1KB Sequential Writes
It’s well known that disk I/O performance can be severely impacted by fragmentation at the file system level. In other words, when a file is allocated space from many small fragments, its performance can be much worse than when its space is allocated from a single contiguous chunk. The impact is most pronounced with sequential I/Os.
Disk defrag tools vendors have been trying very hard to get that message across to you through their aggressive web and email ad campaigns, creating an impression that if you don’t run their file defrag tools regularly, your file system performancewould suffer greatly.
On a traditional directly attached storage, you might as well heed their advice and keep your drives defraged regularly.
However, on drives that are presented from some enterprise disk array over a SAN, the impact of file fragmentation may not be as severe, or not severe at all, and running the defrag tools constantly only serves to burn disk I/Os for nothing or not very much.
To see the impact of file fragmentation on a drive presented from a high end enterprise class disk array, 1KB sequential writes wee tested on a 10GB test file in the following two scenarios:
- The 10GB file was created on a freshly formatted empty drive, thus without any fragmentation at all.
- The freshly formatted drive was first filled to the full capacity with 2MB files, and then some of these 2MB files were randomly deleted to make sufficient room for the 10GB test file. In this case, the 10GB test file was extremely fragmented.
The following chart shows the results of many repeated tests, applying 1KB sequential writes at various load levels:
Clearly, on this drive, severe file fragmentation had no impact on the performance of 1KB sequential writes.
I don’t mean to suggest that file fragmentation does not have any impact on the I/O performance on any SAN/disk arrays. I simply don’t know if there is a significant impact on systems I have had no experience with. For your particular SAN environment, you have to run tests to find out yourself.
The intent of this blog post is to highlight the fact that the impact of file fragmentation may not be as universal as some defrag tools vendor may want to lead you to believe.