diff options
Diffstat (limited to 'lib/libc')
| -rw-r--r-- | lib/libc/sys/mmap.2 | 40 |
1 files changed, 27 insertions, 13 deletions
diff --git a/lib/libc/sys/mmap.2 b/lib/libc/sys/mmap.2 index 30c8960..7dc5520 100644 --- a/lib/libc/sys/mmap.2 +++ b/lib/libc/sys/mmap.2 @@ -169,20 +169,34 @@ maintained whether you use MAP_NOSYNC or not. This option is not portable across UNIX platforms (yet), though some may implement the same behavior by default. .Pp -WARNING! Extending a file with ftruncate(), thus creating a big hole, and -then filling the hole by modifying a shared mmap() can lead to severe -file fragmentation. In order to avoid such fragmentation you should -always pre-allocate the file's backing store by write()ing zero's into the -newly extended area prior to modifying the area via your mmap(). -The fragmentation problem is especially sensitive to MAP_NOSYNC pages, -because pages may be flushed to disk in a totally random order. +WARNING! +Extending a file with +.Xr ftruncate 2 , +thus creating a big hole, and then filling the hole by modifying a shared +.Fn mmap +can lead to severe file fragmentation. +In order to avoid such fragmentation you should always pre-allocate the +file's backing store by +.Fn write Ns ing +zero's into the newly extended area prior to modifying the area via your +.Fn mmap . +The fragmentation problem is especially sensitive to +.Dv MAP_NOSYNC +pages, because pages may be flushed to disk in a totally random order. .Pp -The same applies when using MAP_NOSYNC to implement a file-based shared -memory store. It is recommended that you create the backing store by -write()ing zero's to the backing file rather then ftruncate()ing it. You -can test file fragmentation by observing the KB/t (kilobytes per transfer) -results from an 'iostat 1' while reading a large file sequentially, e.g. -using 'dd if=filename of=/dev/null bs=32k'. +The same applies when using +.Dv MAP_NOSYNC +to implement a file-based shared memory store. +It is recommended that you create the backing store by +.Fn write Ns ing +zero's to the backing file rather then +.Fn ftruncate Ns ing +it. +You can test file fragmentation by observing the KB/t (kilobytes per +transfer) results from an +.Dq Li iostat 1 +while reading a large file sequentially, e.g. using +.Dq Li dd if=filename of=/dev/null bs=32k . .Pp The .Xr fsync 2 |
