fsync(2) manual and hdd write caching

[Ivan Voras]

On 10/27/10 12:11, Bruce Cran wrote:
> On Wed, 27 Oct 2010 02:00:51 -0700
> perryh at pluto.rain.com wrote:
> 
>> Short of mounting synchronously, with the attendant performance
>> hit, would it not make sense for fsync(2) to issue ATA_FLUSHCACHE
>> or SCSI "SYNCHRONIZE CACHE" after it has finished writing data
>> to the drive?  Surely the low-level capability to issue those
>> commands must already exist, else we would have no way to safely
>> prepare for power off.
> 
> mounting synchronously won't help, will it? As I understand it that
> just makes sure that data is sent straight to disk and not left in
> memory; the data will still be stored in the HDD cache for a
> while.

Correct. The problem is actually pretty hard - since AFAIK SoftUpdates
doesn't have "checkpoints" in the sense that it groups writes and all
data "before" can guaranteed to be on-disk, the problem is *when* to
issue BIO_FLUSH requests. One possible solution is to simply decide on a
heuristic like: "ok, doing BIO_FLUSH all the time will destroy
performance, we will only do it for every metadata write". Possibly with
a sysctl tunable or per-mount option.