Messages in this thread | | | Date | Wed, 9 Dec 1998 14:48:09 -0500 (EST) | From | Gregory Maxwell <> | Subject | Re: Dumb question: Which is "better" SCSI or IDE disks? |
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On Wed, 9 Dec 1998, Christopher Smith wrote: > On Wed, 9 Dec 1998, Gregory Maxwell wrote: > > On Tue, 8 Dec 1998, Christopher Smith wrote: > > > On Tue, 8 Dec 1998, Leonard Zhang System Administrator ISD RVIB wrote: > > > > RAID-1 is disk mirror. DPT is cache with RAID. RAID will increase the > > > > disk performance. > > > Ugh.. FYI, RAID does not always increase disk performance, and in > > > particular RAID-1 is guarunteed not to increase disk performance by it's > > What? Join the real world but. Raid-1 doubles read throughput and the > > slowdown of writes is negligable. > A correctly behaving RAID-1 system needs to be able to account for the > fact that given disk will fail, so interleaving reads is not exactly a > good idea.
No. A correctly operating raid understand that modern HDDs have ECC and such and can report when the data is unreadable. There is no need to read the data twice.
> > Almost all other raid levels perform worse (like raid 5 where the fastest > > write takes 2 reads 1 block XOR and 1 write). > You are talking about latency, not thoroughput. Read up on RAID, RAID-0 is > not the only version of RAID with performance advantages. > > > The fastest raid array is a combo of raid-0(striping) and raid-1. > Why would this be faster than just RAID-0?
Take four drives. Here are raid-0 block numbers:
d1 d2 d3 d4 1 2 3 4 5 6 7 8 9 10 11 12
Now read blocks 1,5,9. Wow the performance is that of one drive.
Now raid-0+raid-1
d1 d2 d3 d4 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10
Now read block 1,5,9.. Geez 1,5 are read in parallel! Also comapir reading 1,5,2,6 All are read in parallel.
Raid 1 guanrees a doubling of read throughtput and slightly degrades writeing.
Raid 0 gives you a chance of speed improvement (for 2 drives, 2x is 50%) for both read and write.
Raid0+1 increases your chances of improving speed, and really doesn't hurt write over a two disk raid 0.. The price is lost storage, the advantage is speed and redundancy.
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