Re: [PATCH 1/2] block: Read max write zeroes once for __blkdev_issue_write_zeroes()

["Martin K. Petersen" <martin.petersen@oracle.com>]

Christoph,

>>>> +/*
>>>> + * Pass bio_write_zeroes_limit() return value in @limit, as the return
>>>> + * value may change after a REQ_OP_WRITE_ZEROES is issued.
>>>> + */
>>> I don't think that really helps all that much to explain the issue,
>>> which is about SCSI not having an ahead of time flag that reliably
>>> works for write same support, which makes it clear the limit to 0 on
>>> the first I/O completion.  Maybe you can actually spell this out?
>>
>> Please just tell me what you would like to see, and I will copy verbatim.
>
> Probably just what I just wrote.  Unless Martin can come up with
> better language.

There is no reliable way for the SCSI subsystem to determine whether a
device supports a WRITE SAME operation without actually performing a
write to media. As a result, write_zeroes is enabled by default and will
be disabled if a zeroing operation subsequently fails. This means that
this queue limit is likely to change at runtime.

Something like that, perhaps?

-- 
Martin K. Petersen	Oracle Linux Engineering