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On 2019/2/7 5:41 下午, Andre Noll wrote:

On Thu, Feb 07, 16:16, Coly Li wrote

quoted

From: Coly Li <redacted> 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

quoted

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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