Re: [RFC v1 00/11] vfs: hot data tracking

Zhi Yong Wu
2012-09-17T13:24:35Z

On Mon, Sep 17, 2012 at 5:45 PM, Marco Stornelli
<marco.stornelli@gmail.com> wrote:
> 2012/9/17 <zwu.kernel@gmail.com>:
>> From: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
>>
>> NOTE:
>>
>> The patchset is currently post out mainly to make sure
>> it is going in the correct direction and hope to get some
>> helpful comments from other guys.
>>
>> TODO List:
>>
>> 1.) Need to do scalability or performance tests.
>> 2.) Turn some Micro into tunables
>> TIME_TO_KICK, and HEAT_UPDATE_DELAY
>> 3.) Rafactor hot_hash_is_aging()
>> If you just made the timeout value a timespec and compared
>> the _timespecs_, you would be doing a lot fewer conversions.
>> 4.) Cleanup some unnecessary lock protect
>> 5.) Add more comments to explain how to calc temperature
>>
>> Ben Chociej, Matt Lupfer and Conor Scott originally wrote this code to
>> be very btrfs-specific. I've taken their code and attempted to
>> make it more generic and integrate it at the VFS level.
>>
>> INTRODUCTION:
>>
>> Essentially, this means maintaining some key stats
>> (like number of reads/writes, last read/write time, frequency of
>> reads/writes), then distilling those numbers down to a single
>> "temperature" value that reflects what data is "hot," and using that
>> temperature to move data to SSDs.
>>
>> The long-term goal of these patches is to allow some FSs,
>> e.g. Btrfs to intelligently utilize SSDs in a heterogenous volume.
>> Incidentally, this project has been motivated by
>> the Project Ideas page on the Btrfs wiki.
>>
>> Of course, users are warned not to run this code outside of development
>> environments. These patches are EXPERIMENTAL, and as such they might eat
>> your data and/or memory. That said, the code should be relatively safe
>> when the hottrack mount option are disabled.
>>
>> MOTIVATION:
>>
>> The overall goal of enabling hot data relocation to SSD has been
>> motivated by the Project Ideas page on the Btrfs wiki at
>> <https://btrfs.wiki.kernel.org/index.php/Project_ideas>.
>> It will divide into two steps. VFS provide hot data tracking function
>> while specific FS will provide hot data relocation function.
>> So as the first step of this goal, it is hoped that the patchset
>> for hot data tracking will eventually mature into VFS.
>>
>> This is essentially the traditional cache argument: SSD is fast and
>> expensive; HDD is cheap but slow. ZFS, for example, can already take
>> advantage of SSD caching. Btrfs should also be able to take advantage of
>> hybrid storage without many broad, sweeping changes to existing code.
>>
>> SUMMARY:
>>
>> - Hooks in existing vfs functions to track data access frequency
>>
>> - New rbtrees for tracking access frequency of inodes and sub-file
>> ranges (hot_rb.c)
>> The relationship between super_block and rbtree is as below:
>> super_block->s_hotinfo.hot_inode_tree
>> In include/linux/fs.h, one struct hot_info s_hotinfo is added to
>> super_block struct. Each FS instance can find hot tracking info
>> s_hotinfo via its super_block. In this hot_info, it store a lot of hot
>> tracking info such as hot_inode_tree, inode and range hash list, etc.
>>
>> - A hash list for indexing data by its temperature (hot_hash.c)
>>
>> - A debugfs interface for dumping data from the rbtrees (hot_debugfs.c)
>>
>> - A background kthread for updating inode heat info
>>
>> - Mount options for enabling temperature tracking(-o hottrack, default mean disabled)
>> (hot_track.c)
>>
>> - An ioctl to retrieve the frequency information collected for a certain
>> file
>>
>> - Ioctls to enable/disable frequency tracking per inode.
>>
>> Usage syntax:
>>
>> root@debian-i386:~# mount -o hottrack /dev/sdb /mnt
>> [ 1505.894078] device label test devid 1 transid 29 /dev/sdb
>> [ 1505.952977] btrfs: disk space caching is enabled
>> [ 1506.069678] vfs: turning on hot data tracking
>> root@debian-i386:~# mount -t debugfs none /sys/kernel/debug
>> root@debian-i386:~# ls -l /sys/kernel/debug/vfs_hotdata/
>> total 0
>> drwxr-xr-x 2 root root 0 Aug 8 04:40 sdb
>> root@debian-i386:~# ls -l /sys/kernel/debug/vfs_hotdata/sdb
>> total 0
>> -rw-r--r-- 1 root root 0 Aug 8 04:40 inode_data
>> -rw-r--r-- 1 root root 0 Aug 8 04:40 range_data
>> root@debian-i386:~# vi /mnt/file
>> root@debian-i386:~# cat /sys/kernel/debug/hot_track/sdb/inode_data
>> inode #279, reads 0, writes 1, avg read time 18446744073709551615,
>> avg write time 5251566408153596, temp 109
>> root@debian-i386:~# cat /sys/kernel/debug/hot_track/sdb/range_data
>> inode #279, range start 0 (range len 1048576) reads 0, writes 1,
>> avg read time 18446744073709551615, avg write time 1128690176623144209, temp 64
>> root@debian-i386:~# echo "hot data tracking test" >> /mnt/file
>> root@debian-i386:~# cat /sys/kernel/debug/hot_track/sdb/inode_data
>> inode #279, reads 0, writes 2, avg read time 18446744073709551615,
>> avg write time 4923343766042451, temp 109
>> root@debian-i386:~# cat /sys/kernel/debug/hot_track/sdb/range_data
>> inode #279, range start 0 (range len 1048576) reads 0, writes 2,
>> avg read time 18446744073709551615, avg write time 1058147040842596150, temp 64
>> root@debian-i386:~#
>>
>
> It's a good idea to add a new file under documentation and include
> this kind of information. For example what temp means, how it's worked
> out and how to "read" the avg read/write time (nanoseconds,
> microseconds, jiffies....??)
Good suggestion, i will do, thanks. Do you have comments for other patches?

>
> Marco

--
Regards,

Zhi Yong Wu