On 19 February 2011 05:30, NeilBrown <neilb@xxxxxxx> wrote: > On Sat, 19 Feb 2011 04:55:32 +0000 Mathias BurÃn <mathias.buren@xxxxxxxxx> > wrote: > >> On 19 February 2011 04:49, NeilBrown <neilb@xxxxxxx> wrote: >> > On Sat, 19 Feb 2011 00:47:10 +0000 Mathias BurÃn <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 Â- 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. ÂThe main purpose of the > bitmap is the accelerate resync after an unclean shutdown. ÂThe 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 > > Hi, Yes, you understood me correctly! Sorry, I should've been more clear. That answers my questions, thanks a bunch :-) // Mahtias -- 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
