diff options
Diffstat (limited to 'lib/libc')
| -rw-r--r-- | lib/libc/sys/mmap.2 | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/lib/libc/sys/mmap.2 b/lib/libc/sys/mmap.2 index ab9455d..30c8960 100644 --- a/lib/libc/sys/mmap.2 +++ b/lib/libc/sys/mmap.2 @@ -169,6 +169,21 @@ 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. +.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'. +.Pp The .Xr fsync 2 function will flush all dirty data and metadata associated with a file, |
