RE: [PATCH 3/6] Enable create array with write journal (--write-journal DEVICE).

[Neil Brown <neilb@suse.de>]

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Song Liu <songliubraving@fb.com> writes:
>> 
>> Hmm, perhaps this should instead follow the same command line mechanics
>> as "--write-mostly" whereby all the devices listed after that flag take on that
>> role.  Also, if we anticipate more roles I wonder if the option should be "--
>> role=" that assigns the given role to the device(s) that follow.
>> 
>> Thoughts?
>> 
>> I'm looking at this from the perspective of reviving the SRT patches [1] where
>> a role for a read/write cache device is needed.  Where a read/write-cache is
>> different than a journal.
>> 
>> [1]:
>> https://urldefense.proofpoint.com/v1/url?u=https://lwn.net/Articles/59661
>> 4/&k=ZVNjlDMF0FElm4dQtryO4A%3D%3D%0A&r=J3jGe56dIPfS5TN6DM82U
>> YbbeR1j2viaiSJI40tv6lE%3D%0A&m=dv5znFd%2BTZdHZaSKjzBVqJbdtNBALR0
>> RgzhZsOmsWAU%3D%0A&s=80c812981ff98264cf1bb2b79032863d4e33ba100
>> 093d41956eab3fbd33b5635
>
> I think --write-mostly is used as _hint_ to RAID 1 that differentiates SSD from 
> HDD. While --write-journal turns on "MD_FEATURE_JOURNAL" and specifies 
> journal device. We should not need more than 1 journal device. In this sense, 
> I think current --write-journal works well...

--write-mostly isn't a hint.  It is a clear statement that these devices
should only be read from if there is no other option (the original
justification had nothing to do with SSDs).

However a write journal is a very different thing to the other devices.
For example it is not counted in the --raid-disks and --spare-disks
where as all other devices (whether --write-mostly or not) are.

If anything --write-journal is more like a --bitmap=/some/file than
--write-mostly.

So, I'm happy for --write-journal to stay as it is.

Thanks,
NeilBrown


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