On 9/30/14 9:20 AM, Roberto Spadim wrote: > well, others experiences and comments are wellcome :) > comments about cache (bcache,flashcache,dm cache) are wellcome too Keep in mind that sequential read and write speeds are not everything. For our use pattern, for example (busy Web and mail servers with millions of files that aren't necessarily grouped physically on the disk, even when in the same directory [think maildirs where each file represents one message]), latency is more important -- particularly read latency. Our servers use three-disk RAID 1 arrays. When we replaced one of the spinning hard drives in each array with an SSD (some of which were Samsung 840 Pros), then marked the remaining two spinning disks as "write-mostly", the average read latency on the array dropped from around 12 ms to less than 2 ms. More importantly, the average amount of time any process on a server is waiting in the "D" state (read or write) dropped from 8% to 3%. Note that improving the read performance this way also improves the write performance of the entire array, because when a write occurs, it will never be queued behind a spinning disk read: the spinning disk is more likely to be idle. So our experience confirms that adding a single SSD to a RAID 1 array, then marking the others write-mostly, is a good stopgap measure on the road to replacing all spinning disks with SSDs. It effectively doubled the average performance of our storage. No additional caching layers required. -- Robert L Mathews, Tiger Technologies, http://www.tigertech.net/ -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
