On 02/21/17 09:33, Gandalf Corvotempesta wrote:
Some questions: 1) can I start with a simple replicated volume and then move to a ditributed, replicated by adding more bricks ? I would like to start with 3 disks and then add 3 disks more in next month. seems stupid but this allow me to buy disks from different production batches.
Yes, you'll need to rebalance after you add a dht set so the hash table can utilize the new subvolume(s).
2) let's assume (to keep it simple) a 1GB file with sharding enabled with 100MB size. In a replicated volume with just 1 replicated brick, all shared (and thus the file) are placed on the brick (replicated to 3 servers). What in case of 2 bricks ? Gluster will place shard 1 to 5 on brick1, and 6 to 10 on brick2 or "distribution" only happen for the whole file ? (in example, all shards for file1 are placed on brick1, and all shards for file2 are placed on brick2)
My understanding is that the shards will be distributed using the same distributed hash table algorithm as any other file. (See https://joejulian.name/blog/dht-misses-are-expensive/ )
3) Based on question 2, when accessing a distributed file, gluster will read from all disks increasing the available bandwidth and thourhgput ?
That depends on where your bandwidth bottlenecks are.
4) Still keeping it simple, very simple: let's assume a VM with 10GB disk image placed on a distributed replicated volume. This VM hosts a simple webserver with a simple, but huge, website. Users accessing the website will access different section of the underlaying disk image. These accesses are distributed across the 2 bricks doubling the read performance (and write performance, as I can write on 2 disks at once) ?
If your web servers are hitting the disk for every page load, you're doing it wrong. As for your performance question, you are on the right train of thought.
5) by using ZFS, should I use a redundant ZIL ? What happens in case of ZIL failure? Usually, some date are lost, but Gluster is replicated in a syncronous way, thus loosing a ZIL on a single server should not be an issue, right ? Is gluster able to recover from this automatically ?
I can't answer ZFS questions. I, personally, don't feel it's worth all the hype it's getting and I don't use it.
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