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Lunev" , Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org References: <20210129165030.640169-1-vsementsov@virtuozzo.com> <745c4617-01bc-c888-b6da-95a705cf0c1e@redhat.com> <0669a5e8-bcff-ffb1-23b0-0af9ce20ad27@virtuozzo.com> <476836f5-09d8-976d-bc3c-afb05befddbd@redhat.com> <6a379fe0-cf87-bc5e-ae53-473599ea10c4@openvz.org> From: Max Reitz Message-ID: Date: Wed, 10 Feb 2021 11:00:44 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <6a379fe0-cf87-bc5e-ae53-473599ea10c4@openvz.org> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.57, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.265, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kwolf@redhat.com, qemu-devel@nongnu.org, armbru@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 09.02.21 17:52, Denis V. Lunev wrote: > On 2/9/21 5:47 PM, Max Reitz wrote: >> On 09.02.21 15:10, Vladimir Sementsov-Ogievskiy wrote: >>> 09.02.2021 16:25, Max Reitz wrote: >>>> On 29.01.21 17:50, Vladimir Sementsov-Ogievskiy wrote: >>>>> Hi all! >>>>> >>>>> I know, I have several series waiting for a resend, but I had to >>>>> switch >>>>> to another task spawned from our customer's bug. >>>>> >>>>> Original problem: we use O_DIRECT for all vm images in our product, >>>>> it's >>>>> the policy. The only exclusion is backup target qcow2 image for >>>>> compressed backup, because compressed backup is extremely slow with >>>>> O_DIRECT (due to unaligned writes). Customer complains that backup >>>>> produces a lot of pagecache. >>>>> >>>>> So we can either implement some internal cache or use fadvise somehow. >>>>> Backup has several async workes, which writes simultaneously, so in >>>>> both >>>>> ways we have to track host cluster filling (before dropping the cache >>>>> corresponding to the cluster).  So, if we have to track anyway, let's >>>>> try to implement the cache. >>>> >>>> I wanted to be excited here, because that sounds like it would be >>>> very easy to implement caching.  Like, just keep the cluster at >>>> free_byte_offset cached until the cluster it points to changes, then >>>> flush the cluster. >>> >>> The problem is that chunks are written asynchronously.. That's why >>> this all is not so easy. >>> >>>> >>>> But then I see like 900 new lines of code, and I’m much less excited... >>>> >>>>> Idea is simple: cache small unaligned write and flush the cluster when >>>>> filled. >>>>> >>>>> Performance result is very good (results in a table is time of >>>>> compressed backup of 1000M disk filled with ones in seconds): >>>> >>>> “Filled with ones” really is an edge case, though. >>> >>> Yes, I think, all clusters are compressed to rather small chunks :) >>> >>>> >>>>> ---------------  -----------  ----------- >>>>>                   backup(old)  backup(new) >>>>> ssd:hdd(direct)  3e+02        4.4 >>>>>                                  -99% >>>>> ssd:hdd(cached)  5.7          5.4 >>>>>                                  -5% >>>>> ---------------  -----------  ----------- >>>>> >>>>> So, we have benefit even for cached mode! And the fastest thing is >>>>> O_DIRECT with new implemented cache. So, I suggest to enable the new >>>>> cache by default (which is done by the series). >>>> >>>> First, I’m not sure how O_DIRECT really is relevant, because I don’t >>>> really see the point for writing compressed images. >>> >>> compressed backup is a point >> >> (Perhaps irrelevant, but just to be clear:) I meant the point of using >> O_DIRECT, which one can decide to not use for backup targets (as you >> have done already). >> >>>> Second, I find it a bit cheating if you say there is a huge >>>> improvement for the no-cache case, when actually, well, you just >>>> added a cache.  So the no-cache case just became faster because >>>> there is a cache now. >>> >>> Still, performance comparison is relevant to show that O_DIRECT as is >>> unusable for compressed backup. >> >> (Again, perhaps irrelevant, but:) Yes, but my first point was exactly >> whether O_DIRECT is even relevant for writing compressed images. >> >>>> Well, I suppose I could follow that if O_DIRECT doesn’t make much >>>> sense for compressed images, qemu’s format drivers are free to >>>> introduce some caching (because technically the cache.direct option >>>> only applies to the protocol driver) for collecting compressed writes. >>> >>> Yes I thought in this way, enabling the cache by default. >>> >>>> That conclusion makes both of my complaints kind of moot. >>>> >>>> *shrug* >>>> >>>> Third, what is the real-world impact on the page cache?  