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Subject: Re: bcache on XFS: metadata I/O (dirent I/O?) not getting cached at
 all?
To:     Andre Noll <maan@tuebingen.mpg.de>
Cc:     Nix <nix@esperi.org.uk>, linux-bcache@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org,
        Dave Chinner <david@fromorbit.com>
References: <87h8dgefee.fsf@esperi.org.uk>
 <64f32487-5b8f-f6c2-37a9-84bb0717a9e1@suse.de>
 <20190207094138.GZ24140@tuebingen.mpg.de>
From:   Coly Li <colyli@suse.de>
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Organization: SUSE Labs
Message-ID: <41fef785-1e38-87ea-115a-4cdb7e86ed18@suse.de>
Date:   Thu, 7 Feb 2019 18:23:44 +0800
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On 2019/2/7 5:41 下午, Andre Noll wrote:
> On Thu, Feb 07, 16:16, Coly Li wrote
>> From: Coly Li <colyli@suse.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.

Hi Andre,

Thanks for your review :-)

- -- 

Coly Li
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