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dkim=none (message not signed) header.d=none;redhat.com; dmarc=none action=none header.from=virtuozzo.com; Received: from AM7PR08MB5494.eurprd08.prod.outlook.com (2603:10a6:20b:dc::15) by AM6PR08MB5208.eurprd08.prod.outlook.com (2603:10a6:20b:eb::27) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3977.29; Mon, 29 Mar 2021 20:18:48 +0000 Received: from AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::b403:c1a9:6bb7:133]) by AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::b403:c1a9:6bb7:133%6]) with mapi id 15.20.3977.033; Mon, 29 Mar 2021 20:18:48 +0000 Subject: Re: [PATCH v3 6/6] block/qcow2: use seqcache for compressed writes To: Max Reitz , qemu-block@nongnu.org Cc: qemu-devel@nongnu.org, crosa@redhat.com, ehabkost@redhat.com, kwolf@redhat.com, jsnow@redhat.com References: <20210305173507.393137-1-vsementsov@virtuozzo.com> <20210305173507.393137-7-vsementsov@virtuozzo.com> From: Vladimir Sementsov-Ogievskiy Message-ID: <8c702bb9-416f-ada9-4691-22f37d82b2b8@virtuozzo.com> Date: Mon, 29 Mar 2021 23:18:46 +0300 User-Agent: Mozilla/5.0 (X11; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR03-VE1-obe.outbound.protection.outlook.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_PASS=-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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 12.03.2021 21:15, Max Reitz wrote: > On 05.03.21 18:35, Vladimir Sementsov-Ogievskiy wrote: >> Compressed writes are unaligned to 512, which works very slow in >> O_DIRECT mode. Let's use the cache. >> >> Signed-off-by: Vladimir Sementsov-Ogievskiy >> --- >>   block/coroutines.h     |   3 + >>   block/qcow2.h          |   4 ++ >>   block/qcow2-refcount.c |  10 +++ >>   block/qcow2.c          | 158 ++++++++++++++++++++++++++++++++++++++--- >>   4 files changed, 164 insertions(+), 11 deletions(-) >> [..] >> +static int coroutine_fn >> +qcow2_co_compressed_flush_one(BlockDriverState *bs, bool unfinished) >> +{ >> +    BDRVQcow2State *s = bs->opaque; >> +    int ret; >> +    int64_t align = 1; >> +    int64_t offset, bytes; >> +    uint8_t *buf; >> + >> +    if (!s->compressed_cache) { >> +        return 0; >> +    } >> + >> +    if (!seqcache_get_next_flush(s->compressed_cache, &offset, &bytes, &buf, >> +                                 &unfinished)) >> +    { >> +        return 0; >> +    } >> + >> +    qcow2_inflight_writes_inc(bs, offset, bytes); >> + >> +    /* >> +     * Don't try to align-up unfinished cached cluster, parallel write to it is >> +     * possible! For finished host clusters data in the tail of the cluster will >> +     * be never used. So, take some good alignment for speed. >> +     * >> +     * Note also, that seqcache guarantees that allocated size of @buf is enough >> +     * to fill the cluster up to the end, so we are safe to align up with >> +     * align <= cluster_size. >> +     */ >> +    if (!unfinished) { >> +        align = MIN(s->cluster_size, >> +                    MAX(s->data_file->bs->bl.request_alignment, >> +                        bdrv_opt_mem_align(bs))); >> +    } >> + > > I’d move the pre-write overlap check here, because its purpose is to prevent writes to metadata structures as they are happening, not as they are queued into a cache.  (I’d say.) Hmm. pre-write overlap check usually done under mutex.. Should I add an additional critical section to do overlap check? I'm not sure. > >> +    BLKDBG_EVENT(s->data_file, BLKDBG_WRITE_COMPRESSED); >> +    ret = bdrv_co_pwrite(s->data_file, offset, >> +                         QEMU_ALIGN_UP(offset + bytes, align) - offset, > > I remember you said you wanted to describe more of the properties of the buffer returned by seqcache_get_next_flush().  That would be nice here, because intuitively one would assume that the buffer is @bytes bytes long, which aligning here will exceed.  (It’s fine, but it would be nicer if there was a comment that assured that it’s fine.) > It's here, read several lines above: "Note also, that..." >> +                         buf, 0); >> +    if (ret < 0 && s->compressed_flush_ret == 0) { >> +        /* >> +         * The data that was "written" earlier is lost. We don't want to >> +         * care with storing it somehow to retry flushing later. -- Best regards, Vladimir