bufview(1) bufview(1)
bufview - file system buffer cache activity monitor
bufview [ -options ]
bufview displays periodic information about the file system buffer cache.
It is modeled after the program top(1) which displays information about
running processes.
-b Use batch (or non-interactive mode).
-s delay
Wait delay seconds between updates. The default is 3 seconds.
-d count
Update the display count times and then exit. The default is to
continually update.
-n lines
Display only lines number of buffers. The default is to display as
many buffers as will fit the screen when in interactive mode, or all
buffers when in batch mode.
-f flags
Only display buffers which have the specified flags bits set.
-o order
display buffers using the specified sort order.
-S Do not display system buffers.
-D Do not display data buffers.
bufview accepts display commands interactively. These commands are
currently recognized:
^L Redraw the screen.
h or ?
Display a summary of the commands (help screen).
q Quit.
s Change number of seconds to delay between updates.
S Toggle display of system buffers on or off.
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D Toggle display of file data buffers on or off.
o Specify buffer display order (see details below).
f Display only buffers with specific state flags set (see details
below).
F Ignore buffer state flags when determining which buffers to display.
m Display only buffers from the specified device (mounted file
system).
M Display buffers from all mounted file systems. This is the default
behaviour.
There are a set number of buffer headers allocated by the system when it
starts up, but the memory attached to each buffer changes over time.
In the Irix operating system, a distinction is made between data buffers,
which hold the data from regular files, and system or meta-data buffers,
which hold the file system control data, such as directories, inodes and
file system mapping information.
The file system buffer cache is integrated with the memory page
allocator. Any pages which contain file system data are remembered; if
the pages are released from a buffer header because the buffer is asked
to represent different data, the pages can be reclaimed by another buffer
later if the pages haven't been reused in the meantime. Thus pages of
memory which hold file data can be: attached to buffers; or attached to
one or more user processes (see mmap(2)), separately or at the same time;
or on the free page list.
The first few lines of the display monitor system-wide statistics. These
include buffer counts, memory values, events and time values.
The total number of buffers is displayed as Bufs; at any moment, each of
these buffers is accounted for as either Sys, Data, Empty or Inact. Sys
means that the buffer contains file system meta-data; a Data buffer hold
regular file data. Empty implies that the buffer does not have any data
stored at the moment; Inact means that the buffer is empty and has been
removed from use temporarily to avoid having the buffer cache use too
much memory.
The total amount of system memory is displayed as Mem. The amount of
memory associated with Sys and Data buffers is displayed as SMem and
DMem. The current amount of free memory is displayed as MFree; the
portion of that memory that holds (reclaimable) file system data is shown
as MFreD.
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Various system memory limiters are also shown. The operating system
avoids having the amount of memory in Data buffers grow too large. To
such end, the system tuneable parameter, min_free_pages, specifies how
many pages should be on the free list. If there are fewer, the Data
buffers will be trimmed, but only as low as min_file_pages (see
systune(1)). min_file_pages and min_free_pages are shown, as memory
amounts, in DMin and FrMin.
There are two other limits to the amount of memory the buffer Data cache
can use. There are two system counters known as availrmem and availsmem,
which indicate, respectively, how many physical pages would be available
if push came to shove (that is, if the system paging daemon were to push
every possible user page out of memory and in to backing store); and how
many pages of memory and backing store (swap) have not been reserved by
programs and the kernel. These two counters are expressed as memory
amounts by MaxR and MaxS. In the 6.5 operating system release, Data
buffer pages are not accounted for in either availrmem or availsmem, but
since the Data cache can never be forced to drop below min_file_pages,
the system must ensure that: first, neither availrmem nor availsmem drop
below min_file_pages; and second, that if the Data cache grows beyond
min_file_pages, it does not grow beyond the avail counters. These values
are displayed as a diagnostic tool to help understand whether there are
enough buffers allocated for the system, whether the cache is responding
well enough to various memory pressures, and whether the tuneable
parameters are configured sanely.
If the buffer cache memory must be trimmed, individual buffer headers are
inactivated. Inactivated buffers, displayed as Inact are those which are
both trimmed of their memory and also removed from the buffer free lists
so that won't be immediately refilled to represent other file system
data. When the system decides that the amount of memory allocated to
buffers can grow, and there are no empty buffers available, inactive
buffers are activated (put back on a free list).
Some event counters are also shown. The number of times a buffer is
inactivated is shown as deact; conversly, reactivations are shown as
react.
The number of times a particular buffer is sought is given as gtblk; the
times a sought buffer is found in the cache is found. If a particular
buffer isn't found in the buffer cache, a buffer free list is searched
for the first available buffer that can be reclaimed. Buffers that have
delayed write data or that have special release functions associated with
them are not directly acquired in the search for available buffers;
instead they are written (write) or the release function is called
(relse), and skipped. Buffers have a counter which, when set, allows the
buffer to pass through the free list search unscathed (the counter is
decremented each time it is examined). The counter stky indicated these
events. Note that all event counters are given as events per second
since the display was last updated.
