Date: Thu, 06 Jan 2011 15:45:04 +0200 From: Daniel Kalchev <daniel@digsys.bg> To: freebsd-stable@freebsd.org Subject: Re: ZFS - moving from a zraid1 to zraid2 pool with 1.5tb disks Message-ID: <4D25C760.4070703@digsys.bg> In-Reply-To: <4D25BF91.7070304@my.gd> References: <4D1C6F90.3080206@my.gd> <ifsia5$5ub$2@dough.gmane.org> <4D21E679.80002@my.gd> <84882169-0461-480F-8B4C-58E794BCC8E6@my.gd> <BEBC15BA440AB24484C067A3A9D38D7E0149F32D13E3@server7.acsi.ca> <488AE93A-97B9-4F01-AD0A-0098E4B329C3@my.gd> <AANLkTimezasVY%2BMJjWn2T9sBGQV-JrNmYqRwv_gPYPJP@mail.gmail.com> <CC37553B-EE13-4B5B-AC87-80D0ECC1A2B3@my.gd> <BEBC15BA440AB24484C067A3A9D38D7E0149F32D33F9@server7.acsi.ca> <4D25BF91.7070304@my.gd>
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For pure storage, that is a place you send/store files, you don't really need the ZIL. You also need the L2ARC only if you read over and over again the same dataset, which is larger than the available ARC (ZFS cache memory). Both will not be significant for 'backup server' application, because it's very unlikely to do lots of SYNC I/O (where separate ZIL helps), or serve the same files back (where the L2ARC might help). You should also know that having large L2ARC requires that you also have larger ARC, because there are data pointers in the ARC that point to the L2ARC data. Someone will do good to the community to publish some reasonable estimates of the memory needs, so that people do not end up with large but unusable L2ARC setups. It seems that the upcoming v28 ZFS will help greatly with the ZIL in the main pool.. You need to experiment with the L2ARC (this is safe with current v14 and v15 pools) to see if your usage will see benefit from it's use. Experimenting with ZIL currently requires that you recreate the pool. With the experimental v28 code things are much easier. On 06.01.11 15:11, Damien Fleuriot wrote: > I see, so no dedicated ZIL device in the end ? > > I could make a 15gb slice for the OS running UFS (I don't wanna risk > losing the OS when manipulating ZFS, such as during upgrades), and a > 25gb+ for L2ARC, depending on the disk. > > I can't afford a *dedicated* drive for the cache though, not enough room > in the machine. >
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