linux-ext4 - Selective Data Journaling in ext4

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Date:   Mon, 11 Feb 2019 18:14:18 -0600
From:   Vijay Chidambaram <vijayc@...xas.edu>
To:     linux-ext4@...r.kernel.org, jesus.palos@...xas.edu,
        Theodore Tso <tytso@....edu>
Subject: Selective Data Journaling in ext4

Hi all,

We would like to present an idea to improve the performance of data
journaling in ext4. Data journaling is expensive because data is
written twice: once to the journal and once to the actual file system.
Passing data through the journal provides consistency guarantees that
ordered journaling mode cannot provide (for example, data journaling
prevents a data block from being partially written).

The idea behind Selective Data Journaling is simple: create a new
journaling mode by modifying ordered journaling mode to journal data
blocks which are already part of a file. Data blocks which are newly
allocated are not part of the journal, and are written out before the
journal blocks in accordance with ordered mode's ordering guarantees.
If there is a crash before transaction commit, the only side effect is
un-allocated data blocks getting written with new data.

Selective Data Journaling provides a lot of the benefits of data
journaling, at significantly lower cost. For workloads which mostly
deal with new data blocks (any applications which update files via
atomic rename), Selective Data Journaling can increase performance
significantly.

We came up with Selective Data Journaling during my PhD at the
University of Wisconsin Madison [1]. I haven't inspected the ext4
codebase deeply since then, so this optimization may already exist.
There may also be problems with this approach that we have not
considered -- we are open to discussion. It may also be the case that
nobody uses data journaling so the extra complexity is not worth it.

If this is something you would like to see implemented, my student
Jesus Palos (cced) is interested in doing so. We would like to discuss
how best to implement this if you are interested.

[1] http://research.cs.wisc.edu/adsl/Publications/optfs-sosp13.pdf

Thanks,
Vijay Chidambaram, UT Austin
http://www.cs.utexas.edu/~vijay/