From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: External bitmap, questions Date: Sat, 19 Feb 2011 16:30:18 +1100 Message-ID: <20110219163018.26058ce3@notabene.brown> References: <20110219154924.2218c87c@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Mathias =?ISO-8859-1?B?QnVy6W4=?= Cc: Linux-RAID List-Id: linux-raid.ids On Sat, 19 Feb 2011 04:55:32 +0000 Mathias Bur=E9n wrote: > On 19 February 2011 04:49, NeilBrown wrote: > > On Sat, 19 Feb 2011 00:47:10 +0000 Mathias Bur=E9n > > wrote: > > > >> Hi, > >> > >> >From the mdadm manual: > >> > >> "-b, --bitmap=3D > >> [...] > >> 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 dev= ice 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 externa= l bitmaps > > will work properly. > > > > NeilBrown > > > > >=20 > Thanks, Neil. Are there any differences between using an internal or > external bitmap? Also, does one need a bitmap at all when not plannin= g > to "mess" with the array? (does it provide any other purpose?) The difference between internal and external is simply that one is stor= ed 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 separat= e 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 seconda= ry purpose is to accelerate recovery if you remove and then re-add a devic= e. 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" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html