swap partition vs file for performance?
What is better for performance? A partition closer to the inside of the disk will have slower access times, and we must wait for the drive to switch between the OS and swap partitions.
On the other hand, a swap partition bypasses all of the filesystem allowing writes to the disk directly, which can be faster than a file.
What is the performance trade off?
How much does having a fixed size swapfile make a difference?
Is it a case that it will be longer to change to the swap partition, but performance will be better while it is on the swap partition that if it had been a swap file?
On hard disks, throughput and seeking is often faster towards the beginning of the disk, because that data is stored closer to the outer area of the disk, which has more sectors per cylinder. Thus, creating the swap at the beginning of the disk might improve performance.
For a 2.6 Linux kernel, there is no performance difference between a swap partition and an unfragmented swap file. When a swap partition/file is enabled by swapon, the 2.6 kernel finds which disk blocks the swapfile is stored on, so that when it comes time to swap, it doesn't have to deal with the filesystem at all.
Thus, if the swapfile isn't fragmented, it's exactly as if there were a swap partition at its same location. Or put another way, you'd get identical performance if you used a swap partition raw, or formatted it with a filesystem and then created a swapfile that filled all space, since either way on that disk there is a contiguous region used for swapping, which the kernel uses directly.
So if one creates the swapfile when the filesystem is fresh (thus ensuring it's not fragmented and at the beginning of the volume), performance should be identical to having a swap partition just before the volume. Further, if one created the swapfile say in the middle of the volume, with files on either side, one might get better performance, since there's less seeking to swap.
On Linux, if the swapfile is created unfragmented, and never expanded, it cannot become fragmented, at least with normal filesystems like ext3/4. It will always use the same disk blocks, which are contiguous.
I conclude that about the only benefit of a dedicated swap partition is guaranteed unfragmentation when you need to expand it; if your swap will never be expanded, a file created on a fresh filesystem doesn't require an extra partition.
Actually it doesn't make a lot of difference as long as you don't use sparse files.
Creating a "normal" file with dd will allocate the file (if at all possible) in a single run, while creating a sparse file will tell you that you have a 10GB file lying around but not actually using up all the space. I'm not quite sure wether mkswap won't allocate the space anyway but typically a swap file will grow in time and thus won't allocate a continous sector (as in part of the disk) but rather allocate blocks as needed which lead to fragmentation over time (of course depending on your usage of the disk)
Internally the Linux kernel will access the underlying blocks of a swap file more or less directly - I can't find the link right now what happens under the hood, you have to trust me on this one unless someone will find something more official. All I can come up with right now is:
- a discussion on slashdot
- the wikipedia paging article
- some FAQ on ubuntu
this all applies to the 2.6 line of Linux kernels only.
If you want optimal performance (and what is that, really?... swapping is slow, period. Increase RAM so that you don't swap for best performance), you would want to use a partition.
This is an interesting question and have been reading a lot about the same. Generally a swap partition is better than a file due to the underlying filesystem. But if you are always in need to increase the size of your swap then file is a better option. Untill kernel 2.4 it was considered that a swap partition is faster than a file, but now with the improvements of kernel 2.6 , the performances are almost the same.
Something I found on internet as well.
http://www.go2linux.org/swap-file-vs-swap-partition
and
http://www.sunmanagers.org/pipermail/summaries/2005-November/006913.html