fsync() for FAT file system

[Sebastian Huber]

Hi,

currently the fsync() handler for FAT file systems looks like this:

/* fat_file_datasync --
  *     Synchronize fat-file -  flush all buffered data to the media.
  *
  * PARAMETERS:
  *     mt_entry - mount table entry
  *     fat_fd   - fat-file descriptor
  *
  * RETURNS:
  *     RC_OK on success, or -1 if error occured and errno set appropriately
  */
int
fat_file_datasync(
     rtems_filesystem_mount_table_entry_t *mt_entry,
     fat_file_fd_t                        *fat_fd
     )
{
     int                 rc = RC_OK;
     rtems_status_code   sc = RTEMS_SUCCESSFUL;
     fat_fs_info_t      *fs_info = mt_entry->fs_info;
     uint32_t            cur_cln = fat_fd->cln;
     rtems_bdbuf_buffer *block = NULL;
     uint32_t            sec = 0;
     uint32_t            i = 0;

     if (fat_fd->fat_file_size == 0)
         return RC_OK;

     /*
      * we can use only one bdbuf :( and we also know that cache is useless
      * for sync operation, so don't use it
      */
     rc = fat_buf_release(fs_info);
     if (rc != RC_OK)
         return rc;

     /* for each cluster of the file ... */
     while ((cur_cln & fs_info->vol.mask) < fs_info->vol.eoc_val)
     {
         sec = fat_cluster_num_to_sector_num(mt_entry, cur_cln);
         /* for each sector in cluster ... */
         for ( i = 0; i < fs_info->vol.spc; i++ )
         {
             /* ... sync it */
             sc = rtems_bdbuf_read(fs_info->vol.dd, (sec + i), &block);
             if (sc != RTEMS_SUCCESSFUL)
                 rtems_set_errno_and_return_minus_one( EIO );

             sc = rtems_bdbuf_sync(block);
             if ( sc != RTEMS_SUCCESSFUL )
                 rtems_set_errno_and_return_minus_one( EIO );
         }

         rc = fat_get_fat_cluster(mt_entry, cur_cln, &cur_cln);
         if ( rc != RC_OK )
             return rc;
     }
     return rc;
}

This function will read every cluster of the file into the cache and then 
synchronize it step-by-step.  For unmodified buffers this is a non-operation. 
For modified buffers this will wake-up the swapout task which performs then a 
single buffer write operation.  This is usually quite inefficient.  Firstly we 
do single buffer writes and secondly we may perform a lot of unnecessary read 
operations.

I think we should replace this synchronization procedure with a simple 
rtems_bdbuf_sync_dev(fs_info->vol.dev).  This has the side-effect that also 
buffers not related to the file are synchronized, but I think this is acceptable.

What do you think?

-- 
Sebastian Huber, embedded brains GmbH

Address : Obere Lagerstr. 30, D-82178 Puchheim, Germany
Phone   : +49 89 18 90 80 79-6
Fax     : +49 89 18 90 80 79-9
E-Mail  : sebastian.huber at embedded-brains.de
PGP     : Public key available on request.

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