[Stgt-devel] About the cmd execution in a SCSI device

[Eli Dorfman]

>> I'm going over the code in bs_rdwr_request: the data is read/written
>> from/to the device using pread64/pwrite64. I understand that it treats
>> the device as a file and just reads/writes from/to it with some offset.
>> What I don't understand is if the data is copied between user & kernel
>> space. Eventually, it will go from tgt (user) to scsi-ml (kernel) somehow.
>
> Yes, data is copied between user and kernel spaces, that is, we copy
> data between an user-space buffer and page cache. We could improve
> this area but there are still tons of things that I want to do before
> that.

Can you estimate the performance penalty due to these copies?
Do you have an idea how this could be done?

Eli