You >>>> described that that’s the reason why you need the cache in qemu, >>>> because otherwise the page cache is polluted too much.  How much is >>>> the difference really?  (I don’t know how good the compression ratio >>>> is for real-world images.) >>> >>> Hm. I don't know the ratio.. Customer reported that most of RAM is >>> polluted by Qemu's cache, and we use O_DIRECT for everything except >>> for target of compressed backup.. Still the pollution may relate to >>> several backups and of course it is simple enough to drop the cache >>> after each backup. But I think that even one backup of 16T disk may >>> pollute RAM enough. >> >> Oh, sorry, I just realized I had a brain fart there.  I was referring >> to whether this series improves the page cache pollution.  But >> obviously it will if it allows you to re-enable O_DIRECT. >> >>>> Related to that, I remember a long time ago we had some discussion >>>> about letting qemu-img convert set a special cache mode for the >>>> target image that would make Linux drop everything before the last >>>> offset written (i.e., I suppose fadvise() with >>>> POSIX_FADV_SEQUENTIAL).  You discard that idea based on the fact >>>> that implementing a cache in qemu would be simple, but it isn’t, >>>> really.  What would the impact of POSIX_FADV_SEQUENTIAL be?  (One >>>> advantage of using that would be that we could reuse it for >>>> non-compressed images that are written by backup or qemu-img convert.) >>> >>> The problem is that writes are async. And therefore, not sequential. >> >> In theory, yes, but all compressed writes still goes through >> qcow2_alloc_bytes() right before submitting the write, so I wonder >> whether in practice the writes aren’t usually sufficiently sequential >> to make POSIX_FADV_SEQUENTIAL work fine. >> >>> So >>> I have to track the writes and wait until the whole cluster is >>> filled. It's simple use fadvise as an option to my cache: instead of >>> caching data and write when cluster is filled we can instead mark >>> cluster POSIX_FADV_DONTNEED. >>> >>>> >>>> (I don’t remember why that qemu-img discussion died back then.) >>>> >>>> >>>> Fourth, regarding the code, would it be simpler if it were a pure >>>> write cache?  I.e., on read, everything is flushed, so we don’t have >>>> to deal with that.  I don’t think there are many valid cases where a >>>> compressed image is both written to and read from at the same time. >>>> (Just asking, because I’d really want this code to be simpler.  I >>>> can imagine that reading from the cache is the least bit of >>>> complexity, but perhaps...) >>>> >>> >>> Hm. I really didn't want to support reads, and do it only to make it >>> possible to enable the cache by default.. Still read function is >>> really simple, and I don't think that dropping it will simplify the >>> code significantly. >> >> That’s too bad. >> >> So the only question I have left is what POSIX_FADV_SEQUENTIAL >> actually would do in practice. >> >> (But even then, the premise just doesn’t motivate me sufficiently yet...) >> > POSIX_FADV_SEQUENTIAL influences the amount of the read-ahead > only. > > int generic_fadvise(struct file *file, loff_t offset, loff_t len, int > advice) > { > ..... >     case POSIX_FADV_NORMAL: >         file->f_ra.ra_pages = bdi->ra_pages; >         spin_lock(&file->f_lock); >         file->f_mode &= ~FMODE_RANDOM; >         spin_unlock(&file->f_lock); >         break; >     case POSIX_FADV_SEQUENTIAL: >         file->f_ra.ra_pages = bdi->ra_pages * 2; >         spin_lock(&file->f_lock); >         file->f_mode &= ~FMODE_RANDOM; >         spin_unlock(&file->f_lock); >         break; > > thus the only difference from the usual NORMAL mode would be > doubled amount of read-ahead pages. :/ Thanks for looking it up. Max