The Boitho search engine now uses 7 independed servers. All have 4 sata disk, making it a total of 28 disk. The problem is that we are having a lot of disk crashes. Have lost some 6 disks total now.

Every time that happened we have a system that can find out which pages was on that disk, and recrawle them.

The recrawl is time consuming, sow we are thinking about switching to raid5.

I have newer really tested out raid in a high performance system. According to http://www.pcguide.com/ref/hdd/perf/raid/levels/singleLevel5-c.html “the overhead necessary in dealing with the parity continues to bog down writes”.

How bad is this bog down?

Disk i/o is a big bottleneck to day. To work around this by uses 4 prepossesses in parallel, each indexing data on one disk. Thereby using all 4 disk at ones. If we changes to raid 5 this method will not work.

Have anyone seen any research on this ?

When we become bigger we will use a “redundant array of inexpensive nodes”. Where all data resist on at least 3 independed servers. If one fail we can just add another and copy the data from the two remaining servers. Google uses this approach in the “Google file system”.

We never switched to raid. The time penalty was to high when doing parallel i/o operations. In the test we did we saw that one could do 4 concurrent i/o operations simultaneously when having 4 disk. But when using a cheap raid card (aka fake raid) or software raid there are normally more then one disk involved in each i/o operation. This slows things down.

One could use a real raid controller with battery backed cache, but thus are expensive. Back then I think the raid controller would have cost about 50% of a 1u server with 4 hard disk. Better then to buy more servers and replicate the data.

Greg Lindahl, CTO of blekko have written a god article about this: http://highscalability.com/blog/2012/7/9/data-replication-in-nosql-databases.html

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