On 11/27/2013 10:46 PM, Roman Mamedov wrote: > On Wed, 27 Nov 2013 22:41:49 -0600 > Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> wrote: > >>> when the stripe_cache_size is set 4096. > ... >> 2. A 4KB stripe cache is too small for 6 SSDs, try 8KB > > The stripe cache size setting is specified not in KB, but in pages per disk, > so a value of 4096 on x86 systems means 4096*4096*6 = 96 MB of cache for the > whole array. Thanks Roman for correcting me on that which I know well. Typing a trailing "KB" so often hard wires the brain and fingers I guess. My KBs were intended to be Ks. http://www.spinics.net/lists/raid/msg42370.html On 04/03/13 23:20, Stan Hoeppner wrote: ... > Formula: stripe_cache_size * 4096 bytes * drive_count = RAM usage. To expound on the importance of this, with a handful of SSDs and a value of 8K, throughput tends to plateau, and then slowly decreases as stripe_cache_size is increased. The upper bound of stripe_cache_size gains has not yet been established because the write thread peaks a core with only a few SSDs. Multiple write threads and a larger quantity of SSDs, or much faster SSDs, are needed to explore whether values of 16K-32K provide a meaningful increase in throughput, and whether this is worth the RAM consumed. For instance, with 12 SSDs and stripe_cache_size of 32768: (((32768*4096)*12)/1048576)/1000 = 1.5 GB of RAM is consumed When Shaohua Li completes his threading patch series it may be possible to explore this more thoroughly. -- Stan -- 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
