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[49.186.86.168]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-21177ddd646sm65829975ad.62.2024.11.10.18.04.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 10 Nov 2024 18:04:46 -0800 (PST) Received: from dave by dread.disaster.area with local (Exim 4.96) (envelope-from ) id 1tAJnH-00CzKY-2n; Mon, 11 Nov 2024 13:04:43 +1100 Date: Mon, 11 Nov 2024 13:04:43 +1100 From: Dave Chinner To: Stephen Zhang Cc: djwong@kernel.org, dchinner@redhat.com, leo.lilong@huawei.com, wozizhi@huawei.com, osandov@fb.com, xiang@kernel.org, zhangjiachen.jaycee@bytedance.com, linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, zhangshida@kylinos.cn Subject: Re: [PATCH 0/5] *** Introduce new space allocation algorithm *** Message-ID: References: <20241104014439.3786609-1-zhangshida@kylinos.cn> Precedence: bulk X-Mailing-List: linux-xfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Nov 08, 2024 at 09:34:17AM +0800, Stephen Zhang wrote: > Dave Chinner 于2024年11月4日周一 20:15写道： > > On Mon, Nov 04, 2024 at 05:25:38PM +0800, Stephen Zhang wrote: > > > Dave Chinner 于2024年11月4日周一 11:32写道： > > > > On Mon, Nov 04, 2024 at 09:44:34AM +0800, zhangshida wrote: [snip unnecessary stereotyping, accusations and repeated information] > > AFAICT, this "reserve AG space for inodes" behaviour that you are > > trying to acheive is effectively what the inode32 allocator already > > implements. By forcing inode allocation into the AGs below 1TB and > > preventing data from being allocated in those AGs until allocation > > in all the AGs above start failing, it effectively provides the same > > functionality but without the constraints of a global first fit > > allocation policy. > > > > We can do this with any AG by setting it up to prefer metadata, > > but given we already have the inode32 allocator we can run some > > tests to see if setting the metadata-preferred flag makes the > > existing allocation policies do what is needed. > > > > That is, mkfs a new 2TB filesystem with the same 344AG geometry as > > above, mount it with -o inode32 and run the workload that fragments > > all the free space. What we should see is that AGs in the upper TB > > of the filesystem should fill almost to full before any significant > > amount of allocation occurs in the AGs in the first TB of space. Have you performed this experiment yet? I did not ask it idly, and I certainly did not ask it with the intent that we might implement inode32 with AFs. It is fundamentally impossible to implement inode32 with the proposed AF feature. The inode32 policy -requires- top down data fill so that AG 0 is the *last to fill* with user data. The AF first-fit proposal guarantees bottom up fill where AG 0 is the *first to fill* with user data. For example: > So for the inode32 logarithm: > 1. I need to specify a preferred ag, like ag 0: > |---------------------------- > | ag 0 | ag 1 | ag 2 | ag 3 | > +---------------------------- > 2. Someday space will be used up to 100%, Then we have to growfs to ag 7: > +------+------+------+------+------+------+------+------+ > | full | full | full | full | ag 4 | ag 5 | ag 6 | ag 7 | > +------+------+------+------+------+------+------+------+ > 3. specify another ag for inodes again. > 4. repeat 1-3. Lets's assume that AGs are 512GB each and so AGs 0 and 1 fill the entire lower 1TB of the filesystem. Hence if we get to all AGs full the entire inode32 inode allocation space is full. Even if we grow the filesystem at this point, we still *cannot* allocate more inodes in the inode32 space. That space (AGs 0-1) is full even after the growfs. Hence we will still give ENOSPC, and that is -correct behaviour- because the inode32 policy requires this behaviour. IOWs, growfs and changing the AF bounds cannot fix ENOSPC on inode32 when the inode space is exhausted. Only physically moving data out of the lower AGs can fix that problem... > for the AF logarithm: > mount -o af1=1 $dev $mnt > and we are done. > |<-----+ af 0 +----->|| > |---------------------------- > | ag 0 | ag 1 | ag 2 | ag 3 | > +---------------------------- > because the af is a relative number to ag_count, so when growfs, it will > become: > |<-----+ af 0 +--------------------------------->|| > +------+------+------+------+------+------+------+------+ > | full | full | full | full | ag 4 | ag 5 | ag 6 | ag 7 | > +------+------+------+------+------+------+------+------+ > So just set it once, and run forever. That is actually the general solution to the original problem being reported. I realised this about half way through reading your original proposal. This is why I pointed out inode32 and the preferred metadata mechanism in the AG allocator policies. That is, a general solution should only require the highest AG to be marked as metadata preferred. Then -all- data allocation will then skip over the highest AG until there is no space left in any of the lower AGs. This behaviour will be enforced by the existing AG iteration allocation algorithms without any change being needed. Then when we grow the fs, we set the new highest AG to be metadata preferred, and that space will now be reserved for inodes until all other space is consumed. Do you now understand why I asked you to test whether the inode32 mount option kept the data out of the lower AGs until the higher AGs were completely filled? It's because I wanted confirmation that the metadata preferred flag would do what we need to implement a general solution for the problematic workload. Remeber: free space fragmentation can happen for many reasons - this mysql thing is just the latest one discovered. The best solution is having general mechanisms in the filesystem that automatically mitigate the effects of free space fragmentation on inode allocation. The worst solution is requiring users to tweak knobs... -Dave. -- Dave Chinner david@fromorbit.com