From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Snitzer Subject: Re: dm raid: ensure metadata IO matches device block size. Date: Wed, 15 Oct 2014 09:13:08 -0400 Message-ID: <20141015131307.GA23955@redhat.com> References: <20141015121907.265b3aed@notabene.brown> <20141015025550.GC19683@redhat.com> <20141015144003.7b17752d@notabene.brown> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20141015144003.7b17752d@notabene.brown> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: NeilBrown Cc: Liuhua Wang , Heinz Mauelshagen , device-mapper development , Alasdair G Kergon List-Id: dm-devel.ids On Tue, Oct 14 2014 at 11:40pm -0400, NeilBrown wrote: > On Tue, 14 Oct 2014 22:55:50 -0400 Mike Snitzer wrote: > > > On Tue, Oct 14 2014 at 9:19pm -0400, > > NeilBrown wrote: > > > > > > > > dm_raid_superblock is 512. > > > Reading or writing this on a 512-byte sector works fine. > > > On a 4096-byte sector device, this fails. > > > > > > If we round up rdev->sb_size to match the block size of > > > the device, all IO will work correctly. > > > > > > Reported-by: "Liuhua Wang" > > > Signed-off-by: NeilBrown > > > > > > --- > > > this issue has been discussed already a bit. See email thread > > > Subject: Re: [dm-devel] [PATCH] fix mirror device creation with lvcreate failed > > > I think this is the best fix. It handles boths read and writes, and (I think) > > > at the best level. > > > > > > Thanks, > > > NeilBrown > > > > > > > > > diff --git a/drivers/md/dm-raid.c b/drivers/md/dm-raid.c > > > index 4880b69e2e9e..31bdd73bc368 100644 > > > --- a/drivers/md/dm-raid.c > > > +++ b/drivers/md/dm-raid.c > > > @@ -858,7 +858,8 @@ static int super_load(struct md_rdev *rdev, struct md_rdev *refdev) > > > uint64_t events_sb, events_refsb; > > > > > > rdev->sb_start = 0; > > > - rdev->sb_size = sizeof(*sb); > > > + rdev->sb_size = roundup(sizeof(*sb), > > > + bdev_logical_block_size(rdev->meta_bdev)); > > > > > > ret = read_disk_sb(rdev, rdev->sb_size); > > > if (ret) > > > > Wouldn't it be better to use bdev_physical_block_size()? > > > > Even on a 4K device that emulates 512b logical sectors it is better to > > use the physical block size (4K). > > > _logical_ is the smallest value for which the IO actually works. > And the goal of the change is to make it work. > > I don't object to using _physical_, but it isn't clear to me how I would > justify that as "correct". > > A big question in my mind is: how much space does LVM reserve in this device > for the metadata? It seems reasonable to assume that it reserves at least > 1 logical block. If the API guarantees that at least one physical block is > reserved, then that would justify using _physical_. I'll have to check with Jon and/or Heinz on this point. > A quick look at the code shows that the bitmap superblock is placed 4K after > the start of the metadata. "the code" being the MD kernel code right? Any reason not to export a #define that reflects the space MD reserves and just have dm-raid use that? Starting to feel like hardcoding 4K is the right thing to do given the current code.