This subdirectory contains specific file system, file handle, inode, dentry and quota information.
1，/proc/sys/fs/aio-max-nr
aio-max-nr allows you to change the maximum value/proc/sys/fs/aio-nr can grow to.
2，/proc/sys/fs/aio-nr
aio-nr shows the current system-wide number of asynchronous io requests.
3，/proc/sys/fs/bio_netoops
4，/proc/sys/fs/dentry-state
tatus of the directory cache. Since directory entries are dynamically allocated and deallocated, this file indicates the current status. It holds six values of which only the first three are used:
- nr_dentry - number of dentries currently allocated
- nr_unused - nuber of unused dentries
- age_limit - seconds after the entry may be reclaimed, when memory is short
Last three values are not used and are always zero.
5，/proc/sys/fs/dir-notify-enable
This file controls dnotify, a directory notification mechanism, and will not appear if dnotify support is compiled out. It was introduced with inotify in case some people want to disable dnotify support at runtime.
There are two possible values:
- 1 - dnotify is enabled (default)
- 0 - dnotify is disabled
6，/proc/sys/fs/file-max
The kernel allocates file handles dynamically, but doesn't free them again at this time.
The value in file-max denotes the maximum number of file handles that the Linux kernel will allocate. When you get a lot of error messages about running out of file handles, you might want to raise this limit. The default value is 10% of RAM in kilobytes.
7，/proc/sys/fs/file-nr
Historically, the three values in file-nr denoted the number of allocated file handles, the number of allocated but unused file handles, and the maximum number of file handles. Linux 2.6 always reports 0 as the number of free file handles - this is not an error, it just means that the number of allocated file handles exactly matches the number of used file handles.
8，/proc/sys/fs/inode-nr
The file inode-nr contains the first two items from/proc/sys/fs/inode-state.
9，/proc/sys/fs/inode-state
inode-state contains two actual numbers and five dummy values. The numbers are:
- nr_inodes - denotes the number of inodes the system has allocated. This number will grow and shrink dynamically.
- nr_free_inodes - represents the number of free inodes. Ie. the number of inuse inodes is (nr_inodes - nr_free_inodes).
10，/proc/sys/fs/lease-break-time
This file specifies the grace period (in seconds) that the kernel grants to a process holding a file lease after it has sent a signal to that process notifying it that another process is waiting to open the file. If the lease holder does not remove or downgrade the lease within this grace period, the kernel forcibly breaks the lease.
11，/proc/sys/fs/leases-enable
This file can be used to enable or disable file leases on a system-wide basis. If this file contains the value 0, leases are disabled. A non-zero value enables leases.
12，/proc/sys/fs/nr_open
This denotes the maximum number of file-handles a process can allocate. Default value is 1024*1024 (1048576) which should be enough for most machines. Actual limit depends on RLIMIT_NOFILE resource limit.
13，/proc/sys/fs/overflowgid
Some filesystems only support 16-bit GIDs, although in Linux GIDs are 32 bits. When one of these filesystems is mounted with writes enabled, any GID that would exceed 65535 is translated to a fixed value before being written to disk.
This sysctl allows you to change the value of the fixed GID. The default is 65534.
14，/proc/sys/fs/overflowuid
Some filesystems only support 16-bit UIDs, although in Linux UIDs are 32 bits. When one of these filesystems is mounted with writes enabled, any UID that would exceed 65535 is translated to a fixed value before being written to disk.
This sysctl allows you to change the value of the fixed UID. The default is 65534.
15，/proc/sys/fs/suid_dumpable
The value in this file determines whether core dump files are produced for set-user-ID or otherwise protected/tainted binaries. Three different integer values can be specified:
- 0 (default) - This provides the traditional behaviour. A core dump will not be produced for a process which has changed credentials (by calling seteuid(2), setgid(2), or similar, or by executing a set-user-ID or set-group-ID program) or whose binary does not have read permission enabled.
- 1 ("debug") - All processes dump core when possible. The core dump is owned by the file system user ID of the dumping process and no security is applied. This is intended for system debugging situations only. Ptrace is unchecked.
- 2 ("suidsafe") - Any binary which normally would not be dumped (see "0" above) is dumped readable by root only. This allows the user to remove the core dump file but not to read it. For security reasons core dumps in this mode will not overwrite one another or other files. This mode is appropriate when administrators are attempting to debug problems in a normal environment.
16，/proc/sys/fs/inotify
The following interfaces can be used to limit the amount of kernel memory consumed by the inotify file system event-monitoring mechanism.
16.1 /proc/sys/fs/inotify/max_queued_events
The value in this file is used when an application calls inotify_init(2) to set an upper limit on the number of events that can be queued to the corresponding inotify instance. Events in excess of this limit are dropped, but an IN_Q_OVERFLOW event is always generated.
16.2 /proc/sys/fs/inotify/max_user_instances
This specifies an upper limit on the number of inotify instances that can be created per real user ID.
16.3 /proc/sys/fs/inotify/max_user_watches
This specifies a limit on the number of watches that can be associated with each inotify instance.
17，/proc/sys/fs/epoll
Used to limit the amount of kernel memory consumed by epoll
17.1 /proc/sys/fs/epoll/max_user_watches
This specifies a limit on the total number of file descriptors that a user can register across all epoll instances on the system. The limit is per real user ID. Each registered file descriptor costs roughly 90 bytes on a 32-bit kernel, and roughly 160 bytes on a 64-bit kernel. Currently, the default value for max_user_watches is 1/25 (4%) of the available low memory, divided by the registration cost in bytes.
18，/proc/sys/fs/mqueue/
This subdirectory contains files that are used for controlling the resources used by POSIX message queues.
18.1 /proc/sys/fs/mqueue/msg_default
18.2 /proc/sys/fs/mqueue/msgsize_default
18.3 /proc/sys/fs/mqueue/msg_max
msg_max is a read/write file for setting/getting the maximum number of messages in a queue value. In fact it is the limiting value for another (user) limit which is set in mq_open invocation. This attribute of a queue must be less or equal than msg_max.
18.4 /proc/sys/fs/mqueue/msgsize_max
msgsize_max is a read/write file for setting/getting the maximum message size value (it is every message queue's attribute set during its creation).
18.5 /proc/sys/fs/mqueue/queues_max
queues_max is a read/write file for setting/getting the maximum number of message queues allowed on the system.