On 5/15/2013 1:57 AM, Toke Eskildsen wrote: > On Wed, 2013-05-15 at 08:31 +0200, Shawn Heisey wrote: >> http://wiki.apache.org/solr/SolrPerformanceProblems >> >> I really was serious about reading that page, and not just because I >> wrote it. > > That page makes a clear recommendation of RAM over SSDs. > Have you done any performance testing on this?
For good performance, I don't think SSDs give you the ability to size your machines solely according to your heap requirements. SSD will greatly reduce the cost of a cache miss, but it's still a lot more expensive (timewise) to read from SSD than it is from RAM. Based on the latency numbers of RAM and SSD, you will still need RAM for caching. You might only need 10-50% of your index size for the cache instead of 50-100%. All the above is my best estimate based on what information I have. I don't have access to the required hardware to do performance testing, and it's unlikely that I will have access in the next few years. Donations are welcome! :) Performance testing would be required in order to make a proper determination on whether SSD makes financial sense. There is at least one technical hurdle that I know of with SSD, though I hope it's already been fixed. That's TRIM support in RAID controllers and the software RAID of modern operating systems. If the OS cannot tell the SSD that the space occupied by deleted files is once again available for re-allocation, then performance eventually suffers, and can become even worse than a spinning hard disk. TRIM support is available now for single disks, but those disks are not big enough for many workloads, and single disk volumes will eventually have a catastrophic failure. >From what people have said here on this list, SSDs can do really really amazing things for performance ... but IMHO they cannot eliminate the cache requirement. They may reduce it drastically, of course. Thanks, Shawn
