On Thu, Feb 07, 16:16, Coly Li wrote
> From: Coly Li <colyli@xxxxxxx>
> Date: Thu, 7 Feb 2019 15:54:24 +0800
> Subject: [PATCH] bcache: use (REQ_META|REQ_PRIO) to indicate bio for metadata
>
> In 'commit 752f66a75aba ("bcache: use REQ_PRIO to indicate bio for
> metadata")' REQ_META is replaced by REQ_PRIO to indicate metadata bio.
> This assumption is not always correct, e.g. XFS uses REQ_META to mark
> metadata bio other than REQ_PRIO. This is why Nix reports a regression
> that bcache does not cache metadata for XFS after the above commit.
maybe s/reports a regression/noticed
> Thanks to Dave Chinner, he explains the difference between REQ_META and
> REQ_PRIO from view of file system developer. Here I quote part of his
> explanation from mailing list,
> REQ_META is used for metadata. REQ_PRIO is used to communicate to
> the lower layers that the submitter considers this IO to be more
> important that non REQ_PRIO IO and so dispatch should be expedited.
>
> IOWs, if the filesystem considers metadata IO to be more important
> that user data IO, then it will use REQ_PRIO | REQ_META rather than
> just REQ_META.
>
> Then it seems bios with REQ_META or REQ_PRIO should both be cached for
> performance optimation, because they are all probably low I/O latency
> demand by upper layer (e.g. file system).
>
> So in this patch, when we want to check whether a bio is metadata
> related, REQ_META and REQ_PRIO are both checked. Then both metadata and
> high priority I/O requests will be handled properly.
s/check whether a bio is metadata related/decide whether to bypass the cache
Apart from these two nitpicks, feel free to add my Reviewed-by.
Thanks
Andre
--
Max Planck Institute for Developmental Biology
Max-Planck-Ring 5, 72076 Tübingen, Germany. Phone: (+49) 7071 601 829
http://people.tuebingen.mpg.de/maan/
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