linux-ext4 - Re: [Patch 0/4] RFC : Support for data gradation of a single file.

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Date:   Fri, 6 Apr 2018 18:27:22 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Sayan Ghosh <sgdgp.2014@...il.com>
Cc:     linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        "Bhattacharya, Suparna" <suparna.bhattacharya@....com>,
        niloy ganguly <ganguly.niloy@...il.com>,
        Madhumita Mallick <madhu.cse.ju@...il.com>,
        "Bharde, Madhumita" <madhumita.bharde@....com>
Subject: Re: [Patch 0/4] RFC : Support for data gradation of a single file.

Hi Sayan,

It wasn't clear what was your purpose in posting these patches.  There
are a large number of ways in which they simply aren't ready for
upstream merging.  As a short list:

1)  They are against an ancient version of the kernel (4.7.2).

2)  There are a large number of TODO's in it in the code

3) The boundary between the two different tiers of storage is
currently harded in a header file using a #define (!).

If the goal is to gather comments about the design, I wish you had
presented the problem statement to the ext4 mailig list much earlier.
It might have saved you time in terms since we could have given you
feedback before you had done all of this work on this patch set.

Andreas' comments about making the allocation hints persistent not
making any sense are very much on target.  Once the file is written,
the hints won't be needed at all.

In addition, you should strongly think about some way propagating the
fact that some blocks in device-mapper device are backed by DAX, and
others are not, as a device-mapper interface.  And it might not
necessarily a single break point where below a block number is SSD or
HDD storage, and above a block number it's DAX storage.

The other thing to consider is whether it makes any sense at all to
solve this problem by haing a single file system where part of the
storage is DAX, and part is not.  Why not just have two file systems,
one which is 100% DAX, and another which is 100% HDD/SSD, and store
the data in two files in two different file sytsems?

						- Ted