Date: Thu, 23 Sep 2010 17:10:25 +0100 From: Martin Simmons <martin@lispworks.com> To: freebsd-fs@freebsd.org Subject: Re: what happens to pool if ZIL dies on ZFS v14 Message-ID: <201009231610.o8NGAPkf018898@higson.cam.lispworks.com> In-Reply-To: <AANLkTi=O26MxE_8kEk_Es7H=QZDVbE5bypT2XvkurAoY@mail.gmail.com> (message from David Brodbeck on Wed, 22 Sep 2010 10:19:22 -0700) References: <AANLkTi=vYVG300nhMjkcLju=kQhBdPJDqyaXR0mG84%2Bp@mail.gmail.com> <4C9385B0.2080909@shatow.net> <AANLkTin0LwQz%2BWi5cBOcHuVqyOz3%2BfFR7YC_=f2L5CyX@mail.gmail.com> <AANLkTinbPK1rNK5hg=t7N=sqFLuh8sNrZT9DFC_ppXWF@mail.gmail.com> <20100917161847.GA58503@icarus.home.lan> <AANLkTikEgrFGGUVUW8dQWGH44K41jPG=PwXXzsT5fYdV@mail.gmail.com> <201009221300.o8MD0Cbm030033@higson.cam.lispworks.com> <AANLkTik00NQX=FiUemGBKdogXUQHPS2rvT-XSV30VCNq@mail.gmail.com> <AANLkTi=O26MxE_8kEk_Es7H=QZDVbE5bypT2XvkurAoY@mail.gmail.com>
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>>>>> On Wed, 22 Sep 2010 10:19:22 -0700, David Brodbeck said: > > On Wed, Sep 22, 2010 at 10:14 AM, David Brodbeck <gull@gull.us> wrote: > > On Wed, Sep 22, 2010 at 6:00 AM, Martin Simmons <martin@lispworks.com> wrote: > >>>>>>> On Tue, 21 Sep 2010 15:38:22 -0700, David Brodbeck said: > >>> > >>> If you don't have a separate log device, synchronous writes are very > >>> slow with the ZIL enabled. This isn't such a big deal unless you're > >>> using NFS, where essentially every write is synchronous. > >> > >> Is that true for all versions of NFS? In my experience (on 8.0-RELEASE), > >> NFSv2 is indeed synchronous, but NFSv3 does asynchronous flushing (for a > >> variety of different client OSes). > > > > It does allow clients to request asynchronous flushing. My statement > > that "essentially every write is synchronous" was a bit of an > > overstatement; the problem comes when the client issues a COMMIT, > > which happens frequently when doing some operations, such as > > extracting tar files. These are the operations that can get quite > > slow when using NFS with the ZIL enabled and no separate log device. > > By "quite slow," I mean several minutes to extract a tar file that > > takes less than a minute with the ZIL disabled. > > I should add that there's a very good, if somewhat > OpenSolaris-centric, explanation of the issue here: > http://blogs.sun.com/roch/entry/nfs_and_zfs_a_fine > > The problem shows up more with ZFS because it enforces proper cache > semantics, while many other filesystems do not. This isn't always a > satisfactory explanation to users who expect to be able to untar files > in a reasonable amount of time, however. ;) Thanks, I see what you mean. FWIW, both Linux and FreeBSD 7.3 nfs clients appear to be running asynchronously by default (zpool iostat 1 shows bursty writes while extracting with tar), whereas Solaris 10 clients appear to be doing very slow synchronous operations. __Martin
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