>>> Also on RAID-0 and RAID-1? I have only used HW RAID without bbu, so I >>> have no experience with that. However you can tweak your filesystem and >>> disks to do write caching too, especially if you have plenty of RAM. Of >>> course this means living dangerously, so when you need performance and >>> data safety HW RAID with bbu is the only option of course. >> Why did you think I specifically said hardware raid that comes with bbu >> memory caches? > > Because you said "Hardware raid that comes with bbu write caches normally have > a speed boost in addition to the extra data safety too."? ;-) RAID-0/RAID-1 with a filesystem using lots of cache in memory will not be faster in every scenario. Applications that call fsync will bypass the memory cache and thus be reduced to disk speed rather than memory speed in the case of hardware raid + bbu memory cache. > >> RAM will always be faster than disk so if the hardware >> raid stores stuff first in a bbu memory cache, it will always beat >> software raid even on raid0,raid1. > > Does'nt Linux have a memory cache on it's own? Putting lot's of RAM into a > machine, using a fast filesystem, i.e. XFS and tuning it to not write > everything to disc immideately should give equal performance. It should be > even better, because you can put much more RAM into a machine than on the > RAID controller. People do not turn off fsync. They may mount async but that does not affect fsync calls from databases, mtas, and other data important applications. > > Of course this does not give you the same level of data safety. Far from it! Another way could to get a NVM card or BBU RAM drive for the journal of filesystems that support full data journalling but the price difference makes getting a RAID card a no brainer in most cases. > >> Hardware raid without a bbu memory cache will always be slower on raid5 >> even if the raid processor is beefy enough. > > I use HW RAID with a memory cache but without bbu which gives the same level > of performance. Of course this is dangerous and might eat the data when the > machine dies. However with redundant power supply and UPS this is not so > likely (and yes, we have backups). > Yes, so long as you have a large enough memory cache, hardware raid 5 will be faster than software raid.