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From:   Chris Chilvers <chilversc@gmail.com>
Date:   Mon, 28 Feb 2022 19:24:07 +0000
Message-ID: <CAAmbk-f=Y_k1h-5b3yrGUj36LtL+ZU=J-sC01mrDj83wyKBMkg@mail.gmail.com>
Subject: [BUG] FS-Cache write but no read when using sync
To:     linux-nfs@vger.kernel.org
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While trying the new FS-Cache implementation using the 5.17-rc5 kernel on
Ubuntu 21.10 I ran into an issue where it appears that FS-Cache was not being
used when the sync mount option is enabled.

While monitoring /proc/fs/fscache/stats it was observed there were high writes
to the cache volume, but no reads:

    IO     : rd=0 wr=344713665

Further tracing of /sys/kernel/debug/tracing/events/fscache/fscache_access shows
a large number of io_write operations, but no io_read operations.

The volume did fill up and perform culling as expected.

When the mount option was changed from sync to async the cache performed as
expected and a mixture of io_write and io_read was observed.

-- Chris