Detect block size for quota in Linux
The limit placed on disk quota in Linux is counted in blocks. However, I found no reliable way to determine the block size. Tutorials I found refer to block size as 512 bytes, and sometimes as 1024 bytes.
I got confused reading a post on LinuxForum.org for what a block size really means. So I tried to find that meaning in the context of quota.
I found a "Determine the block size on hard disk filesystem for disk quota" tip on NixCraft, that suggested the command:
dumpe2fs /dev/sdXN | grep -i 'Block size'
or
blockdev --getbsz /dev/sdXN
But on my system those commands returned 4096, and when I checked the real quota bloc开发者_如何学Ck size on the same system, I got a block size of 1024 bytes.
Is there a scriptable way to determine the quota block size on a device, short of creating a known sized file, and checking it's quota usage?
The filesystem blocksize and the quota blocksize are potentially different. The quota blocksize is given by the BLOCK_SIZE
macro defined in <sys/mount.h>
(/usr/include/sys/mount.h):
#ifndef _SYS_MOUNT_H
#define _SYS_MOUNT_H 1
#include <features.h>
#include <sys/ioctl.h>
#define BLOCK_SIZE 1024
#define BLOCK_SIZE_BITS 10
...
The filesystem blocksize for a given filesystem is returned by the statvfs
call:
#include <stdio.h>
#include <sys/statvfs.h>
int main(int argc, char *argv[])
{
char *fn;
struct statvfs vfs;
if (argc > 1)
fn = argv[1];
else
fn = argv[0];
if (statvfs(fn, &vfs))
{
perror("statvfs");
return 1;
}
printf("(%s) bsize: %lu\n", fn, vfs.f_bsize);
return 0;
}
The <sys/quota.h>
header includes a convenience macro to convert filesystem blocks to disk quota blocks:
/*
* Convert count of filesystem blocks to diskquota blocks, meant
* for filesystems where i_blksize != BLOCK_SIZE
*/
#define fs_to_dq_blocks(num, blksize) (((num) * (blksize)) / BLOCK_SIZE)
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