Re: scsi vs ide performance on fsync's

[Steve Lord]

> 
> 
> We're doing some mysql benchmarking.  For some reason it seems that ide
> drives are currently beating a scsi raid array and it seems to be related
> to fsync's.  Bonnie stats show the scsi array to blow away ide as
> expected, but mysql tests still have the idea beating on plain insert
> speeds.  Can anyone explain how this is possible, or perhaps explain how
> our testing may be flawed?
> 

The fsync system call does this:

        down(&inode->i_sem);
        filemap_fdatasync(inode->i_mapping);
        err = file->f_op->fsync(file, dentry, 0);
        filemap_fdatawait(inode->i_mapping);
        up(&inode->i_sem);

the f_op->fsync part of this calls file_fsync() which does:

	sync_buffers(dev, 1);

So it looks like fsync is going to cost more for bigger devices. Given the
O_SYNC changes Stephen Tweedie did, couldnt fsync look more like this:

	down(&inode->i_sem);
        filemap_fdatasync(ip->i_mapping);
        fsync_inode_buffers(ip);
        filemap_fdatawait(ip->i_mapping);
	up(&inode->i_sem);

Steve Lord