Enables large directory caching.

Unsigned integer

1 (enabled)

0 (disabled), 1 (enabled)

Yes, but do not change this tunable dynamically. It is possible to enable it if originally disabled, or to disable it if originally enabled. However, enabling, disabling, and then enabling this parameter might lead to stale directory caches.

No

Directory caching has no known problems, but if problems occur, then set dnlc_dir_enable to 0 to disable caching.

Unstable

Minimum number of entries before caching for one directory.

Unsigned integer

40

0 to MAXUINT (no maximum)

Entries

Yes, it can be changed at any time.

No

If performance problems occur with caching small directories, then increase dnlc_dir_min_size. Note that individual file systems might have their own range limits for caching directories. For instance, UFS limits directories to a minimum of ufs_min_dir_cache bytes (approximately 1024 entries), assuming 16 bytes per entry.

Unstable

Maximum number of entries cached for one directory.

Unsigned integer

MAXUINT (no maximum)

0 to MAXUINT

Yes, it can be changed at any time.

No

If performance problems occur with large directories, then decrease dnlc_dir_max_size.

Unstable

Maximum amount of memory for caching I/O buffers. The buffers are used for writing file system metadata (superblocks, inodes, indirect blocks, and directories). Buffers are allocated as needed until the amount to be allocated would exceed bufhwm. At this point, enough buffers are reclaimed to satisfy the request.

For historical reasons, this parameter does not require the ufs: prefix.

Signed integer

2% of physical memory

80 Kbytes to 20% of physical memory

Kbytes

No. Value is used to compute hash bucket sizes and is then stored into a data structure that adjusts the value in the field as buffers are allocated and deallocated. Attempting to adjust this value without following the locking protocol on a running system can lead to incorrect operation.

If bufhwm is less than 80 Kbytes or greater than the lesser of 20% of physical memory or twice the current amount of kernel heap, it is reset to the lesser of 20% of physical memory or twice the current amount of kernel heap. The following message appears on the system console and in the /var/adm/messages file.

"binit: bufhwm out of range (value attempted). Using N."

Value attempted refers to the value entered in /etc/system or by using the kadb -d command. N is the value computed by the system based on available system memory.

Since buffers are only allocated as they are needed, the overhead from the default setting is the allocation of a number of control structures to handle the maximum possible number of buffers. These structures consume 52 bytes per potential buffer on a 32-bit kernel and 104 bytes per potential buffer on a 64-bit kernel. On a 512 Mbyte 64-bit kernel this consumes 104*10144 bytes, or 1 Mbyte. The header allocations assumes buffers are 1 Kbyte in size, although in most cases, the buffer size is larger.

The amount of memory, which has not been allocated in the buffer pool, can be found by looking at the bfreelist structure in the kernel with a kernel debugger. The field of interest in the structure is bufsize, which is the possible remaining memory in bytes. Looking at it with the buf macro by using mdb:

# mdb -k
Loading modules: [ unix krtld genunix ip nfs ipc ]
> bfreelist$<buf
bfreelist:
[ elided ]
bfreelist + 0x78:	bufsize			[ elided ]
				 	      75734016

bufhwm on this system, with 6 Gbytes of memory, is 122277. It is not directly possible to determine the number of header structures used since the actual buffer size requested is usually larger than 1 Kbyte. However, some space might be profitably reclaimed from control structure allocation for this system.

The same structure on the 512 Mbyte system shows that only 4 Kbytes of 10144 Kbytes has not been allocated. When the biostats kstat is examined with kstat -n biostats, it is seen that the system had a reasonable ratio of buffer_cache_hits to buffer_cache_lookups as well. This indicates that the default setting is reasonable for that system.

Unstable

Number of quota structures for the UFS file system that should be allocated. Relevant only if quotas are enabled on one or more UFS file systems. Because of historical reasons, the ufs: prefix is not needed.

Signed integer

((maxusers x 40) / 4) + max_nprocs

0 to MAXINT

Quota structures

No

None. Excessively large values hang the system.

When the default number of quota structures is not enough. This situation is indicated by the following message displayed on the console or written in the message log.

dquot table full

Unstable