From mboxrd@z Thu Jan 1 00:00:00 1970 From: Neil Brown Subject: Re: RAID6 and crashes (reporting back re. --bitmap) Date: Tue, 15 Jun 2010 07:24:49 +1000 Message-ID: <20100615072449.43ab5c9d@notabene.brown> References: <4C1128C2.4020105@meetinghouse.net> <001501cb091f$07ac4e40$1704eac0$@woodlea.com> <20100611145019.6f1a5e7d@notabene.brown> <201006131628.35037.bernd.schubert@fastmail.fm> <20100614090520.2ae5a961@notabene.brown> <20100614151425.11f91562@natsu> <20100614194742.4a383a3f@notabene.brown> <20100614175327.57f05bb1@natsu> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20100614175327.57f05bb1@natsu> Sender: linux-raid-owner@vger.kernel.org To: Roman Mamedov Cc: Bernd Schubert , linux-raid@vger.kernel.org List-Id: linux-raid.ids On Mon, 14 Jun 2010 17:53:27 +0600 Roman Mamedov wrote: > On Mon, 14 Jun 2010 19:47:42 +1000 > Neil Brown wrote: > > > When you create an array an specify that a bitmap be added at the same time, > > there is more flexibility for size and location of the bitmap. It can easily > > be more that 3K in that case. > > > > So presumably this array was created with a bitmap, rather than created > > without a bitmap and had a bitmap added later with --grow. Correct? > > Yes, as far as I remember. However, it seems a bit unfortunate to have any > significant difference between adding the bitmap right when creating the > array, and adding it later. For that reason, I'd suggest reserving more space > in the metadata than 3K, even if the bitmap isn't requested - and even if it > won't be added later, then that space could prove useful for something else > that might require it later. Maybe 64, 128 or 256K - still miniscule compared > to the array size, and could provide some nice flexibility for the future. > Yes. An I'm fairly sure mdadm does always reserve space. The point is that (until very recently) it wasn't possible to tell the kernel where to add a bitmap to an active array - just that it should add one. So it could only add it at a place that it was certain would be usable. That is the place I described. It is now possible to give the kernel more details of the bitmap to add. I just need to teach mdadm how to choose the best space and how to tell the kernel about it. NeilBrown