On Mon, Nov 15, 2010 at 6:13 AM, Bogdan Cristea
Hi
I intend to use openSUSE for a real time application which requires to write from memory to hard disk 330 MB of data continuously in about 2.56 s.
That's about 130 MB/sec. That's really fast. If bursting it to kernel cache counts, you can do it with a single disk and plenty of RAM. If you really need to write to continuously at that speed, you need to really pay attention to your storage system design. I don't believe you will find any SATA or SSD standalone drives that have the ability to run that fast continuously, even with purely sequential writes. You may find a SAS drive that can. I don't know their performance criteria very well, but I'd be surprised to see that speed even with SAS drives. More realistically, you will need to create a raid 0 with 3 or 4 disks. The cost is not that high, and you should get good performance. (A 4-disk raid-0 should theoretically be 4x as fast as a single disk.) Note that a 4 disk raid 0 is 4 times as unreliable as a single disk. So if reliability is a concern, now you need to create a 8-disk raid 10. At $100 per disk, that's less than $1K, so not a bad option. Obviously you need a MB with 8-ports (or more) to make that doable.
When I write only 330 MB, the write time using Round Robin real-time scheduler is about 1.5 s, almost two times better than using the default scheduler.
I'm willing to bet your benchmark is not timing the full write process. ie. It is not forcing cache's to flush prior to calling the writes done.
If I use 10 times 330 MB data and then I average the write time I get about 7 s regardless of the scheduling policy.
Yes, now the caching is no longer giving you falsely fast results.
My first thought is that the disk defragmentation should be held responsible for such performance penalty. Is this the right explanation ?
No, 130 MB/sec is simply not achievable with a single disk.
Are there ways to improve the write time when dealing with large amounts of data ?
As I said, you need raid. Note that for sequential writes rotating disk should be just as fast as SSD, so don't spend money on that unless you have benchmarks to show its faster.
thanks -- Bogdan Cristea http://sites.google.com/site/cristeab/ --
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