Date: Sat, 18 May 2013 08:40:00 -0400 From: Paul Kraus <paul@kraus-haus.org> To: kpneal@pobox.com Cc: Liste FreeBSD <freebsd-questions@freebsd.org> Subject: Re: ZFS install on a partition Message-ID: <4F46D2E7-308C-45C9-B3A0-B2870E69AC19@kraus-haus.org> In-Reply-To: <20130518044900.GA30652@neutralgood.org> References: <F744BBF1-D98C-47BF-9546-14D1A9CB3733@todoo.biz> <5FB78660-EDA4-40DB-8A0D-B0DE23A0FB6C@kraus-haus.org> <20130518044900.GA30652@neutralgood.org>
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On May 18, 2013, at 12:49 AM, kpneal@pobox.com wrote: > On Fri, May 17, 2013 at 08:03:30PM -0400, Paul Kraus wrote: >> On May 17, 2013, at 6:24 PM, "bsd@todoo.biz" <bsd@todoo.biz> wrote: >>> 3. Should I avoid using ZFS since my system is not well tuned and It = would be asking for trouble to use ZFS in these conditions.=20 >>=20 >> No. One of the biggest benefits of ZFS is the end to end data = integrity. >> IF there is a silent fault in the HW RAID (it happens), ZFS will = detect >> the corrupt data and note it. If you had a mirror or other redundant = device, >> ZFS would then read the data from the *other* copy and rewrite the = bad >> block (or mark that physical block bad and use another). >=20 > I believe the "copies=3D2" and "copies=3D3" option exists to enable = ZFS to > self heal despite ZFS not being in charge of RAID. If ZFS only has a = single > LUN to work with, but the copies=3D2 or more option is set, then if = ZFS > detects an error it can still correct it. Yes, but =85. What the "copies=3D<n>" parameter does is tell ZFS = to make that many copies of every block written on the top level device. = So if you set copies=3D2 and then write a 2MB file, it will take up 4MB = of space since ZFS will keep two copies of it. ZFS will attempt to put = them on different devices if it can, but there are no guarantees here. = If you have a single vdev stripe and you lose that one device, you = *will* lose all your data (assuming you did not have another backup copy = someplace else). On the other hand, if the single device develops some = bad blocks, with copies=3D2 you will *probably* not lose data as there = will be other copies of those disk blocks elsewhere to recover from. =46rom my experience on the ZFS Discuss lists, the place people = seem to use copies=3D<more than 1> are on laptops where they only have = one drive and copies=3D<more than1> is better than no protection at all, = it is just not complete protection. > This option is a dataset option, is inheritable by child datasets, and = can > be changed at any time affecting data written after the change. To get = the > full benefit you'll therefore want to set the option before putting = data > into the relevant dataset. You can change it any time and it will only effect data written = from that point on. This can be useful if you have both high value data = band low value and you can control when each is written. For example, = you leave copies=3D1 for most of the time, then you want to save your = wedding photos, so you set copies=3D3 and write all the wedding photos, = you then set copies=3D1. You will have three copies of the wedding = photos and one copy of everything else. -- Paul Kraus Deputy Technical Director, LoneStarCon 3 Sound Coordinator, Schenectady Light Opera Company
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