Create Local Disk

The sample program presented in Virtual Disk API Sample Code creates virtual disk of type MONOLITHIC_SPARSE, in other words one big file, not pre-allocated. This is the default because modern file systems, in particular NTFS, support files larger than 2GB, and can hold more than 2GB of total data. This is not true of legacy file systems, such as FAT16 on MS-DOS and early Windows, or the ISO9660 file system for writing files on CD, or NFS version 2, or Linux kernel 2.4. All are limited to 2GB per volume. FAT and FAT32 were extended to 4GB in NT 3.51.

However, a SPLIT virtual disk might be safer than the MONOLITHIC variety, because if something goes wrong with the underlying host file system, some data might be recoverable from uncorrupted 2GB extents. VMware products do their best to repair a damaged VMDK, but having a split VMDK increases the chance of salvaging files during repair. On the downside, SPLIT virtual disk involves higher overhead (more file descriptors) and increases administrative complexity.

When required for a FAT16 or early Linux file system, you can create SPLIT_SPARSE virtual disk. The change is simple: the line highlighted in boldface. The sample program could be extended to have an option for this.

static void DoCreate(void)
{
   VixDiskLibAdapterType adapter = strcmp(appGlobals.adapterType, "scsi") == 0 ?
                       VIXDISKLIB_ADAPTER_SCSI_BUSLOGIC : VIXDISKLIB_ADAPTER_IDE;
   VixDiskLibCreateParams createParams;
   VixError vixError;
   createParams.adapterType = adapter;
   createParams.capacity = appGlobals.mbSize * 2048;
   createParams.diskType = VIXDISKLIB_DISK_SPLIT_SPARSE;
   vixError = VixDiskLib_Create(appGlobals.connection, appGlobals.diskPath, &createParams, NULL, NULL);
   CHECK_AND_THROW(vixError);
}

This one-line change to DoCreate() causes creation of 200MB split VMDK files (200MB being the capacity set on the previous line) unless the -cap command-line argument specifies otherwise.