Question to the Storage Advisors, from Neal G.: How many disk IOs does it take to write one file? I have a RAID10 array writing a 120KB file. Is this one IO for the entire file, or is it one IO per sector on the disk/write cache (which would then be approx 240 IOs)?

Neal, it really depends on the operating system and how the application is writing the file. Plus, besides the file write, there are plenty of other disk accesses such as directory, FAT or inode reads and writes.

But your question made me curious, so I did a quick experiment with SysInternals’ Disk Monitor utility which traps IO type (read vs write), transfer length, and disk number. Then I simply copied a 120KB file from one drive to another, and captured the IOs to the destination drive.

Using Windows XP Pro it was nice to see that the OS issued large writes - 64KB when possible. So the 120KB file was written using just two IOs. But I also saw about a dozen other short (16KB or less) reads and writes to other sections of the disk, which I assume are directory and FAT updates.

Do a search for SysInternals and you will find Disk Monitor. Microsoft bought them last year, but I believe their tools are still free.

Enjoy,
TT

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