December 6, 2004 at 2:10 pm
My system administrator is telling me to clean up our SQL Server disk drive because he go the following message:
The F: disk is at or near capacity. You may need to delete some files.
He is getting the message because the F drive dropped below 10 % capacity. Our F drive has 67.8 GB of total storage space. Therefore we have 6+ gig of free space and we are not really growing much on a daily basis.
He also says when a disk only has 25 percent free space is when degradation starts. Therefore I need to clean up some space.
I'm trying to confirm his claim about degradation with some documentation. Does anyone know where I can find an article that discusses disk degradation and at what point is starts when a disk starts filling up?
Gregory A. Larsen, MVP
December 7, 2004 at 9:10 am
Just a thought.
I have never heard of disk degradation because of space issues. The only closest thing i can think about is that when the disk fills up and with very less free space, this can result in greater fragmentation while trying to allocate space.
December 7, 2004 at 9:22 am
Check out the book "SQL Server 2000 Performance Tuning" (Edward Whalen, Marcilina Garcia, Steve Adrien DeLuca, Dean Thompson). Chapter 8 has quite a bit about queueing theory and the "knee of the curve", where utilization starts skyrocketing. For processors, it is at about 75%, for disks, about 85%, so your sysadmin sounds correct.
December 7, 2004 at 9:53 am
I heard this in a NetIQ lecture a couple years ago as well. There are two things at work. One is the fragmentation problems as well as the greater likelihood of head movement since most of the disk is full.
The other thing has to do with simple space issues. The disk fills from the inside out, so as you approach the outside, access slows due to physical time it takes to rotate the larger area to read data. He suggested you keep the disks 50% free, although ackowledged that's not always feasable.
December 7, 2004 at 10:32 am
Your system administrator probably referred to performance degradation as explained in Fred Williams reply above. You may also read Microsoft SQL Server 2000 Administrator's Companion Part 2 Chapter 6 Topic
"When you are sizing a disk subsystem, always apply the 85 percent usage rule to both the size of the database and to the number of I/Os per second that users will generate.... Also remember that 85 percent is the absolute maximum usage the disk should ever see. In practice, use a number lower that 85 percent."
Yelena
Regards,Yelena Varsha
December 8, 2004 at 8:42 am
From all the replies above, what i understood is that it talks about about disk usage utilization and disk queing. I agree with the disk usage concept and you might want to have th usage below 80-85%. But Greg's question is below.
"He also says when a disk only has 25 percent free space is when degradation starts. Therefore I need to clean up some space. "
The above, with what i understand explains about performance degradation with free space and not disk utilization. Am i missing something ?
December 9, 2004 at 10:05 am
Sa24,
The article I mentioned says "both size and number of IOs" And it have graphs. Anyway, if the drive is 75 % full it will be filled in completely fast.
Reletaed Joke:
Pessimist (sysadmin): this disk is half-full
Optimist (dba): the disk is half-empty
R&D engineer: the disk is twice as big as it should be.
Regards,Yelena Varsha
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