linux-kernel - Re: bcache on XFS: metadata I/O (dirent I/O?) not getting cached at all?

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Message-ID: <20190207094138.GZ24140@tuebingen.mpg.de>
Date:   Thu, 7 Feb 2019 10:41:38 +0100
From:   Andre Noll <maan@...bingen.mpg.de>
To:     Coly Li <colyli@...e.de>
Cc:     Nix <nix@...eri.org.uk>, linux-bcache@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dave Chinner <david@...morbit.com>
Subject: Re: bcache on XFS: metadata I/O (dirent I/O?) not getting cached at
 all?

On Thu, Feb 07, 16:16, Coly Li wrote
> From: Coly Li <colyli@...e.de>
> 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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