Date: Mon, 24 Jan 2011 17:51:30 +0100 From: Olivier Smedts <olivier@gid0.org> To: =?UTF-8?Q?=C5=A0imun_Mikecin?= <numisemis@gmail.com> Cc: freebsd-fs@freebsd.org Subject: Re: Write cache, is write cache, is write cache? Message-ID: <AANLkTi=SrEWnHxGLD-K1UZweJboEWuKysKr68L1rUSQ=@mail.gmail.com> In-Reply-To: <AANLkTi=K0xYJciWgtzTkMP-BhnOOfc5UeUgX--sCVLma@mail.gmail.com> References: <1ABA88EDF84B6472579216FE@Octa64> <20110122111045.GA59117@icarus.home.lan> <AANLkTik_rii-F_QWTP3OdyTS0gx1tDxv6--2LGGF6Ear@mail.gmail.com> <20110124144236.GA19500@icarus.home.lan> <AANLkTi=K0xYJciWgtzTkMP-BhnOOfc5UeUgX--sCVLma@mail.gmail.com>
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2011/1/24 =C5=A0imun Mikecin <numisemis@gmail.com>: > 2011/1/24 Jeremy Chadwick <freebsd@jdc.parodius.com> > >> The term "flush" means many different things. =C2=A0fsync(2), for exampl= e, >> behaves differently on UFS than it does on ZFS. =C2=A0People think that >> "flush" means "guarantee the data written was written to disk", but >> ensuring an actual ATA/SCSI command completes **and** has had its data >> written to the platters is an entirely different beast (IMO) than "flush >> kernel buffers to disk and hope for the best". >> > > AFAIK, BIO_FLUSH should complete when data was written to disk (not befor= e), > or in the case of battery backed cache: when data was written to battery > backed cache. > AFAIK, ZFS doesn't only "flush kernel buffers" (like UFS does), but also > executes BIO_FLUSH every X seconds (X=3D5 if my memory serves me well). S= o, > you shouldn't loose data that was written before BIO_FLUSH executed. I thought ZFS also took care of executing BIO_FLUSH after each write to ZIL, because every bit in ZFS expect writes to be ordered, and some specific writes to be synced to platters, else your FS is gone if ZIL is corrupted in some way. The way ZFS works is the reason there's fsck-like repair tool for it. Of course, this does not work if you don't use a propre controller or disk, but it should behave that way for most, no ? > =C2=A0 With single disks, all I've seen are read/write errors which can't= be > >> repaired. =C2=A0"zpool status" will actually show what files got affecte= d as >> a result of the issue, though sometimes "zpool scrub" needs to be run >> before this can be detected. > > > Adding redudancy does make your data safer, but if everything is working = as > explained, sudden power-loss shouldn't be the cause of those errors even > with single disks. Correct me if you think that I'm mistaken. > _______________________________________________ > freebsd-fs@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-fs > To unsubscribe, send any mail to "freebsd-fs-unsubscribe@freebsd.org" > --=20 Olivier Smedts=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=C2=A0 _ =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 ASCII ri= bbon campaign ( ) e-mail: olivier@gid0.org=C2=A0 =C2=A0 =C2=A0 =C2=A0 - against HTML email & = vCards=C2=A0 X www: http://www.gid0.org=C2=A0 =C2=A0 - against proprietary attachments / \ =C2=A0 "Il y a seulement 10 sortes de gens dans le monde : =C2=A0 ceux qui comprennent le binaire, =C2=A0 et ceux qui ne le comprennent pas."
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