Did two tests, one with async and one with softdep, on amd64,
5.9-CURRENT, UFS.
(Checked "dd"'s sources and there is no fsync() anywhere in there.
The bufcache setting was 90, 3GB free RAM, pushed 2GB of data using "dd"
to disk.
It took 12 and 15 seconds respectively, which is the harddrive's write
speed - the buffer cache of course would have absorbed this in 0
seconds.)
So, both runs showed that OpenBSD *not* does any write caching to talk
about, at all.
This means if a program wants write caching, it needs to implement it
itself.
Good to know.
Tinker
On 2016-02-13 23:47, Tinker wrote:
Hi,
How much of my file writing, and filesystem operations such as
creating a new file/directory, will land in OpenBSD's disk/write cache
without touching the disk before return of the respective operation to
my program, for softdep or async UFS media and I never fsync() ?
This is relevant to know for any usecase where there may be a big
write load to a magnet disk *and* there's lots of RAM and "sysctl
kern.bufcachepercent" is high.
If those ops will be done in a way that is synchronous with the magnet
disk, the actual fopen(), fwrite(), fread() (for re-read of the data
that's been written but still only is in the OS RAM CACHE) etc. might
be so slow that a program would need to implement its own write cache
for supporting even small spikes in write activity.
Sorry for the fuss but neither "man" nor googling taught me anything.
Thanks!!
Tinker
