On Sat, 19 Feb 2011 04:55:32 +0000 Mathias Burén <mathias.buren@xxxxxxxxx> wrote: > On 19 February 2011 04:49, NeilBrown <neilb@xxxxxxx> wrote: > > On Sat, 19 Feb 2011 00:47:10 +0000 Mathias Burén <mathias.buren@xxxxxxxxx> > > wrote: > > > >> Hi, > >> > >> >From the mdadm manual: > >> > >> "-b, --bitmap= > >> [...] > >> Note: external bitmaps are only known to work on ext2 and ext3. > >> Storing bitmap files on other filesystems may result in serious > >> problems." > >> > >> I was planning to store the external bitmap on an ext4 partition. Will > >> this be a problem, or is the warning there because it hasn't been > >> tested enough but no problems found? > > > > External bitmaps use 'BMAP' to find where the file lives on the device and > > then writes directly to the device - not through the filesystem. > > > > So as long as there is no tail-packing to block migration happening it should > > work fine. > > > > I haven't looked inside ext4 but I am fairly confident that external bitmaps > > will work properly. > > > > NeilBrown > > > > > > Thanks, Neil. Are there any differences between using an internal or > external bitmap? Also, does one need a bitmap at all when not planning > to "mess" with the array? (does it provide any other purpose?) The difference between internal and external is simply that one is stored in the array - a copy on each device - and the other is stored externally to the array - a single copy in a file. External are slightly harder to work with as you need somewhere separate to store the bitmap (it cannot be in the array) and you need to tell mdadm where to find it (so it needs to be listed in mdadm.conf). I don't know what you mean by "mess" with the array. The main purpose of the bitmap is the accelerate resync after an unclean shutdown. The secondary purpose is to accelerate recovery if you remove and then re-add a device. Maybe that is what you mean by "mess with". So if you don't want to "mess" with the array and you are certain that your machine will never crash, then you don't really need a bitmap. NeilBrown -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
