On Mon, 13 Jan 2014 11:15:13 +0100 Gionatan Danti <g.danti@xxxxxxxxxx> wrote: > On 01/13/2014 10:45 AM, NeilBrown wrote: > > On Mon, 13 Jan 2014 09:52:50 +0100 Gionatan Danti <g.danti@xxxxxxxxxx> wrote: > > > >> Hi Neil, > >> let me recap from a previous message: > >> > >> >FAR LAYOUT > >> >md(4) states: > >> >"The first copy of all data blocks will be striped across the early >part > >> >of all drives in RAID0 fashion, and then the next copy of all blocks > >> >will be striped across a later section of all drives, always ensuring > >> >that all copies of any given block are on different drives" > >> > > >> >The "on different drives" part let me wonder _how_ are chunks > >> >distributed. On a 4-disk array, I can imagine some different schemas: > >> > > >> >1) A1 A2 A3 A4 > >> > .. .. .. .. > >> > A4 A1 A2 A3 > >> > > >> >2) A1 A2 A3 A4 > >> > .. .. .. .. > >> > A2 A1 A4 A3 > >> > > >> >The first schema is the one depicted by SuSe documentation [1], while > >> >the second is the one described by Wikipedia [2]. > >> > > >> >Question 1: as the two schema have different reliability > >> >characteristics, which is really used? > >> > >> SuSe entry: > >> https://www.suse.com/documentation/sles11/stor_admin/data/raidmdadmr10cpx.html#b7cynnk > >> > >> Wikipedia entry: > >> http://en.wikipedia.org/wiki/Linux_MD_RAID_10#LINUX-MD-RAID-10 (see how > >> far layout is depicted) > >> > >> Keld kindly told me that the SuSe is simply not updated, as it depict a > >> situation changed with newer kernels. So my two questions: > > > > I cannot see an important difference between the two pages you reference. > > Both appear to be correct. > > Mmm... they seem different to me. > > SeSe FAR Layout: > > sda1 sdb1 sdc1 sde1 > 0 1 2 3 > 4 5 6 7 > . . . > 3 0 1 2 > 7 4 5 6 > > Notice how (for example) sdb1 is coupled both to sda1 (0,4) and > sdc1(1,5). If sdb1 fails, any sda1 or sdc1 failure lead to data loss. > > Now, Wikipedia FAR Layout: > > 4 drives (sda1, sdb1, sdc1, sdd1) > -------------------- > A1 A2 A3 A4 > A5 A6 A7 A8 > A9 A10 A11 A12 > .. .. .. .. > A2 A1 A4 A3 > A6 A5 A8 A7 > A10 A9 A12 A11 > .. .. .. .. > > Notice now how a single disk (eg: sdb1) is coupled to only another > _single_ disk (eg: sda1). In this case, if sdb1 fails, you had to lose > sda1 to have a data loss. Losing sdc1 or sdd1 will _not_ lead to data loss. > Thanks for being explicit - it is much easier to answer explicit questions :-) Yes, they are different. So the wikipedia article is wrong, or at least misleading. That is not what the "f2" layout looks like. The md driver does support that layout. I don't know yet what mdadm will call it, but it won't be called "f2". So this change: http://en.wikipedia.org/w/index.php?title=Non-standard_RAID_levels&diff=501908270&oldid=501604733 was wrong. NeilBrown
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