Messages in this thread |  | | From | Andreas Dilger <> | Date | Tue, 9 Oct 2001 21:35:26 -0600 | Subject | Re: dynamic swap prioritizing |
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On Oct 10, 2001 00:01 +0200, Xuan Baldauf wrote: > I have a linux box with 3 harddisks of different > characteristics (size, seek time, throughput), each capable > of holding a swap partition. Sometimes, one harddisk is > driven heavily (e.g. database application), sometimes, the > other harddisk is busy.
Daniel Phillips was working on something which may be useful in this regard. Basically, he was trying to determine how "busy" a disk was, so that if there are dirty pages to be written on an "idle" disk they would be written immediately. The theory is that if you wait longer, the disk may be busy with other I/O and you have "wasted" the resource of disk bandwidth doing nothing.
Similarly, if you knew how busy each disk with a swap partition was, you could swap to the most idle disk (assuming equal speed) or at least take this into account if the speeds are different.
If this is to be generally useful, it would be good to find things like max sequential read speed, max sequential write speed, and max seek time (at least). Estimates for max sequential read speed and seek time could be found at boot time for each disk relatively easily, but write speed may have to be found only at runtime (or it could all be fed in to the kernel from user space from benchmarks run previously).
Once we had data like that, it would be relatively easy to keep track of the queue depth for each device to determine "business" and estimated time to an empty queue and make intelligent disk I/O scheduling decisions (e.g. which MD RAID 1 disk to read from, which disk to swap to, guaranteed I/O rate for XFS, etc).
Cheers, Andreas -- Andreas Dilger \ "If a man ate a pound of pasta and a pound of antipasto, \ would they cancel out, leaving him still hungry?" http://www-mddsp.enel.ucalgary.ca/People/adilger/ -- Dogbert
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