Date: Mon, 22 Jun 2009 00:25:53 +0200 From: "Ronald Klop" <ronald-freebsd8@klop.yi.org> To: "Dan Naumov" <dan.naumov@gmail.com>, "FreeBSD-STABLE Mailing List" <freebsd-stable@freebsd.org>, freebsd-fs@freebsd.org Subject: Re: ufs2 / softupdates / ZFS / disk write cache Message-ID: <op.uvwa9fm18527sy@82-170-177-25.ip.telfort.nl> In-Reply-To: <cf9b1ee00906201429y7ec68afdse66be30fc2f75e8f@mail.gmail.com> References: <cf9b1ee00906201429y7ec68afdse66be30fc2f75e8f@mail.gmail.com>
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On Sat, 20 Jun 2009 23:29:26 +0200, Dan Naumov <dan.naumov@gmail.com> wrote: > I have the following setup: > > A single consumer-grade 2tb SATA disk: Western Digital Green (model > WDC WD20EADS-00R6B0). This disk is setup like this: > > 16gb root partition with UFS2 + softupdates, containing mostly static > things: > /bin /boot /etc /root /sbin /usr /var and such > > a 1,9tb non-redundant zfs pool on top of a slice, it hosts things like: > /DATA, /home, /usr/local, /var/log and such. > > What should I do to ensure (as much as possible) filesystem > consistency of the root filesystem in the case of the power loss? I > know there have been a lot of discussions on the subject of > consumer-level disks literally lying about the state of files in > transit (disks telling the system that files have been written to disk > while in reality they are still in disk's write cache), in turn > throwing softupdates off balance (since softupdates assumes the disks > don't lie about such things), in turn sometimes resulting in severe > data losses in the case of a system power loss during heavy disk IO. > > One of the solutions that was often brought up in the mailing lists is > disabling the actual disk write cache via adding hw.ata.wc=0 to > /boot/loader.conf, FreeBSD 4.3 actually even had this setting by > default, but this was apparently reverted back because some people > have reported a write performance regression on the tune of becoming > 4-6 times slower. So what should I do in my case? Should I disable > disk write cache via the hw.ata.wc tunable? As far as I know, ZFS has > a write cache of it's own and since the ufs2 root filesystem in my > case is mostly static data, I am guessing I "shouldn't" notice that > big of a performance hit. Or am I completely in the wrong here and > setting hw.ata.wc=0 is going to adversely affect the write performance > on both the root partition AND the zfs pool despite zfs using it's own > write cache? > > Another thing I have been pondering is: I do have 2gb of space left > unused on the system (currently being used as swap, I have 2 swap > slices, one 1gb at the very beginning of the disk, the other being 2gb > at the end), which I could turn into a GJOURNAL for the root > filesystem... Using gjournal is a very trusted way for a good balance in consistency and speed. I don't know about any performance impact of having the journal at the other 'end' of the disk than where the fs is. You can try, because switching back is possible. Ronald.
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