On Thu, Sep 26, 2013 at 02:55:38PM +0000, miaou sami wrote: > OK, that's clear. > Nice space simulator btw :-) you should add a link somewhere in btrfs wiki...
There is one, linked from the first line of the relevant section in the FAQ. Hugo. > Thanks > ---------------------------------------- > > Date: Thu, 26 Sep 2013 14:46:05 +0100 > > From: h...@carfax.org.uk > > To: miaous...@hotmail.com > > CC: linux-btrfs@vger.kernel.org > > Subject: Re: [raidX vs single/dup] > > > > On Thu, Sep 26, 2013 at 01:40:57PM +0000, miaou sami wrote: > >> Thank you, it is quite clear now. > >> > >> > >> I guess that on multi device, raid0 vs single would be a matter of > >> performance vs ease of low level hardware data recovery. > >> > >> > >> The wiki > >> https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices > >> says: > >> "When you have drives with differing sizes and want to use the full > >> capacity of each drive, you have to use the single profile for the data > >> blocks." > >> Let's assume the following configuration: 1x10GB disk and 2x5GB disks > >> --> Does it mean I cannot use the full capacity AND have a duplication of > >> my data in the configuration above? (full capacity would be 10GB here) > > > > No, that will give you the full usable space. A 20 GB drive and two > > 5 GB drives would not, though. > > > >> --> If I try to setup either -d raid1 or -d dup on that > >> configuration, what will I get? > > > > Try it for yourself in the space simulator: > > > > http://carfax.org.uk/btrfs-usage/ > > > >> --> Is there any behavior difference between raid1 / dup in that case? > > > > If you have multiple disks, I think DUP gets automatically upgraded > > to RAID-1 (i.e. the "different copies on different devices" > > requirement is enforced). So, no. > > > >> --> Can raid1 ensure that data are always duplicated on different devices > >> AND take advantage of all available space? > > > > Depends on the relative sizes of the devices. If your largest > > device is bigger than the rest put together, then you'll lose some > > space. > > > > Hugo. > > > >> Regards, > >> Sam > >> > >> > >> ---------------------------------------- > >>> Date: Thu, 26 Sep 2013 13:32:33 +0100 > >>> From: h...@carfax.org.uk > >>> To: miaous...@hotmail.com > >>> CC: linux-btrfs@vger.kernel.org > >>> Subject: Re: [raidX vs single/dup] > >>> > >>> On Thu, Sep 26, 2013 at 12:22:49PM +0000, miaou sami wrote: > >>>> Hi btrfs guys, > >>>> > >>>> could someone explain to me the differences in mkfs.btrfs: > >>>> > >>>> - between -d raid0 and -d single > >>> > >>> In RAID0, data is striped across all the devices, so the first 64k > >>> of a file will go on device 1, the next 64k will go on device 2, and > >>> so on. With single, files are allocated linearly on one device. > >>> > >>> (This is assuming smallish files, a filesystem with lots of space. > >>> Even with single, files can still end up being scattered around over > >>> multiple devices -- but with RAID0, even non-fragmented files are > >>> striped) > >>> > >>>> - between -m raid1 and -m dup > >>> > >>> In both cases, there are two copies of each metadata block. With > >>> RAID1, it *requires* the two copies to live on different devices. With > >>> DUP, it allows the two copies to live on the same device (e.g. if > >>> there's only one device). > >>> > >>>> - between -m raid0 and -m single > >>> > >>> As for -draid0 and -dsingle, but for metadata instead of data. > >>> > >>>> My understanding is that raidX should be used in case of multi > >>>> devices and single/dup should be used in case of single device to > >>>> allow duplication, but it is not 100% clear to me... > >>> > >>>> As btrfs raid concepts are quite different from traditionnal raid, > >>>> shouldn't we use the words "stripped" and "mirrored" instead of > >>>> raid0/raid1? or even "single" and "duplicated"? > >>>> Then there would be no difference between single/raid0 and > >>>> duplicated/raid1... > >>> > >>> But there _are_ differences between them, as explained above. :) > >>> > >>> I posted a patch a while ago to change the names to something more > >>> logical and expressive, but it didn't get merged. > >>> > >>> Hugo. > >>> > > -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- The trouble with you, Ibid, is you think you know everything. ---
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