From: smallm@panix.com   
      
   Richard Kettlewell  writes:   
      
   > Rich  writes:   
   >> Mike Small  wrote:   
   >>> I'm noticing places in both Slackware's installer and   
   >>> debian-installer where a mkfs run is immediately followed by a   
   >>> sync...   
   >>>   
   >>> [snip]   
   >>>   
   >>> Is this just a known thing among those who've been around?  mkfs   
   >>> isn't truly done and with all bits on disk when it exits?   
   >>   
   >> It is not just mkfs.  Linux caches writes in RAM, with the data   
   >> actually being committed to disk some time later.   
   >>   
   >> If you want to be sure a write has hit the disk (note, due to on disk   
   >> caches, you can never really be /sure/) you need to tell the kernel to   
   >> write the cached RAM data out to disk -- that is what 'sync' does.   
   >>   
   >> Those sync's are not strictly necessary, but are likely present for   
   >> safety's sake.  Run the mkfs, then try to make certian all the writes   
   >> have been at least sent to the disk controller.   
   >   
   > They are redundant, at least for ext4, since mkfs.ext4 calls fsync on   
   > the target device. I’m not going to check other filesystems but the   
   > situation is likely to be the same.   
      
   Suppose the ext4 fs was on an encrypted volume on top of a dm raid,   
   i.e. two levels of device mapper. Is it possible that the fsync on the   
   crypt device wouldn't carry through to the raid?   
      
   My reason for looking at what Slackware and Debian do came from seeing a   
   script at work fail intermittently. It does the following:   
      
       1. create an ext4 fs   
       2. mount it   
       3. copy files to it   
       4. unmount it   
       5. remount it at a different point   
       6. access copied files.   
      
   This sequence sometimes fails at step 5 and rarely in step 6.  Typically   
   it does not fail. Could be a hardware error or something else (I'm   
   always looking for a new thing to blame udevd on, for instance), but it   
   sure would be nice (but not reassuring) if only a sync or two was what   
   was needed.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)