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Subject: Re: [ANNOUNCE] Reiser5: Data Tiering. Burst Buffers. Speedup
 synchronous modifications
To:     Pavel Machek <pavel@ucw.cz>
Cc:     ReiserFS development mailing list 
        <reiserfs-devel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
References: <4f919dee-5b72-9269-2bd0-6818a7167864@gmail.com>
 <20200530101354.GA630@duo.ucw.cz>
From:   Edward Shishkin <edward.shishkin@gmail.com>
Message-ID: <a0a2176a-9abc-ed26-6f6c-726936f90607@gmail.com>
Date:   Sun, 31 May 2020 20:34:25 +0200
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On 05/30/2020 12:13 PM, Pavel Machek wrote:
> Hi!
> 
> 
>> For example, you can use proxy device to store hot data only. With
>> such strategy new logical blocks (which are always "cold") will always
>> go to the main storage (in contrast with Burst Buffers, where new
>> logical blocks first get written to the proxy disk). Once in a while
>> you need to scan your volume in order to push colder data out, and
>> pull hotter data in the proxy disk. Reiser5 contains a common
>> interface for this. It is possible to maintain per-file, or even per-
>> blocks-extent "temperature" of data (e.g. as a generation counter),
> 
> Would it be possible to offer userland interface for this? I can
> probably say that mp3/video files should be cold, while some source
> files should be hot, etc...
> 
> Best regards,
> 									Pavel
> 

Hi Pavel,

Yes, it is possible. One just needs to add an ioctl handler for regular
files managed by a plugin with STRIPED_FILE_PLUGIN_ID. That handler is
to set user-defined "temperature" to a file.

Also we'll need an additional on-disk file attribute (32 (or 64?)-bit
field in the private part of inode) to store the "temperature" in. It
can be added by standard way via implementation of respective stat-data
extension in the file reiser4/plugin/item/static_stat.c

Finally, we'll need to handle temperature in the common migration
procedure balance_volume_asym(), which is responsible for clearing up
the proxy device. It should look like this:

...

if (!IS_ERR(inode) && inode_file_plugin(inode)->balance &&
     file_is_cold_enough(inode)) {
		reiser4_iget_complete(inode);
		/*
		 * migrate data blocks of this file
		 */
...

Currently it works as if all files are "cold" (i.e. migrates
everything).

Once I find the current stuff more-or-less stable I'll add temperature
support and send the patch.

Thanks,
Edward.