I was going through an article that explained the hibernation procedure in Microsoft Windows. The main points that I get out of it are
- Windows dumps the whole RAM (after processing it maybe) in the
hiberfil.sysfile. - During boot up, the hibernation file is read, and the contents are loaded in the RAM.
My question is when I am usually copying a file of size, say, 1 GB, it takes about 2 minutes to complete.
However, when Windows is writing the hibernation file (during the hibernation procedure), the whole process takes maybe 10-15 seconds.
Why is there such a difference in writing speed?
My RAM size is 4 GB. (I am not talking about fast boot technology.)
Benchmarks:
- Copying 1 GB file from Disk 1 to Disk 2 (external): 2.3 minutes.
- Hibernating the system: 15 seconds.
Best Answer
This is probably an three-fold answer.
One thing that may be at play here is the new Hybrid Shutdown in Windows which effectively closes your applications, logs you off and then proceeds to hibernate the core of the operating system. Already having this data saved out would mean it does not need to "re-hibernate" it potentially.
The second thing would be that the hibernation would not need to save out memory pages that are either paged out to the swap file or are not in use (this would be one reason to aggressively fill the swap file and keep data in memory as well).
The third would be that the hibernation file data is also compressed. Combine that with my second point and if you have only a small set of data to export that contains highly compressible data (executables generally get compressed well) then the amount of data to go out to the hibernation file can be substantially smaller than the working set of data. Note that, as stated in the comments, file caches and other unnecessary buffer data could easily be discarded with no ill effect to reduce the amount of data to be dumped in the hibernation file.
Additionally, current hard drives are quite fast. With a disk that has a sustained write in the order of 100 MB/s you would be able to write out (uncompressed) 4 GB of RAM in under one minute. As hibernation can be done as the last thing after suspending all user processes and before suspending the CPU the OS will generally have the full write speed of the disk. This is one thing your simple benchmark will not have, and copying from disk to disk will potentially be slower than simply writing RAM out to disk.
Combine these things and the amount of data to be written to the hibernation file could be quite small, potentially of the order of 1 GB and would probably be written to one large continuous block in under 10 seconds.