Re: [PATCH 02/18] xstat: Add a pair of system calls to make extended file stats available [ver #6]

On Friday 16 July 2010, David Howells wrote:

Mark Harris [off-list ref] wrote:
So how about using up the dead space for what Steve French wanted:

        | One hole that this reminded me about is how to return the superblock
        | time granularity (for NFSv4 this is attribute 51 "time_delta" which
        | is called on a superblock not on a file).  We run into time rounding
        | issues with Samba too.

By doing something like:

        struct xstat_time {
                signed long long        tv_sec;
                unsigned int            tv_nsec;
                unsigned short          tv_granularity;
                unsigned short          tv_gran_units;
        };

I like that!

Where tv_granularity is the minimum granularity for tv_sec and tv_nsec given
as a quantity of tv_gran_units.  tv_gran_units could then be a constant, such
as:

        XSTAT_NANOSECONDS_GRANULARITY
        XSTAT_MICROSECONDS_GRANULARITY
        XSTAT_MILLISECONDS_GRANULARITY
        XSTAT_SECONDS_GRANULARITY
        XSTAT_MINUTES_GRANULARITY
        XSTAT_HOURS_GRANULARITY
        XSTAT_DAYS_GRANULARITY

So, for example, FAT times are a 2s granularity, so FAT would set
tv_granularity to 2 and tv_gran_units to XSTAT_SECONDS_GRANULARITY.

You could also define the tv_gran_units to be power-of-ten nanoseconds,
making it a decimal floating point number like 

enum {
	XSTAT_NANOSECONDS_GRANULARITY = 0,
	XSTAT_MICROSECONDS_GRANULARITY = 3,
	XSTAT_MILLISECONDS_GRANULARITY = 6,
	XSTAT_SECONDS_GRANULARITY = 9,
};

That would make it easier to define an xstat_time_before() function, though
it means that you could no longer do XSTAT_MINUTES_GRANULARITY and
higher directly other than { .tv_gran_units = 10, .tv_granularity = 6, }.

We could even support picosecond granularity if we made tv_nsec a 5-byte
field (tv_psec):

        struct xstat_time {
                signed long long        tv_sec;
                unsigned long long      tv_gran_units : 8;
                unsigned long long      tv_granularity : 16;
                unsigned long long      tv_psec : 48;
        };

but that's probably excessive.  Does any filesystem we currently support need
that?

I wouldn't even go that far if we needed sub-ns (I don't think we do), because
that breaks old compilers that cannot do bit fields.

	Arnd