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

[Jan Tulak <jtulak@xxxxxxxxxx>]

On Fri, Feb 19, 2016 at 4:43 PM, Brian Foster <bfoster@xxxxxxxxxx> 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 ||
      Âmp->m_logbsize > XLOG_MAX_RECORD_BSIZE ||
      Â!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. :-)

â

Â

> There is no check for a minimal value, so I can do -l su=1 (or su=0). Are
> there some caveatâs (other than performance) with such a small value? Can
> it be that we are missing a check? Because
> XLOG_BIG_RECORD_BSIZE
> â and XLOG_MAX_RECORD_BSIZE are used and the upper bound is limited.â..
>

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..?

âSounds 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@xxxxxxxxxxÂ/ jan@xxxxxxxx