Choosing an Optimal Extent Size for a Version 1.0 Volume Group

When creating a Version 1.0 volume group, the vgcreate command may fail and display a message that the extent size is too small or that the VGRA is too big. In this situation, you must choose a larger extent size and run vgcreate again.

Increasing the extent size increases the data area marked stale when a write to a mirrored logical volume fails. That can increase the time required to resynchronize stale data. Also, more space may be allocated to each logical volume because the space is allocated in units of extent size. Therefore, the optimal extent size is the smallest value that can be used to successfully create the volume group with the desired configuration parameters.

The minimum extent size for a volume group is calculated using the maximum number of logical volumes (MAXLVs) and physical volumes (MAXPVs) in the volume group and the maximum number of physical extents (MAXPXs) per each physical volume.

For a volume group with bootable physical volumes, the metadata must fit within 768 KB. Therefore, running vgcreate with a set of values for MAXLVs, MAXPVs and MAXPXs that succeed on a volume group without bootable physical volumes may fail on a volume group with bootable physical volumes. In this situation, if you must add a bootable physical volume to a volume group, recreate the volume group by giving lesser values for these arguments. By far the biggest factor in the size of the metadata is the values for MAXPVs and MAXPXs. Alternatively, convert the bootable physical volume to a normal physical volume by running pvcreate on that physical volume without the –B option and then adding it to the volume group. For a physical volume that is already part of a volume group, you can use vgmodify to change a physical volume from a bootable to a normal physical volume.

Sample Shell Script

The following shell script creates and compiles a small program that displays the minimum extent size for a given volume group:

#!/usr/bin/sh
cat << EOF > vgrasize.c
#include <stdio.h>

#define BS 1024 /* Device block Size */
#define roundup(val, rnd) (((val + rnd - 1) / rnd) * rnd)

main(int argc, char *argv[])
{
    int i, length, lvs, pvs, pxs;
    if (argc != 4) {
        /* Usage example:
         * Maximum LVs in the VG = 255
         * Maximum PVs in the VG = 16
         * Maximum extents per PV = 2500
         *
         * $ vgrasize 255 16 2500
         */
        printf("USAGE: %s MAXLVs MAXPVs MAXPXs\n", argv[0]);
        exit(1);
    }
    lvs = atoi(argv[1]);
    pvs = atoi(argv[2]);
    pxs = atoi(argv[3]);
    length = 16 + 2 * roundup(2 +
                     (roundup(36 + ((3 * roundup(pvs, 32)) / 8) +
                             (roundup(pxs, 8) / 8) * pvs, BS) +
                      roundup(16 * lvs, BS) +
                      roundup(16 + 4 * pxs, BS) * pvs) / BS, 8);

    if (length > 768) {
        printf("Warning: A bootable PV cannot be added to a VG \n"
        "created with the specified argument values. \n"
        "The metadata size %d Kbytes, must be less \n"
        "than 768 Kbytes.\n"
        "If the intention is not to have a boot disk in this \n"
        "VG then do not use '-B' option during pvcreate(1M) \n"
        "for the PVs to be part of this VG. \n", length);
    }

    length = roundup(length, 1024) / 1024;

    if (length > 256 ) {
        printf("Cannot configure a VG with the maximum values"
        " for LVs, PVs and PXs\n");
        exit(1);
    }

    for (i = 1; i < length ; i = i << 1) { }

    printf("\nMinimum extent size for this configuration = %d MB\n", i);

    exit(0);
}
EOF
make vgrasize