On Friday 16 July 2010, David Howells wrote:
> Mark Harris <mhlk@xxxxxx> 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
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