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Date:   Sat, 3 Oct 2020 00:49:05 -0400
From:   "Theodore Y. Ts'o" <tytso@mit.edu>
To:     Ritesh Harjani <riteshh@linux.ibm.com>
Cc:     linux-ext4@vger.kernel.org, jack@suse.cz, dan.j.williams@intel.com,
        anju@linux.vnet.ibm.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCHv3 1/1] ext4: Optimize file overwrites
Message-ID: <20201003044905.GF23474@mit.edu>
References: <cover.1600401668.git.riteshh@linux.ibm.com>
 <88e795d8a4d5cd22165c7ebe857ba91d68d8813e.1600401668.git.riteshh@linux.ibm.com>
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On Fri, Sep 18, 2020 at 10:36:35AM +0530, Ritesh Harjani wrote:
> In case if the file already has underlying blocks/extents allocated
> then we don't need to start a journal txn and can directly return
> the underlying mapping. Currently ext4_iomap_begin() is used by
> both DAX & DIO path. We can check if the write request is an
> overwrite & then directly return the mapping information.
> 
> This could give a significant perf boost for multi-threaded writes
> specially random overwrites.
> On PPC64 VM with simulated pmem(DAX) device, ~10x perf improvement
> could be seen in random writes (overwrite). Also bcoz this optimizes
> away the spinlock contention during jbd2 slab cache allocation
> (jbd2_journal_handle). On x86 VM, ~2x perf improvement was observed.
> 
> Reported-by: Dan Williams <dan.j.williams@intel.com>
> Suggested-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>

Thanks, applied.

					- Ted