Re: What is the meaning of XLOG_MIN_RECORD_BSIZE? (Missing a check?)

[Dave Chinner <david@fromorbit.com>]

On Fri, Feb 19, 2016 at 11:29:13AM -0500, Brian Foster wrote:
> On Fri, Feb 19, 2016 at 05:16:28PM +0100, Jan Tulak wrote:
> > On Fri, Feb 19, 2016 at 4:43 PM, Brian Foster <bfoster@redhat.com> wrote:
> > > On Fri, Feb 19, 2016 at 02:34:52PM +0100, Jan Tulak wrote:
> > > > There is no check for a minimal value, so I can do -l su=1 (or su=0). Are
> > > > there some caveat​s (other than performance) with such a small value? Can
> > > > it be that we are missing a check? Because
> > > > XLOG_BIG_RECORD_BSIZE
> > > > ​ and XLOG_MAX_RECORD_BSIZE are used and the upper bound is limited.​..
> > > >
> > >
> > > On a quick test, it looks like mkfs just ignores certain log stripe unit
> > > values that aren't block aligned. I'd probably expect this to behave
> > > similar to the '-d su' option and complain about invalid input..?
> > 
> > ​Sounds logical and like what I expected and didn't found. I will send a
> > patch adding this check... the only question is, what should be the minimal
> > value? Should I check it against block size and forbid smaller sizes?
> > Aligning a stripe unit with length 1024 on 4096 blocks doesn't looks like a
> > nice thing. :-)
> > (And on a quick check, it seems that -d su is doing just that.)
> 
> The man page says it must be a multiple of the fsb size. Indeed, '-d su'
> complains about anything that is less than 1 FSB, so I would just go
> with that. :)

Keep in mind that a value of 0 in the superblock is completely
acceptible, in which case the kernel treats the log stripe unit as
being a single sector (i.e. same as a v1 log). See, for example,
xlog_sync() where it works out the padding to use for the log buffer
write.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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