Re: [PATCH v7 00/11] Introduce Zone Append for writing to zoned block devices

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On 17/04/2020 18:04, Theodore Y. Ts'o wrote:
> What sort of reordering can take place due to I/O schedulers and or
> racing write appends from different CPU's?  Is it purely the
> userspace's responsibility to avoid racings writes to a particular
> zone?

For normal writes (i.e.: REQ_OP_WRITE) the zone will be locked by the 
I/O scheduler (namely mq-deadline as it's currently the one which does 
have support for zoned block devices). For REQ_OP_ZONE_APPEND we have a 
trylock scheme in the SCSI emulation of ZONE_APPEND, which is fine, as 
SCSI disks are single queue only.

A scenario that can of cause happen is concurring REQ_OP_ZONE_APPEND 
writes from different CPUs to the same zone. This is fine *iff* the sum 
of all writes stays within the free space of the zone. If one if the 
writes will cross a zone boundary it'll get EIO, obviously.

For "userspace's responsibility", I'd re-phrase this as "a consumer's 
responsibility", as we don't have an interface which aims at user-space 
yet. The only consumer this series implements is zonefs, although we did 
have an AIO implementation for early testing and io_uring shouldn't be 
too hard to implement.




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