CoW not enabled by default and inplace_data_updates option does not exist

[ShawnZhong]

It seems that the documents need to be updated to reflect the change in ccf810c.

The document says that there is a module option called inplace_data_updates.

linux-nova/Documentation/filesystems/nova.txt

Line 82 in 976a4d1

* inplace_data_updates: Update data in place rather than with COW (default: 0)

This option does not exist in the code and CoW is not enabled by default:

linux-nova/fs/nova/super.c

Lines 54 to 73 in 976a4d1

	module_param(measure_timing, int, 0444);
	MODULE_PARM_DESC(measure_timing, "Timing measurement");
	module_param(metadata_csum, int, 0444);
	MODULE_PARM_DESC(metadata_csum, "Protect metadata structures with replication and checksums");
	module_param(wprotect, int, 0444);
	MODULE_PARM_DESC(wprotect, "Write-protect pmem region and use CR0.WP to allow updates");
	module_param(data_csum, int, 0444);
	MODULE_PARM_DESC(data_csum, "Detect corruption of data pages using checksum");
	module_param(data_parity, int, 0444);
	MODULE_PARM_DESC(data_parity, "Protect file data using RAID-5 style parity.");
	module_param(dram_struct_csum, int, 0444);
	MODULE_PARM_DESC(dram_struct_csum, "Protect key DRAM data structures with checksums");
	module_param(nova_dbgmask, int, 0444);
	MODULE_PARM_DESC(nova_dbgmask, "Control debugging output");

In order to enable CoW, one needs to mount with option -o data_cow, and the message nova: Enable copy-on-write updates should be printed to dmesg

linux-nova/fs/nova/super.c

Lines 278 to 281 in 976a4d1

	case Opt_data_cow:
	set_opt(sbi->s_mount_opt, DATA_COW);
	nova_info("Enable copy-on-write updates\n");
	break;