ext4: Improve the documentation for ext4's /proc tunables

The   "procs_blocked" line gives  the  number of  processes currently blocked,

waiting for I/O to complete.

1.9 Ext4 file system parameters

------------------------------

Ext4 file system have one directory per partition under /proc/fs/ext4/

# ls /proc/fs/ext4/hdc/

group_prealloc  max_to_scan  mb_groups  mb_history  min_to_scan  order2_req

stats  stream_req

mb_groups:

This file gives the details of multiblock allocator buddy cache of free blocks

mb_history:

Multiblock allocation history.

stats:

This file indicate whether the multiblock allocator should start collecting

statistics. The statistics are shown during unmount

group_prealloc:

The multiblock allocator normalize the block allocation request to

group_prealloc filesystem blocks if we don't have strip value set.

The stripe value can be specified at mount time or during mke2fs.

max_to_scan:

How long multiblock allocator can look for a best extent (in found extents)

min_to_scan:

How long multiblock allocator  must look for a best extent

order2_req:

Multiblock allocator use  2^N search using buddies only for requests greater

than or equal to order2_req. The request size is specfied in file system

blocks. A value of 2 indicate only if the requests are greater than or equal

to 4 blocks.

stream_req:

Files smaller than stream_req are served by the stream allocator, whose

purpose is to pack requests as close each to other as possible to

produce smooth I/O traffic. Avalue of 16 indicate that file smaller than 16

filesystem block size will use group based preallocation.

Information about mounted ext4 file systems can be found in

/proc/fs/ext4.  Each mounted filesystem will have a directory in

/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or

/proc/fs/ext4/dm-0).   The files in each per-device directory are shown

in Table 1-10, below.

Table 1-10: Files in /proc/fs/ext4/<devname>

..............................................................................

 File            Content                                        

 mb_groups       details of multiblock allocator buddy cache of free blocks

 mb_history      multiblock allocation history

 stats           controls whether the multiblock allocator should start

                 collecting statistics, which are shown during the unmount

 group_prealloc  the multiblock allocator will round up allocation

                 requests to a multiple of this tuning parameter if the

                 stripe size is not set in the ext4 superblock

 max_to_scan     The maximum number of extents the multiblock allocator

                 will search to find the best extent

 min_to_scan     The minimum number of extents the multiblock allocator

                 will search to find the best extent

 order2_req      Tuning parameter which controls the minimum size for 

                 requests (as a power of 2) where the buddy cache is

                 used

 stream_req      Files which have fewer blocks than this tunable

                 parameter will have their blocks allocated out of a

                 block group specific preallocation pool, so that small

                 files are packed closely together.  Each large file

                 will have its blocks allocated out of its own unique

                 preallocation pool.

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Summary