RE: 4.8.2: device/sysdep_LINUX.c

[Aaron Scamehorn]

Hi Martin,

I don't think your assumption about f_bsize == f_frsize is valid.  And
if it is, what is the point of multiplying by (usage.f_frsize /
usage.f_bsize), which is always 1?

I don't see anything in the POSIX docs for statvfs stating that this
math is necessary, nor that f_bsize == f_frsize is a rule.

I didn't see anything in the GNU coreutils package (df, du, stat, etc)
doing this math either.

One place that this assumption breaks is for an nfs mounted partition;
The default blocksize is 8K, usually larger (32K).

Please look at the attached test program, and it's associated output.

I think you'll see that statvfs is ultimately making the same system
call that statfs does (as evidenced by the strace output), which gives
you the proper f_blocks without any additional math necessary.

Thanks,
Aaron


-----Original Message-----
From: address@hidden
[mailto:address@hidden On
Behalf Of Martin Pala
Sent: Friday, November 17, 2006 4:28 PM
To: The monit developer list
Subject: Re: 4.8.2: device/sysdep_LINUX.c

Hi,

the blocks size is computed according to statvfs structure description
(see 'man statvfs'):

--8<--
          struct statvfs {
            unsigned long  f_bsize;    /* file system block size */
            unsigned long  f_frsize;   /* fragment size */
            fsblkcnt_t     f_blocks;   /* size of fs in f_frsize units
*/
            fsblkcnt_t     f_bfree;    /* # free blocks */
            fsblkcnt_t     f_bavail;   /* # free blocks for non-root */
            fsfilcnt_t     f_files;    /* # inodes */
            fsfilcnt_t     f_ffree;    /* # free inodes */
            fsfilcnt_t     f_favail;   /* # free inodes for non-root */
            unsigned long  f_fsid;     /* file system ID */
            unsigned long  f_flag;     /* mount flags */
            unsigned long  f_namemax;  /* maximum filename length */
          };
--8<--

f_blocks has to be recomputed this way since it describes the number of
fragments, not blocks (the f_bsize and f_frsize match on all linuxes
which i ever saw even with different block sizes).

Martin


Aaron Scamehorn wrote:
> Hello,
> 
> In device_usage_sysdep:
> 
> I'm not quite sure what the following is supposed to be calculating:
> 
> inf->f_blocks=          usage.f_blocks * usage.f_frsize /
usage.f_bsize;
> 
> 
> I think you're incorrectly mixing block size & fragment sizes.  I 
> don't think they have anything to do with each other.
> 
> You're probably just getting lucky, because your f_frsize & f_bsize 
> both = 4K.
> 
> However, on large filesystems, where f_bsize might be 32K, you'll get 
> crazy usage numbers.
> 
> I think you just want:
> 
> inf->f_blocks=          usage.f_blocks ;
> 
> 
> Thanks,
> Aaron
> 
> 
> 25        if(statvfs( dir, &usage) != 0) {
> (gdb) n
> (gdb) print usage
> $1 = {f_bsize = 32768, f_frsize = 4096, f_blocks = 8811322, f_bfree = 
> 8053030, f_bavail = 7605440,
>   f_files = 35815424, f_ffree = 35800806, f_favail = 35800806, f_fsid 
> = 0, __f_unused = 0, f_flag = 0,
>     f_namemax = 255, __f_spare = {0, 0, 0, 0, 0, 0}}
> 
> 
> _______________________________________________
> monit-dev mailing list
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statfs.c
Description: statfs.c

output.txt
Description: output.txt