Public bug reported: I'm opening a raw disk device (like /dev/sdb) with O_DIRECT and then doing write() calls. The device is actually USB mass storage. Looking at the USB packets with Wireshark, I see that the write() calls are being split into multiple SCSI writes. That part is okay I guess. The problem is that these writes can happen in reverse order. Suppose the write() call is writing to sectors 100 to 200. The first SCSI write may write 150 to 200 and the second SCSI write may write 100 to 149.
Correctness does appear to be maintained. I only had a problem with correctness because I'm abusing the protocol for writing to a hacked digital photo frame via https://github.com/dreamlayers/st220x/blob/master/libst2205/st2205.c . The photo frame doesn't fully process the protocol and only looks at the data as a series of USB packets. However, this is bad for performance with hard disks. Surely if you have a sequential write, the optimal way to do that write is sequentially from start to finish. Since changing the scheduler for that block device from cfq to noop fixes this problem, I assume it has to do with the elevator algorithm use to optimize hard disk access. But surely such optimization only makes sense between contiguous sequential writes, and not for splitting up one such write. ProblemType: Bug DistroRelease: Ubuntu 17.10 Package: linux-image-4.13.0-19-generic 4.13.0-19.22 ProcVersionSignature: Ubuntu 4.13.0-19.22-generic 4.13.13 Uname: Linux 4.13.0-19-generic x86_64 NonfreeKernelModules: nvidia_uvm nvidia AlsaVersion: Advanced Linux Sound Architecture Driver Version k4.13.0-19-generic. ApportVersion: 2.20.7-0ubuntu3.6 Architecture: amd64 AudioDevicesInUse: USER PID ACCESS COMMAND /dev/snd/controlC0: bgjenero 2116 F.... pulseaudio Card0.Amixer.info: Card hw:0 'Intel'/'HDA Intel at 0xfa200000 irq 30' Mixer name : 'Realtek ALC889A' Components : 'HDA:10ec0885,1458a002,00100101' Controls : 63 Simple ctrls : 26 CurrentDesktop: XFCE Date: Mon Dec 11 05:23:12 2017 HibernationDevice: RESUME=UUID=a44c3385-e1ba-4456-91ec-be27c892ff11 IwConfig: eth0 no wireless extensions. lo no wireless extensions. MachineType: Gigabyte Technology Co., Ltd. P35-DS3R ProcFB: ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.13.0-19-generic root=UUID=52543c1b-1764-4766-a504-f4b94422fa6e ro RelatedPackageVersions: linux-restricted-modules-4.13.0-19-generic N/A linux-backports-modules-4.13.0-19-generic N/A linux-firmware 1.169.1 RfKill: Error: [Errno 2] No such file or directory: 'rfkill': 'rfkill' SourcePackage: linux UpgradeStatus: Upgraded to artful on 2017-09-30 (71 days ago) dmi.bios.date: 06/19/2009 dmi.bios.vendor: Award Software International, Inc. dmi.bios.version: F13 dmi.board.name: P35-DS3R dmi.board.vendor: Gigabyte Technology Co., Ltd. dmi.chassis.type: 3 dmi.chassis.vendor: Gigabyte Technology Co., Ltd. dmi.modalias: dmi:bvnAwardSoftwareInternational,Inc.:bvrF13:bd06/19/2009:svnGigabyteTechnologyCo.,Ltd.:pnP35-DS3R:pvr:rvnGigabyteTechnologyCo.,Ltd.:rnP35-DS3R:rvr:cvnGigabyteTechnologyCo.,Ltd.:ct3:cvr: dmi.product.name: P35-DS3R dmi.sys.vendor: Gigabyte Technology Co., Ltd. ** Affects: linux (Ubuntu) Importance: Undecided Status: New ** Tags: amd64 apport-bug artful -- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux in Ubuntu. https://bugs.launchpad.net/bugs/1737522 Title: cfq scheduler can write parts of sequential write in reverse order Status in linux package in Ubuntu: New Bug description: I'm opening a raw disk device (like /dev/sdb) with O_DIRECT and then doing write() calls. The device is actually USB mass storage. Looking at the USB packets with Wireshark, I see that the write() calls are being split into multiple SCSI writes. That part is okay I guess. The problem is that these writes can happen in reverse order. Suppose the write() call is writing to sectors 100 to 200. The first SCSI write may write 150 to 200 and the second SCSI write may write 100 to 149. Correctness does appear to be maintained. I only had a problem with correctness because I'm abusing the protocol for writing to a hacked digital photo frame via https://github.com/dreamlayers/st220x/blob/master/libst2205/st2205.c . The photo frame doesn't fully process the protocol and only looks at the data as a series of USB packets. However, this is bad for performance with hard disks. Surely if you have a sequential write, the optimal way to do that write is sequentially from start to finish. Since changing the scheduler for that block device from cfq to noop fixes this problem, I assume it has to do with the elevator algorithm use to optimize hard disk access. But surely such optimization only makes sense between contiguous sequential writes, and not for splitting up one such write. ProblemType: Bug DistroRelease: Ubuntu 17.10 Package: linux-image-4.13.0-19-generic 4.13.0-19.22 ProcVersionSignature: Ubuntu 4.13.0-19.22-generic 4.13.13 Uname: Linux 4.13.0-19-generic x86_64 NonfreeKernelModules: nvidia_uvm nvidia AlsaVersion: Advanced Linux Sound Architecture Driver Version k4.13.0-19-generic. ApportVersion: 2.20.7-0ubuntu3.6 Architecture: amd64 AudioDevicesInUse: USER PID ACCESS COMMAND /dev/snd/controlC0: bgjenero 2116 F.... pulseaudio Card0.Amixer.info: Card hw:0 'Intel'/'HDA Intel at 0xfa200000 irq 30' Mixer name : 'Realtek ALC889A' Components : 'HDA:10ec0885,1458a002,00100101' Controls : 63 Simple ctrls : 26 CurrentDesktop: XFCE Date: Mon Dec 11 05:23:12 2017 HibernationDevice: RESUME=UUID=a44c3385-e1ba-4456-91ec-be27c892ff11 IwConfig: eth0 no wireless extensions. lo no wireless extensions. MachineType: Gigabyte Technology Co., Ltd. P35-DS3R ProcFB: ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.13.0-19-generic root=UUID=52543c1b-1764-4766-a504-f4b94422fa6e ro RelatedPackageVersions: linux-restricted-modules-4.13.0-19-generic N/A linux-backports-modules-4.13.0-19-generic N/A linux-firmware 1.169.1 RfKill: Error: [Errno 2] No such file or directory: 'rfkill': 'rfkill' SourcePackage: linux UpgradeStatus: Upgraded to artful on 2017-09-30 (71 days ago) dmi.bios.date: 06/19/2009 dmi.bios.vendor: Award Software International, Inc. dmi.bios.version: F13 dmi.board.name: P35-DS3R dmi.board.vendor: Gigabyte Technology Co., Ltd. dmi.chassis.type: 3 dmi.chassis.vendor: Gigabyte Technology Co., Ltd. dmi.modalias: dmi:bvnAwardSoftwareInternational,Inc.:bvrF13:bd06/19/2009:svnGigabyteTechnologyCo.,Ltd.:pnP35-DS3R:pvr:rvnGigabyteTechnologyCo.,Ltd.:rnP35-DS3R:rvr:cvnGigabyteTechnologyCo.,Ltd.:ct3:cvr: dmi.product.name: P35-DS3R dmi.sys.vendor: Gigabyte Technology Co., Ltd. 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