From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cn.fujitsu.com ([222.73.24.84]:3148 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1753342Ab2FKD6J (ORCPT ); Sun, 10 Jun 2012 23:58:09 -0400 Message-ID: <4FD56E71.9070101@cn.fujitsu.com> Date: Mon, 11 Jun 2012 12:05:05 +0800 From: Liu Bo MIME-Version: 1.0 To: WeiFeng Liu CC: linux-btrfs Subject: Re: Two questions about update_global_block_rsv() References: <20120607144622.96FFDE674A@smtp.hushmail.com> In-Reply-To: <20120607144622.96FFDE674A@smtp.hushmail.com> Content-Type: text/plain; charset=UTF-8 Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 06/07/2012 10:46 PM, WeiFeng Liu wrote: > Help two questions about update_global_block_rsv() > > update_global_block_rsv(fs_info) { > > ... > block_rsv = &fs_info->global_block_rsv; > sinfo = block_rsv->space_info; > > num_bytes = sinfo->bytes_used + sinfo->bytes_pinned + > sinfo->bytes_reserved + sinfo->bytes_readonly + > sinfo->bytes_may_use; > > if (sinfo->total_bytes > num_bytes) { > num_bytes = sinfo->total_bytes - num_bytes; > block_rsv->reserved += num_bytes; > sinfo->bytes_may_use += num_bytes; > } > ... > } > > 1. I don't quite understand the lines: > block_rsv->reserved += num_bytes; > sinfo->bytes_may_use += num_bytes; > > why aren't they like these? > block_rsv->reserved = num_bytes; > sinfo->bytes_may_use = num_bytes; > > assuming that nothing else is changed, and we just call > update_global_block_rsv() for several times, then block_rsv->reserved will > increase constantly, and so sinfo->bytes_may_use; can anyone tell me what it > means? I mean if nothing else is changed the two variables should keep their > values unchanged also, is that right? a bit puzzlement to me. > the following if-statement tells the truth: if (block_rsv->reserved >= block_rsv->size) { ... } > 2. Is the meaning of sinfo->bytes_may_use that bytes which is available for use > or that bytes which will likely/unlikely be used? > You need to understand how a block_rsv works, both block_rsv and space_info are a kind of _container_: Introducing metadata reseravtion contexts has two major advantages. First, it makes metadata reservation more traceable. Second, it can reclaim freed space and re-add them to the itself after transaction committed. here is a basic model: +--+--->size +--+--->size +--------------+ | | | | | | | | | | | | +--+--->reserved +--+--->reserved +--------------+-->may_used | | | | | | | | | | | | | | | | | | +--+ +--+ +--------------+ block_rsv + block_rsv + ... = space_info(flag=4) A B | | +--> reserved meta bytes Hope that this helps. thanks, liubo > thanks > > WeiFeng Liu > > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >