Messages in this thread | | | Subject | Re: How do I self diagnose slow write to HDD performance issue | From | Valdis.Kletnieks@vt ... | Date | Tue, 21 Oct 2008 23:07:25 -0400 |
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On Tue, 21 Oct 2008 14:34:01 CDT, Jonathan Johnson said:
> Before I setup the array I tested the SATA 2 drives connected to the mobo SATA connect and got approx 60mb/s > write and even better read speeds per drive.
Remember that streaming read/write speed is usually only a small part of the total disk performance story. Sure, if you don't have to move the heads much, you can go 60 megabytes/sec. However, even the *hottest* drives out there have a hard time sustaining 200 seeks per second.
In other words, every time you have to move the heads, it costs you 300Kbytes/sec of throughput (1/200th of that 60M).
And bonnie++ probably moves the heads a lot.
To *add* insult to injury, if you're running in RAID6 mode, each time you write to the RAID set, it has to do *several* I/Os to do all the parity computations. So you take a 300K hit for each of those I/Os.
There's a *reason* why RAID1 is still used - you can (assuming a smart enough controller) launch both the write and the mirroring write at the same time in parallel, taking little or no performance penalty.
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