From mboxrd@z Thu Jan 1 00:00:00 1970 From: Shaohua Li Subject: Re: [PATCH] md/raid5: write an empty meta-block when creatinglogsuper-block Date: Thu, 27 Oct 2016 15:21:45 -0700 Message-ID: <20161027222145.h6wwtls6a5qynb7u@kernel.org> References: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Zhengyuan Liu Cc: Song Liu , linux-raid , liuzhengyuang521 List-Id: linux-raid.ids On Thu, Oct 27, 2016 at 10:05:06PM +0800, Zhengyuan Liu wrote: > Sorry for the unclear expression. > > The log might look like this before we did a recovery : > | mb1 | mb2 | mb3 | | | | > last_checkpoint = mb1'postion, last_cp_seq = mb1'seq > After we did a recovery(we would write a empty meta block emb at log tail): > | mb1 | mb2 | mb3 | emb | | | > last_checkpoint = emb'position, last_cp_seq = mb1'seq + 11 > Then we write two meta blocks and suppose crash happens: > | mb1 | mb2 | mb3 | emb | mb4 | mb5 | > last_checkpoint = emb'position, last_cp_seq = mb1'seq + 11 > Now we did another recovery after restart and suppose mb4 was invalid: > | mb1 | mb2 | mb3 | emb | mb4 | mb5 | > last_checkpoint = emb'position, last_cp_seq = mb1'seq + 11 > Since mb4 was invalid, we would stop recovering mb5 which should be discarded. > After recovery, log_start points to mb4 and we wouldn't write a empty meta block > because condition "ctx.seq > log->last_cp_seq + 1" doesn't satisfy. If we are going to > write a valid meta block and crash happens again, the new meta block will fall into > position of mb4 and recovery process may do a recovery to mb5 since it's seq > is matched. > > What I try to say is that if the first meta block ,not only the mid one, we written was > invalid, the log recovery could bring problem here too . I think the condition for > write a empty meta block should like this: > - if (ctx.seq > log->last_cp_seq + 1) { > + if (ctx.seq > log->last_cp_seq） { Get it, thanks! That's correct, as long as we recover one block, we should rewirte the empty meta block. I'll queue a patch for this. Thanks, Shaohua