On Thu, 9 Dec 2004, Henrik Krohns wrote:
On Thu, Dec 09, 2004 at 03:58:07PM +0800, Ow Mun Heng wrote:On Thu, 2004-12-09 at 13:52, Lucia Di Occhi wrote:Do not use RAID.
Do you mean SOFTWARE RAID? Hardware striping will definately improve performance.
To address this assertion, You will frequently find linux software raid to have superior performance to hardware raid subsystems especially if you have lots of bandwidth available to your controllers. that something to need to benchmark for your self but when you compare that rate at which you processor (a 3GHZ xeon for example) can xor data which is order of 4GB/s vs the risc core on your raid controller which is probably order of 200-300MB/s, that probably not an issue on a 32 bit 33mhz pci bus but it can be with 2 x 64bit 66mhz buses or more.
Frankly I'm not sure. Most ppl on the list does not encourage using RAID. I only run the cache on 1 SATA disk.
The performance isssue with raid is when you have cache objects larger than your stripes chuck size you are serializing requests across multiple disks. That means you're moving two or more spindles to read or write the same object when you could move only one. The fastest hard-drive you can buy (maxtor atlas 15kII) has a read-service time (access plus rotational latency) of around 5.5ms which means that even under ideal conditions it can only access the media around 181 times a second. Add in the additional overhead of additional writes on raid 1 or 1+0 (twice as many) or the xored parity on raid 4 or 5 (about 1/8 more in most cases) a you have more overhead...
The question becomes is it faster to stripe say 3 x 15k rpm disks or have three seperate cache dirs one on each disk. squid performance is really measured on a basis of number of objects per second (or transactions if you're a database person) not throughput bytes in-out. This issomething you may need to benchmark on your hardware in order to be shure of what you're seeing.
The issue of squid disk io performance and tuning gets a lot of treatment in Duane's excellent squid book in chapter 8 advanced disk cache topics, and appendix D filesystem performance beenchmarks. ISBN 0-596-00162-2
People should be more specific than just "dont use raid!". Modern hardware raid-boxes with GB's of cache are blazingly fast even on RAID-5. Ofcourse you shouldn't spend such money for cache which you can lose without problems, but if you have some free space on some raid-box why not use it..
Cheers, Henrik
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