Date: Sat, 16 Jan 2010 15:45:31 +0100 From: Pieter de Goeje <pieter@degoeje.nl> To: freebsd-questions@freebsd.org Cc: Mike Clarke <jmc-freebsd2@milibyte.co.uk> Subject: Re: Newbie gmirror questions Message-ID: <201001161545.31616.pieter@degoeje.nl> In-Reply-To: <201001152334.52978.jmc-freebsd2@milibyte.co.uk> References: <201001152334.52978.jmc-freebsd2@milibyte.co.uk>
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On Saturday 16 January 2010 00:34:52 Mike Clarke wrote: > I'm about to upgrade to more disk space and I'm tempted use this as an > opportunity to get two disks and implement gmirror. Before I go ahead > there's a few aspects of mirroring I'm not sure about and would > appreciate some advice. > > I'm using grub for multi booting. Does this introduce any problems if I > want to boot into Windows or Linux on one of the other partitions? Gmirror stores the metadata at the last sector of each disk. So this shouldn't be a problem. But other operating systems might overwrite this data if you're not careful during the paritioning. > > The gmirror manpage describes the procedure for handling kernel dumps > using the prefer balance algorithm in the early stages of booting and > then switching to round-robin in the /etc/rc.local script. It then goes > on to say that "If on the next boot a component with a higher priority > will be available, the prefer algorithm will choose to read from it and > savecore(8) will find nothing". Does this only arise if I've made some > change to the configuration of the mirror between the dump and the > reboot or is there some instances when the priority automatically > changes? Priority never changes automatically. > > Some of the articles I've read about gmirror suggest setting the balance > to round-robin while others just leave this at the default setting of > split. Am I right in assuming that round-robin would give better > performance, and does it make much noticeable difference in real terms. > In particular am I likely to see a reduction in performance using > gmirror compared with what I would get with just a normal single disk. Assuming you have two or more regular HDDs, I can recommend updating to 8-STABLE and using the "load" algorithm. It has had some major improvements lately, and is now the default. It should give equal or better read performance in comparison to a single disk in all cases. The performance of "split" and "round-robin" is very dependent on the access patterns and stripe size (for split). > > Finally, recent articles say to set kern.geom.debugflags to 17 when > creating a mirror on a mounted drive while older articles say to set it > to 16. Although I'll probably be creating the mirror on my disks before > copying my system onto them so I don't really need to worry about > setting this flag but I'm curious to know the difference between using > the two values. The sysctl is a bitfield, so 17 (0x11) enables some extra stuff compared to 16 (0x10). See geom(4), section DIAGNOSTICS for more details. -- Pieter de Goeje
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