uninterruptible sleep 187

In a message on Fri, 13 Jan 2006 20:12:15 GMT, wrote :

I-O wait includes paging and disk I-O in general. If your machine is memory poor and-or you have processes gobbling up memory, your process could be paging. It could hang in that state for some time, depending on what else is going on. Processes that are not paging, but doing normal disk I-O to the same physical disk as the paging disk could also land in D state. With IDE mbutt storage devices you have to note that concurrent I-O on the same controller is not possible. So if the system disk isdev-hda and the CD-ROM isdev-hdb (master and slave on the same controller) and something is paging hard, a program that is just accessing the CD-ROM might land in D state. Also if the CD in the CD-ROM is 'dirty' or 'scratch', etc and-or you are reading a big file off it and some process starts paging, you will get processes in D state. Oh, if you are copying a large file from a CD-ROM to a hard drive *on the same controller* and don't happen to have enough free physical memory for the buffer size you have selected, you might have interesting problems... Usually, the kernel works everything out, eventually. It *might* take awhile. Short of true hardware failure (disk crash, toasted CD-ROM drive, etc.), I've never seen these programs take on a 'permanent' condition.

Also: I-O waits include NFS. NFS is *notorious* for getting processes 'stuck' in D state:

1) There is a known bug in RHEL 3.0 (and WBL 3.0) kernels where if you run out of physical memory and are using NFS file systems there is a condition where the kernel cannot allocate an NFS-based ACL and NFS operations from then on go into D mode. Cure is a reboot. The system is generally fine otherwise. (The is a mainly a problem on *client* machines, mainly with SMP kernels -- watch out on P4Ht machines.)

2) If the NFS server in question has gone south or there is a network problem somewhere between the client and the server, NFS operations on the client will go into D mode. In modern systems these may time out or clear up once the server is accessible again. Older versions of NFS might require a reboot of the client to clear things up.