From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tomasz Chmielewski Subject: Re: how to synchronize two devices (RAID-1, but not really?) Date: Tue, 15 May 2007 13:21:27 +0200 Message-ID: <464997B7.4040600@wpkg.org> References: <4649760A.1060805@wpkg.org> <17993.34959.150157.200536@notabene.brown> <46498E60.9060407@wpkg.org> <17993.38016.174124.287952@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <17993.38016.174124.287952@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Neil Brown schrieb: > On Tuesday May 15, mangoo@wpkg.org wrote: >> Neil Brown schrieb: >> >> (...) >> >>> An external bitmap means that if the link goes down, it keeps track of >>> which blocks are in sync and which aren't, and when the link comes >>> back up you re-add the missing device and the rebuild continues where >>> it left off. >>> mdadm --build /dev/md22 --level=1 --bitmap=/root/mybitmap \ >>> --write-behind --raid-disks=2 /dev/localdevice --write-mostly /dev/remotedevice >> One more question - is there a way to estimate the size of the bitmap >> file? Does it depend on the size of the array? >> >> What bitmap file size can I expect for a 600 GB array? > > Due to internal implementation details, mdadm limits the size of the > bitmap to 2^20 bits. So the file will be 100K +/- 50%. > That will lead to about 1Meg per bit. If you have a failure, this > might mean you end up re-copying several megabytes more than you > really need to, but that should add up to less than one second. Good, I was wondering if ~200 MB left I have on a filesystem would be enough :) -- Tomasz Chmielewski http://wpkg.org