On Thu, Feb 28, 2008 at 04:56:18PM -0600, Kenneth Marshall wrote:
> It may be that the software RAID 5 is your problem. Without the
> use of NVRAM for a cache, all of the writes need all 3 disks.
> That will cause quite a bottle-neck.
It's much worse than that. Since metadata updates are almost certainly
smaller than the stripe size, evety metadata update will look like this:
- read the full stripe (i.e. read from ALL disks)
- calculate the new parity
- write back the modification & the new parity
That sure as hell will kill your performance. Move at least the matadata
partition to a RAID1 or RAID10 array. With Linux, you can do RAID10 even
with just 3 disks, but you will of course loose 1/2 disk capacity
compared to RAID5.
Gabor
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MTA SZTAKI Computer and Automation Research Institute
Hungarian Academy of Sciences
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