Sam Lang wrote:
On Jul 7, 2008, at 10:24 AM, Phil Carns wrote:
There are a couple of options for dealing with the case where
num_groups > num_dfiles if we want to keep the current defaults:
1) Transparently reduce the num_groups within the distribution
calculation. From the user's perspective the params wouldn't change;
the distribution would just automatically fall back to acting like
simple stripe. We would just need to add this to the top of a few of
the dist functions:
if(num_groups > server_ct)
{
num_groups = server_ct;
}
2) Return an explicit error at create time. The varstrip distribution
has this behavior now; it actually prevents you from creating a file
if the dfile count isn't plausible for the distribution parameters.
The way to do this is to implement the get_num_dfiles() function in
twod_stripe so that it can perform a safety check and return an error
at create time if the user tries to use too high of a num_groups
setting. On the command line you would get a generic errno (EINVAL or
whatever) but we could put a descriptive message in the logs.
Option 1) seems reasonable to me; it is a small code change, we get to
keep the existing desirable default parameters, and the single server
case just transparently falls back to simple stripe.
There's a third option, and what I've been proposing -- instead of
changing the actual parameter, adjust the value during the
logical_to_physical calculation.
-sam
No, that's actually the same thing I was proposing, I just didn't
realize that you meant the same thing until just now :) The code
snippet above would not change the parameter in the directory hints.
"num_groups" is a variable within the logical_to_physical_offset() function.
-Phil
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