On 05/04/2014 12:24 AM, Marc MERLIN wrote: > > Gotcha, thanks for confirming, so -m raid1 -d raid0 really only protects > against metadata corruption or a single block loss, but otherwise if you > lost a drive in a 2 drive raid0, you'll have lost more than just half > your files. > >> The scenario you mentioned at the beginning, "if I lose a drive, >> I'll still have full metadata for the entire filesystem and only >> missing files" is more applicable to using "-m raid1 -d single". >> Single is not geared towards performance and, though it doesn't >> guarantee a file is only on a single disk, the allocation does mean >> that the majority of all files smaller than a chunk will be stored >> on only one disk or the other - not both. > Ok, so in other words: > -d raid0: if you one 1 drive out of 2, you may end up with small files > and the rest will be lost > > -d single: you're more likely to have files be on one drive or the > other, although there is no guarantee there either. > > Correct? > > Thanks, > Marc This often seems to confuse people and I think there is a common misconception that the btrfs raid/single/dup features work at the file level when in reality they work at a level closer to lvm/md. If someone told you that they lost a device out of a jbod or multi disk lvm group(somewhat analogous to -d single) with ext on top you would expect them to lose data in any file that had a fragment in the lost region (lets ignore metadata for a moment). This is potentially up to 100% of the files but this should not be a surprising result. Similarly, someone who has lost a disk out of a md/lvm raid0 volume should not be surprised to have a hard time recovering any data at all from it. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