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The current time is also shown, as well as the current system clock tick
value.
Note that all event counters are labeled with names that contain no
upper-case letters, and that all memory values are labeled with names
that contain an upper-case 'M'. With memory values, a 'K' represents
1024 bytes, an 'M' is 1024 K and so on. Buffer counters are given as
decimal values, and their labels all start with upper-case characters.
DISPLAY ORDER
The display sort order and any pruning flags are also displayed. The
sort order determines in what order buffers are to be displayed; and
pruning flags, if specified, indicated that bufview should only display
buffers with the indicated flags set. The following sort keys can be
specified:
m display the files with the most buffers attached
l display the files with the least buffers attached
b display the biggest buffers first
s display the smallest buffers first
n display the newest buffers first
o display the oldest buffers first
Note that bufview can be in either aggregate mode or itemized mode. When
in aggregate mode, all files (for regular file data) or devices (for
system data) that are currently using buffers are represented as a single
buffer; in itemized mode, individual buffers are presented. By default,
bufview runs in aggregate mode and sorts using m and b as the sort keys.
The display order can be changed, either interactively or as a
command-line option. Either way, the last order specifier given becomes
the first sort key.
If any pruning flags are requested, only those buffers which have the
corresponding flags set will be displayed. The following pruning flags
can be specified:
dw display delayed write buffers
bsy display in-use (busy) buffers
as display buffers being used for asynchronous reads/writes
na display NFS buffers being used for async reads/writes
da display buffers without allocated backing-store
nc display NFS3 buffers without committed backing-store
swp display buffers being used to swap user pages
inact display inactive buffers
ino display inode buffers
inomap display inode map buffers
dir_bt display buffers containing directory btrees
map display buffers containing maps
attr_bt display attribute btree buffers
agi display buffers containing AG header (inode allocation)
agf display allocation group header buffers
agfl display allocation group free block array buffers
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dquot display quota buffers containing dquot structures
These, too, can be specified as command-line options or demanded
interactively. Further, pruning flags can be removed (wholesale) with
the F interactive command.
If a display order or flag specifier is given that is incompatible with
the current mode, the mode changes and any incompatible sort or display
directives are removed. For example, the n display order and all pruning
flags are incompatible with aggregate mode; if any of these are
specified, m or l will be removed as a sort key. Similarly, if either m
and l are demanded, bufview will convert to aggregate mode, and any
pruning flags or itemized sort keys will be deleted.
The information displayed about buffers changes depending whether bufview
is in aggregate mode or itemized mode. In either mode, the internal node
number of the file is displayed under the column VNUMBER for data
buffers, or listed simply as a 'system' file if the buffer represents a
meta-data buffer.
In aggregate mode, the next column is NAME. Only data file names are
given, and only those whose names are brief enough to be included in the
system's internal directory name cache (currently 31 bytes). The names
displayed by bufview are truncated. A file whose name is too large to
fit in the name cache is displayed as '?'; if the system cannot determine
the name of the file because it would have had to sleep waiting for an
internal lock, the file names is displayed as '??'. In itemized mode the
column NAME/REF shows either the (possibly truncated) file name or, for
meta-data buffers, a pseudo-reference counter which, if non-zero, allows
a buffer a free trip though the buffer free list.
The column DEVICE gives the last component(s) of the file system path
name of the device from which the file system for the given buffer's file
is mounted.
The column FSTYP gives an indication of the type of file system that is
being managed by the buffer.
In aggregate mode, the next column is NBUF, which is the number of
buffers currently holding data or meta-data for the particular file or
file system, respectively. The aggregate buffer size and the amount of
buffer memory which is delayed-write is shown as SIZE and DELWRI.
(Delayed write means that the buffer has been altered but the new
contents have not yet been written to backing store.)
The final two columns in aggregate mode, LOW and HIGH, represent the
lowest and highest bytes represented by the buffers mapping the object.
For data buffers, they refer to logical offsets within the file; for
meta-data buffers, offsets within the file system.
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In itemized mode, the index of the particular buffer in the system's
buffer array is given as BUF. Its size is given next as SIZE, and OFFSET
the offset within the file or file system of the first byte mapped by the
buffer.
The column AGE shows the clock tick stored in the buffer. This value is
set when the buffer is first created and reset whenever the buffer is
accessed or modified.
Lastly, interesting buffer flags are shown under FLAGS. The list of
flags which might be displayed are exactly those which can be specified
as display pruning flags.
/etc/mtab list of mounted file systems
The buffer cache notion of a device number does not match that of the
mounted file system for NFS file systems, so the device number, not its
name, is displayed.
top(1), osview(1)
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