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ConcurrentLinuxSr.md
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ConcurrentLinuxSr.md
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# The problems with concurrent burns on Linux
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Beginning with high version numbers of Linux kernel 2.6 and up to
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at least kernel 4.3.0 there is a performance problem when more than
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one optical drive is used for burning or reading via libburn at the same time.
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The overall throughput is less than the throughput of the slowest drive
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when it is used as only one.
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This affects SATA and USB attached drives alike. Most probably also IDE
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attached ones.
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## Kernel theory
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The most plausible explanation for the problem is the introduction
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of a global lock for all optical drives which is held during the
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whole transaction time of `ioctl(SG_IO)`.
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This ioctl is used by libburn to send SCSI commands to the burner
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drive. So each single burner blocks the other libburn processes
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until it has ended its transaction.
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See the problematic changeset which affected `drivers/scsi/sr.c` among
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many other kernel parts:
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https://lkml.org/lkml/2010/9/14/338
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and the summary of consequences with remedy proposal:
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http://marc.info/?l=linux-scsi&m=135705061804384&w=2
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Obviously the deprecated device file family `/dev/sg*` does not have such
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a lock.
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# Consequences for libburn
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## Using /dev/sg* against all good advise
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The libburn call `burn_preset_device_open()`, cdrskin option
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`drive_scsi_dev_family=`, and xorriso command `-scsi_dev_family`
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can cause libburn to use `/dev/sg*` instead of `/dev/sr*`.
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Without such special setting a `/dev/sg*` address given by the user would
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be mapped to the corresponding `/dev/sr*`.
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These runs of version 1.4.4 or higher can coexist without throughput
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problems:
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```
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xorriso -scsi_dev_family sg -dev /dev/sr0 -map ./data /data
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```
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```
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xorriso -as cdrecord drive_scsi_dev_family=sg dev=/dev/sr1 -v image2.iso
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```
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```
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cdrskin drive_scsi_dev_family=sg dev=/dev/sr3 -v image3.iso
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```
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### /dev/sg with older versions of cdrskin
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A bug in libburn-0.5.2 to 1.4.2 prevented automatic conversion from `/dev/sr*`
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addresses to `/dev/sg*` addresses if `drive_scsi_dev_family=sg` is given.
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xorriso up to version 1.4.2 did not offer the opportunity to use `/dev/sg*`.
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Nevertheless it is possible to let older cdrskin versions work with `/dev/sg*`:
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```
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$ cdrskin drive_scsi_dev_family=sg --devices
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...
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0 dev='/dev/sg2' rwrw-- : 'HL-DT-ST' 'BDDVDRW GGC-H20L'
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1 dev='/dev/sg3' rwrw-- : 'HL-DT-ST' 'DVDRAM GH24NSC0'
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2 dev='/dev/sg4' rwrw-- : 'ASUS ' 'BW-16D1HT'
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3 dev='/dev/sg5' rwrw-- : 'HL-DT-ST' 'BD-RE GGW-H20L'
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4 dev='/dev/sg6' rwrw-- : 'Optiarc ' 'BD RW BD-5300S'
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```
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```
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$ cdrskin drive_scsi_dev_family=sg dev=/dev/sg2 -v image1.iso
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$ cdrskin drive_scsi_dev_family=sg dev=/dev/sg3 -v image2.iso
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$ cdrskin drive_scsi_dev_family=sg dev=/dev/sg6 -v image3.iso
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```
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Meanwhile the releases on [release page](Releases)
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have the bug fixed and xorriso augmented by a new command.
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## Staying with /dev/sr* as bad as it is
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Despite the fine performance of `/dev/sg*` it remains desirable to use
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`/dev/sr*` because `/dev/sg` is deprecated by the kernel developers,
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the translation from `/dev/sr` to `/dev/sg` is not guaranteed to work in future,
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and `/dev/sr*` is better coordinated with `mount(8)` and `open(2)` for
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`read(2)`.
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So long lasting SCSI transactions via `/dev/sr*` cause long
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delays of communication with the other drives. Some of these delays
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cannot be avoided.
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E.g. the SCSI command to eject the tray needs several seconds to be
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completed.
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Another long delay can be circumvented, though:
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If the buffer of a drive is nearly full and a WRITE command
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arrives, then the drive may delay the reply until there is room
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in the buffer for the next WRITE command.
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Depending on the firmware and buffer size, this may last quite some
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time. Further the time granularity of lock waiting seems quite coarse.
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These long waiting WRITE commands can be avoided if libburn keeps
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itself from filling the drive buffer to a level where the firmware
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would begin to delay the reply.
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xorriso command `-modesty_on_drive`, cdrskin option
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`modesty_on_drive=`, and libburn call `burn_drive_set_buffer_waiting()`
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apply a strategy which stems from the days of IDE when the slow CD drive
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clogged the IDE controller which it shared with a HDD.
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Example:
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```
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-modesty_on_drive on:min_percent=90:max_percent=95
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```
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Some drive firmwares work well with partly empty buffer, some don't.
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Regrettably there is no similar remedy known for reading from /dev/sr*
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via ioctl(SG_IO).
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### Parameter considerations
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Those drives which can work well with `-modesty_on_drive` still may need
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adjustment of the modesty parameters.
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The threshold for delaying further WRITE commands is controlled by
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parameter `max_percent`. The threshold for resuming to send WRITE
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commands is `min_percent`.
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So if `min_percent=90:max_percent=95` does not show the desired effect
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of reaching about 80 to 90 percent of the throughput of an undisturbed
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burn run of a single burner, then one has to try variations.
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Keep in mind that `min_percent` has to stay lower than `max_percent`.
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Drives have been observed to report their buffer fill too low. This
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risks to send a long blocking WRITE command by mistake.
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A lower `max_percent` percent value may compensate.
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```
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-modesty_on_drive on:min_percent=75:max_percent=80
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```
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Drives have been observed to become even slower by waiting for more
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input before writing if their buffer is not full enough.
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A higher `min_percent` may help.
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```
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-modesty_on_drive on:min_percent=95:max_percent=97
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```
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Drives have been observed to slow down their write speed if the
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input comes in small amounts with some short waiting time inbetween.
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A larger gap between `min_percent` and `max_percent` may help
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to get larger write amounts without intermediate waiting.
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```
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-modesty_on_drive on:min_percent=75:max_percent=95
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```
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Finally there are drives which perform poorly with stream recording if
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their buffer is not completely full. This means that they will not
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be able to burn DVD-RAM, BD-RE, or formatted BD-R at full nominal speed
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if `-modesty_on_drive` is set to `on`.
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