From: Zhi Yong Wu <wuzhy@xxxxxxxxxxxxxxxxxx> Add one doc for VFS hot tracking feature Signed-off-by: Zhi Yong Wu <wuzhy@xxxxxxxxxxxxxxxxxx> --- Documentation/filesystems/00-INDEX | 2 + Documentation/filesystems/hot_tracking.txt | 262 ++++++++++++++++++++++++++++ 2 files changed, 264 insertions(+), 0 deletions(-) create mode 100644 Documentation/filesystems/hot_tracking.txt diff --git a/Documentation/filesystems/00-INDEX b/Documentation/filesystems/00-INDEX index 8c624a1..b68bdff 100644 --- a/Documentation/filesystems/00-INDEX +++ b/Documentation/filesystems/00-INDEX @@ -118,3 +118,5 @@ xfs.txt - info and mount options for the XFS filesystem. xip.txt - info on execute-in-place for file mappings. +hot_tracking.txt + - info on hot data tracking in VFS layer diff --git a/Documentation/filesystems/hot_tracking.txt b/Documentation/filesystems/hot_tracking.txt new file mode 100644 index 0000000..a39a96d --- /dev/null +++ b/Documentation/filesystems/hot_tracking.txt @@ -0,0 +1,262 @@ +Hot Data Tracking + +September, 2012 Zhi Yong Wu <wuzhy@xxxxxxxxxxxxxxxxxx> + +CONTENTS + +1. Introduction +2. Motivation +3. The Design +4. How to Calc Frequency of Reads/Writes & Temperature +5. Git Development Tree +6. Usage Example + + +1. Introduction + + The feature adds experimental support for tracking data temperature +information in VFS layer. 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 the feature 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. + +2. 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. + + +3. The Design + +These include the following parts: + + * Hooks in existing vfs functions to track data access frequency + + * New radix-trees for tracking access frequency of inodes and sub-file +ranges + The relationship between super_block and radix-tree is as below: +hot_info.hot_inode_tree + Each FS instance can find hot tracking info s_hotinfo. +In this hot_info, it store a lot of hot tracking info such as hot_inode_tree, +inode and range list, etc. + + * A list for indexing data by its temperature + + * A debugfs interface for dumping data from the radix-trees + + * A background kthread for updating inode heat info + + * Mount options for enabling temperature tracking(-o hot_track, +default mean disabled) + * An ioctl to retrieve the frequency information collected for a certain +file + * Ioctls to enable/disable frequency tracking per inode. + +Let us see their relationship as below: + + * hot_info.hot_inode_tree indexes hot_inode_items, one per inode + + * hot_inode_item contains access frequency data for that inode + + * hot_inode_item holds a heat list node to index the access +frequency data for that inode + + * hot_inode_item.hot_range_tree indexes hot_range_items for that inode + + * hot_range_item contains access frequency data for that range + + * hot_range_item holds a heat list node to index the access +frequency data for that range + + * hot_info.heat_inode_map indexes per-inode heat list nodes + + * hot_info.heat_range_map indexes per-range heat list nodes + + How about some ascii art? :) Just looking at the hot inode item case +(the range item case is the same pattern, though), we have: + +heat_inode_map hot_inode_tree + | | + | V + | +-------hot_comm_item--------+ + | | frequency data | ++---+ | list_head | +| V ^ | V +| ...<--hot_comm_item-->... | | ...<--hot_comm_item-->... +| frequency data | | frequency data ++-------->list_head----------+ +--------->list_head--->..... + hot_range_tree hot_range_tree + | + heat_range_map V + | +-------hot_comm_item--------+ + | | frequency data | + +---+ | list_head | + | V ^ | V + | ...<--hot_comm_item-->... | | ...<--hot_comm_item-->... + | frequency data | | frequency data + +-------->list_head----------+ +--------->list_head--->..... + + +4. How to Calc Frequency of Reads/Writes & Temperature + +1.) hot_average_update() + + This function does the actual work of updating the frequency numbers, +whatever they turn out to be. FREQ_POWER determines how many atime +deltas we keep track of (as a power of 2). So, setting it to anything above +16ish is probably overkill. Also, the higher the power, the more bits get +right shifted out of the timestamp, reducing precision, so take note of that +as well. + + The caller should have already locked freq_data's parent's spinlock. + + FREQ_POWER, defined immediately below, determines how heavily to weight +the current frequency numbers against the newest access. For example, a value +of 4 means that the new access information will be weighted 1/16th (ie 2^-4) +as heavily as the existing frequency info. In essence, this is a kludged- +together version