Re: [PATCH 2/2] fs/ext4,jbd2: Add support for passing write-hint with journal.

[Jens Axboe <axboe@kernel.dk>]

On 12/10/18 7:12 AM, Jan Kara wrote:
> On Mon 10-12-18 18:20:04, Kanchan Joshi wrote:
>> This patch introduces "j_writehint" in JBD2 journal,
>> which is set based by Ext4 depending on "journal_writehint"
>> mount option (inspired from "journal_ioprio").
> 
> Thanks for the patch! It would be good to provide the explanation you have
> in the cover letter in this patch as well so that it gets recorded in git
> logs.
> 
> Also I don't like the fact that users have to set the hint via a mount
> option for this to be enabled. OTOH the WRITE_FILE_<foo> hints defined in
> fs.h are generally supposed to be used by userspace so it's difficult to
> pick anything if we don't know what the userspace is going to do. I'd argue
> it's even difficult for the sysadmin to pick any good value even if he
> actually knows that he might benefit from setting some. Jens, is there
> some reasonable way for fs to automatically pick some stream value for its
> journal?

I think we have two options here:

1) It's _probably_ safe to assume that journal data is short lived. While
   hints are all relative to the specific use case, the size of the journal
   compared to the rest of the drive is most likely very small. Hence a
   default of WRITE_LIFE_SHORT is probably a good idea.

2) We add a specific internal life time hint for fs journals.

#2 makes the most sense to me, but requires a bit more work...

-- 
Jens Axboe