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

[Jan Tulak]

On Fri, Feb 19, 2016 at 4:43 PM, Brian Foster <bfoster at redhat.com> wrote:

> On Fri, Feb 19, 2016 at 02:34:52PM +0100, Jan Tulak wrote:
> > Hi guys,
> >
> > what is the meaning of XLOG_MIN_RECORD_BSIZE in libxfs/xfs_log_format.h?
> > It is not used anywhere. I thought it might be related to -l su/sunit
> > option, but seeing tests with -l su=4096 (the macro is set to 16k), it
> > looks more like a forgotten value.
> >
>
> It's the minimum log buffer size allowed in the kernel. It's used in
> xfs_super.c at mount time to validate the logbsize option:
>
>         if (mp->m_logbsize != -1 &&
>             mp->m_logbsize !=  0 &&
>             (mp->m_logbsize < XLOG_MIN_RECORD_BSIZE ||
>              mp->m_logbsize > XLOG_MAX_RECORD_BSIZE ||
>              !is_power_of_2(mp->m_logbsize))) {
>                 xfs_warn(mp,
>                         "invalid logbufsize: %d [not 16k,32k,64k,128k or
> 256k]",
>                         mp->m_logbsize);
>                 return -EINVAL;
>         }
>
> I suspect it's not relevant in userspace.
>

This is ok, then. Thank you for pointing me to kernel space, I didn't
realised I should check it there too. :-)
​


>
> > 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.)

Thanks,
Jan

-- 
Jan Tulak
jtulak at redhat.com / jan at tulak.me
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