Why Linux returns the device size for 1 block more when it could be read??

fa3a · 01-29-2004, 05:18 AM

The problem I observed on RedHat Linux is that ioctl() returns the device size for 1 block more then it could be read in a reality.

Code:

E.x. ioctl(devfile, BLKGETSIZE, &blksize) returns blksize=226
but
dd if=/dev/hdb6 of=hdb6.img shows that it was written 225 blocks!!!

does anybody know why it's designed in that way? or is it a bug?
I use RHEL and kernel 2.4

Thanks!

fa3a · 01-29-2004, 06:23 AM

The problem seems to be in the device geometry.. sometimes system returns for 512 bytes (1 block) more then we really use:

Code:

=> hdparm -g /dev/hdb6

/dev/hdb6:
 geometry     = 119150/16/63, sectors = 205569, start = 69207327

=> fdisk -l /dev/hdb | grep hdb6
/dev/hdb6         68659     68862    102784+  83  Linux

102784*2=205568

so system detects 205569 but we really use 205568...