. --=20 Karl Denninger karl@denninger.net <mailto:karl@denninger.net> /The Market Ticker/ /[S/MIME encrypted email preferred]/ --------------ms000209070104010805030705 Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 Content-Disposition: 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