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Thunderbird/102.5.0 Subject: Re: (PING) btrfs sequential 8K read()s from compressed files are not merging Content-Language: en-US To: Dimitrios Apostolou , Qu Wenruo Cc: Christoph Hellwig , linux-btrfs@vger.kernel.org References: <0db91235-810e-1c6e-7192-48f698c55c59@gmx.net> <4b16bd02-a446-8000-b10e-4b24aaede854@gmx.net> <62b24a0c-08e9-5dfd-33a6-a34dd93b1727@gmx.net> From: Anand Jain In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SI2PR01CA0035.apcprd01.prod.exchangelabs.com (2603:1096:4:192::13) To PH0PR10MB5706.namprd10.prod.outlook.com (2603:10b6:510:148::10) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH0PR10MB5706:EE_|SA1PR10MB6319:EE_ X-MS-Office365-Filtering-Correlation-Id: a9b98fff-3a3e-4df8-6428-08dba9b86235 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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definitions=main-2308310000 X-Proofpoint-GUID: -NI3OTtWozlsdHx4PV4_-d1Mbr1Ur3Cj X-Proofpoint-ORIG-GUID: -NI3OTtWozlsdHx4PV4_-d1Mbr1Ur3Cj Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org > # dd if=/dev/zero of=blah bs=1G count=16 > 16+0 records in > 16+0 records out > 17179869184 bytes (17 GB, 16 GiB) copied, 14.2627 s, 1.2 GB/s > > I verified the file is well compressed: > > # compsize blah > Processed 1 file, 131073 regular extents (131073 refs), 0 inline. > Type       Perc     Disk Usage   Uncompressed Referenced > TOTAL        3%      512M          16G          16G > zstd         3%      512M          16G          16G > > I'm surprised that such a file needed 128Kextents and required 512MB of > disk space (the filesystem is mounted with compress=zstd:3) but it is what > it is. On to reading the file: > > # dd if=blah of=/dev/null bs=512k > 32768+0 records in > 32768+0 records out > 17179869184 bytes (17 GB, 16 GiB) copied, 7.40493 s, 2.3 GB/s > ### iostat showed 30MB/s to 100MB/s device read speed > > # dd if=blah of=/dev/null bs=32k > 524288+0 records in > 524288+0 records out > 17179869184 bytes (17 GB, 16 GiB) copied, 8.34762 s, 2.1 GB/s > ### iostat showed 30MB/s to 90MB/s device read speed > > # dd if=blah of=/dev/null bs=8k > 2097152+0 records in > 2097152+0 records out > 17179869184 bytes (17 GB, 16 GiB) copied, 18.7143 s, 918 MB/s > ### iostat showed very variable 8MB/s to 60MB/s device read speed > ### average maybe around 40MB/s > > > Also worth noting is the IO request size that iostat is reporting. For > bs=8k it reports a request size of about 4 (KB?), while it's order of > magnitudes higher for all the other measurements in this email. > The sector size is 4k, and the compression block size is 128k. There will be a lot more read IO, which may not be mergeable for reads with lower block sizes. > > ==== Same test with uncompressable file > > I performed the same experiments with a urandom-filled file. I assume here > that btrfs is detecting the file can't be compressed, so it's treating it > differently. This is what the measurements are showing here, that the > device speed limits are reached in all cases > (this host has an HDD with limit 200MB/s). > > # dd if=/dev/urandom of=blah-random bs=1G count=16 > 16+0 records in > 16+0 records out > 17179869184 bytes (17 GB, 16 GiB) copied, 84.0045 s, 205 MB/s > > # compsize blah-random > Processed 1 file, 133 regular extents (133 refs), 0 inline. > Type       Perc     Disk Usage   Uncompressed Referenced > TOTAL      100%       15G          15G          15G > none       100%       15G          15G          15G > > # dd if=blah-random of=/dev/null bs=512k > 32768+0 records in > 32768+0 records out > 17179869184 bytes (17 GB, 16 GiB) copied, 87.82 s, 196 MB/s > ### iostat showed 180-205MB/s device read speed > > # dd if=blah-random of=/dev/null bs=32k > 524288+0 records in > 524288+0 records out > 17179869184 bytes (17 GB, 16 GiB) copied, 88.3785 s, 194 MB/s > ### iostat showed 180-205MB/s device read speed > > # dd if=blah-random of=/dev/null bs=8k > 2097152+0 records in > 2097152+0 records out > 17179869184 bytes (17 GB, 16 GiB) copied, 88.7887 s, 193 MB/s > ### iostat showed 180-205MB/s device read speed The heuristic will disable compression on the file if the data is incompressible, such as that from /dev/urandom. Generally, to test compression in fstests, we use the 'dd' command as below. od /dev/urandom | dd iflag=fullblock of=.. bs=.. count=.. Thanks, Anand