linux-ext4 - Why not always do copy-out in jbd2_journal_write_metadta

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Message-ID: <usGdshd4UQ6wFx4jVDkzY1xddFnUnT7NXj6IWA-uVczdSc5_TmczQvtmULxNVQ3reUPTmyQd2YeLhPa_L2uPbaNdfBPVlLz7rQQ9oC7aSkw=@protonmail.com>
Date:   Mon, 22 Oct 2018 10:34:48 +0000
From:   Fisher <fisherthepooh@...tonmail.com>
To:     "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Why not always do copy-out in jbd2_journal_write_metadta_buffer

Hi,

Recently I was testing my storage's performance and found that there were

periodic performance drops when I ran sequential write benchmark.

After profiling the duration in each step, I found that the dropping

performance was due to wait_on_bit_io(&bh->b_state, BH_Shadow, TASK_UNINTERRUPTIBLE)

in do_get_write_access(). And this is what made me confused, if my understanding was right,

I thought buffer_shadow(bh) stands for buffer not being copied-out that's why we should wait.

But why don't we do copy-out in jbd2_journal_write_metadta_buffer()? and if we do

do the copy-out, does that mean we don't have to set_buffer_shadow because it refers to

buffer not copied-out?

I made a test, when a buffer_head goes into jbd2_journal_write_metadta_buffer(), as long as

it belongs to metadata, then force it to do copy-out and do not set_buffer_shadow, then there

will be no periodic performance drops. Is this test reasonable?

Thanks,