Date: Sat, 29 Mar 2014 01:16:25 +0400 (MSK) From: Dmitry Morozovsky <marck@rinet.ru> To: Freddie Cash <fjwcash@gmail.com> Cc: "freebsd-fs@freebsd.org" <freebsd-fs@freebsd.org> Subject: Re: zfs l2arc warmup Message-ID: <alpine.BSF.2.00.1403290115030.60856@woozle.rinet.ru> In-Reply-To: <CAOjFWZ7h5080%2BzEvSfzxgENwP%2BPXEXKPXdEDHAtbx5RAxxWT0g@mail.gmail.com> References: <CAFfb-hpi20062%2BHCrSVhey1hVk9TAcOZAWgHSAP93RSov3sx4A@mail.gmail.com> <CALfReydi_29L5tVe1P-aiFnm_0T4JJt72Z1zKouuj8cjHLKhnw@mail.gmail.com> <CAFfb-hpZos5-d3xo8snU1aVER5u=dSFRx-B-oqjFRTkT83w0Kg@mail.gmail.com> <20140328005911.GA30665@neutralgood.org> <CAFfb-hr=wR6nxqL%2B4tn-y2eQEw4n_g7rZoK9rRLnm_Ldcm1TZQ@mail.gmail.com> <alpine.BSF.2.00.1403290032070.60856@woozle.rinet.ru> <CAOjFWZ7h5080%2BzEvSfzxgENwP%2BPXEXKPXdEDHAtbx5RAxxWT0g@mail.gmail.com>
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On Fri, 28 Mar 2014, Freddie Cash wrote: > > > > Have you measured to see if, or do you otherwise know for sure, that > > you > > > > really do need a ZIL? I suggest not adding a ZIL unless you are certain > > > > you need it. > > > Yes, I only recently realized that too, and I'm really not sure if a > > > zil is required. > > > Some small portion of files (som hundre MB's) are served over nfs from > > > the same server, if I understand it right a zil will help for nfs > > > stuff (?) , but I'm not sure if it's any gain of having a zil today. > > > On the other hand, a zil doesn't have to be big, I can simply buy a > > > 128GB ssd which are cheap today . > > > > Please don't forget that, unlike L2ARC, if you lost ZIL during sync write, > > you're effectively lost the pool. > > > > ?Nope. Not even close. > > The ZIL is only ever read at boot time. If you lose the ZIL between the > time the data is written to the ZIL and the time the async write of the > data is actually done to the pool ... and the server is rebooted at that > time, then you get an error message at pool import. > > You can then force the import of the pool, losing any *data* in the ZIL, > but nothing else. > > It used to be (back in the pre-ZFSv?13-ish days) that if you lost the ZIL > while there was data in it that wasn't yet written to the pool, the pool > would fault and be gone. Hence the rule-of-thumb to always mirror the ZIL. > > Around ZFSv14-ish, the ability to import a pool with a missing ZIL was > added. > > Remember the flow of data in ZFS: > async write request --> TXG --> disk > sync write request --> ZIL > \--> TXG --> disk > > All sync writes are written to the pool as part of a normal async TXG after > its written sync to the ZIL. And the ZIL is only ever read during pool > import. > > ?[Note, I'm not a ZFS developer so some of the above may not be 100% > accurate, but the gist of it is.]? Ah, thanks, I stand corrected. Great we're tighten the window we could possibly lose precious data. -- Sincerely, D.Marck [DM5020, MCK-RIPE, DM3-RIPN] [ FreeBSD committer: marck@FreeBSD.org ] ------------------------------------------------------------------------ *** Dmitry Morozovsky --- D.Marck --- Wild Woozle --- marck@rinet.ru *** ------------------------------------------------------------------------
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