[Bug 43484] Re: poor disk performance during heavy io

[Ryan Lortie]

The disks are serialata so I don't know how to check for UDMA or if this question is applicable.

Both drives are 250GB Western Digital drives with 8 or 16 MB cache (can't remember).

Model: WDC WD2500JD-00H

Here's a test case that always works for me:

desrt at moonpix:~$ dd if=/dev/zero of=big bs=4M count=1k


while that's running, try to do some stuff.  Open up vi and save a small file to the disk.  It's _really slow_.  It reliably takes as much as 20-30 seconds to save a one-line file to disk.

One note: if you save in vi just after starting the 'dd' command then it only takes a few seconds.  It starts to get longer and longer as 'dd' has been running for a while.

One idea:

Maybe the disk driver is submitting a whole whack of tagged entries to the drive in order to latency hide.  When the new request comes in (from vi) then no matter what the IO elevator in the kernel says to do the new request gets stuck behind all the other tagged requests already in queue.

Another possibility is that the drive is using the on-disk cache memory to implement its own IO elevator that is grouping requests too agressively (resulting in vi being croweded out).

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poor disk performance during heavy io
https://launchpad.net/bugs/43484