Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Buffered atomic writes

[Jan Kara <jack@suse.cz>]

Hi!

On Fri 13-02-26 19:02:39, Ojaswin Mujoo wrote:
> Another thing that came up is to consider using write through semantics 
> for buffered atomic writes, where we are able to transition page to
> writeback state immediately after the write and avoid any other users to
> modify the data till writeback completes. This might affect performance
> since we won't be able to batch similar atomic IOs but maybe
> applications like postgres would not mind this too much. If we go with
> this approach, we will be able to avoid worrying too much about other
> users changing atomic data underneath us. 
> 
> An argument against this however is that it is user's responsibility to
> not do non atomic IO over an atomic range and this shall be considered a
> userspace usage error. This is similar to how there are ways users can
> tear a dio if they perform overlapping writes. [1]. 

Yes, I was wondering whether the write-through semantics would make sense
as well. Intuitively it should make things simpler because you could
practially reuse the atomic DIO write path. Only that you'd first copy
data into the page cache and issue dio write from those folios. No need for
special tracking of which folios actually belong together in atomic write,
no need for cluttering standard folio writeback path, in case atomic write
cannot happen (e.g. because you cannot allocate appropriately aligned
blocks) you get the error back rightaway, ...

Of course this all depends on whether such semantics would be actually
useful for users such as PostgreSQL.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR