Jeremy Zawodny's blog

Disk wait monitoring in Linux?

Any Linux sysadmins reading this?

In FreeBSD, I can use systat to find out how busy a given disk is. It looks like this:

Disks   da0   da1 pass0 pass1   md0
KB/t  26.67 22.10  0.00  0.00  0.00
tps       1     8     0     0     0
MB/s   0.02  0.18  0.00  0.00  0.00
% busy    0     4     0     0     0
queue     0     0     0     0     0

See, disk da1 was 4% busy during that 5 second snapshot. (I ran systat -v 5).

That's very useful when diagnosing a disk-bound MySQL server that's not doing lots of I/O but ends up waiting for lots of disk seeks on a slow RPM drive.

How do I do that in Linux?

I don't want to know how much I/O it's doing--that's easy... I want to know how often it's servicing an I/O request ('cause it may spend a lot of time seeking). I need to know how busy the disk is--even if it's not doing any I/O at the moment.

vmstat will give me I/O figures (read & write) but not % busy.

On Solaris, I'll use iostat to look at the average wait time for disk requests. It turns out that Linux can do the same thing. The await value tells you this.

However, I ran iostat -d -k -x 5 but got no data--just headers. It seems that -x only works in post 2.5 kernels. Damn.

Hmm.

My questions are two-fold:

1. Can I get those stats for a 2.4.xx kernel? How?
2. Can I get something like FreeBSD's systat % busy value in Linux? How?

Thanks to any pointers you might offer.

Update: Steve tried iostat -x on his RedHat 8 box and got meaningful output. Now I'm really puzzled.

Rude Blog Readers

No, I'm not talking about aggregators that suck down too much bandwidth. It seems that comments are being disabled in the Surfin' Safari blog.

I have decided to permanently disable comments in my Safari blog. Despite my repeated insistent attempts to explain that my blog is not a bug database, people still come here complaining about every random Web site that doesn't work. When I blog about a topic like "Standards Charts" instead of getting relevant feedback, I get more bug reports. In the real world this would be clearly impolite, somewhat akin to interrupting someone in the middle of a conversation in order to babble about a completely different subject, so what makes you think it's any less rude to do it in my blog comments?

Always refer to rule #1: People are stupid. (Even computer people. Especially some computer people.)