of a weighted average, since we can't afford to keep all of +the information that it would take to get a _real_ weighted average. + +2.) Some Micro explaination + + The following comments explain what exactly comprises a unit of heat. +Each of six values of heat are calculated and combined in order to form an +overall temperature for the data: + + * NRR - number of reads since mount + * NRW - number of writes since mount + * LTR - time elapsed since last read (ns) + * LTW - time elapsed since last write (ns) + * AVR - average delta between recent reads (ns) + * AVW - average delta between recent writes (ns) + + These values are divided (right-shifted) according to the *_DIVIDER_POWER +values defined below to bring the numbers into a reasonable range. You can +modify these values to fit your needs. However, each heat unit is a u32 and +thus maxes out at 2^32 - 1. Therefore, you must choose your dividers quite +carefully or else they could max out or be stuck at zero quite easily. +(E.g., if you chose AVR_DIVIDER_POWER = 0, nothing less than 4s of atime +delta would bring the temperature above zero, ever.) + + Finally, each value is added to the overall temperature between 0 and 8 +times, depending on its *_COEFF_POWER value. Note that the coefficients are +also actually implemented with shifts, so take care to treat these values +as powers of 2. (I.e., 0 means we'll add it to the temp once; 1 = 2x, etc.) + + * AVR/AVW cold unit = 2^X ns of average delta + * AVR/AVW heat unit = HEAT_MAX_VALUE - cold unit + + E.g., data with an average delta between 0 and 2^X ns will have a cold +value of 0, which means a heat value equal to HEAT_MAX_VALUE. + +3.) hot_temp_calc() + + This function is responsible for distilling the six heat +criteria, which are described in detail in hot_tracking.h) down into a single +temperature value for the data, which is an integer between 0 +and HEAT_MAX_VALUE. + + To accomplish this, the raw values from the hot_freq_data structure +are shifted various ways in order to make the temperature calculation more +or less sensitive to each value. + + Once this calibration has happened, we do some additional normalization and +make sure that everything fits nicely in a u32. From there, we take a very +rudimentary kind of "average" of each of the values, where the *_COEFF_POWER +values act as weights for the average. + + Finally, we use the HEAT_HASH_BITS value, which determines the size of the +heat list array, to normalize the temperature to the proper granularity. + + +5. Git Development Tree + + This feature is still on development and review, so if you're interested, +you can pull from the git repository at the following location: + + https://github.com/wuzhy/kernel.git hot_tracking + git://github.com/wuzhy/kernel.git hot_tracking + + +6. Usage Example + +1.) To use hot tracking, you should mount like this: + +$ mount -o hot_track /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 + +2.) Mount debugfs at first: + +$ mount -t debugfs none /sys/kernel/debug +$ ls -l /sys/kernel/debug/hot_track/ +total 0 +drwxr-xr-x 2 root root 0 Aug 8 04:40 sdb +$ ls -l /sys/kernel/debug/hot_track/sdb +total 0 +-rw-r--r-- 1 root root 0 Aug 8 04:40 rt_stats_inode +-rw-r--r-- 1 root root 0 Aug 8 04:40 rt_stats_range + +3.) View information about hot tracking from debugfs: + +$ echo "hot tracking test" > /mnt/file +$ cat /sys/kernel/debug/hot_track/sdb/rt_stats_inode +inode #279, reads 0, writes 1, avg read time 18446744073709551615, +avg write time 5251566408153596, temp 109 +$ 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 + +$ echo "hot data tracking test" >> /mnt/file +$ cat /sys/kernel/debug/hot_track/sdb/rt_stats_inode +inode #279, reads 0, writes 2, avg read time 18446744073709551615, +avg write time 4923343766042451, temp 109 +$ 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 + +4.) Check temp sorting result of some nodes: + +$ cat /sys/kernel/debug/hot_track/loop0/hot_spots_inode +inode #5248773, reads 0, writes 244, +avg read time 18446744073709, avg write time 822, temp 111 +inode #878523, reads 0, writes 1, +avg read time 18446744073709, avg write time 5278036898, temp 109 +inode #878524, reads 0, writes 1, +avg read time 18446744073709, avg write time 5278036898, temp 109 + +5.) Tune some hot tracking parameters as below: + +$ cat /proc/sys/fs/hot-kick-time +300 +$ echo 360 > /proc/sys/fs/hot-kick-time +$ cat /proc/sys/fs/hot-kick-time +360 +$ cat /proc/sys/fs/hot-update-delay +300 +$ echo 360 > /proc/sys/fs/hot-update-delay +$ cat /proc/sys/fs/hot-update-delay +360 + -- 1.7.6.5 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
