On tis, 2017-08-08 at 10:24 -0300, FERNANDO FREDIANI wrote: > That's something on the way RAID works, regardless what most > 'super-ultra' powerfull hardware controller you may have. RAID 5 or > 6 > will never have the same write performance as a RAID 10 o 0 for > example. > Writeback caches can deal with bursts well but they have a limit > therefore there will always be a penalty compared to what else you > could > have. > > If you have a continuous stream of data (a big VM deployment or a > large > data copy) there will be a continuous write and that will likely fill > up > the cache making the disks underneath the bottleneck. > That's why on some other scenarios, like ZFS people have multiple > groups > of RAID 6 (called RAIDZ2) so it improves the write speeds for these > type > of scenarios.
Just pointing out that it is commonly known as RAID 60, outside of the ZFS lingo: https://en.wikipedia.org/wiki/Nested_RAID_levels#RAID_60 /K > > In the scenario given in this thread with just 3 servers, each with > a > RAID 6 there will be a bare limit on the write performance specially > for > streammed data for most powerfull your hardware controller can do > write-back. > > Also I agree the 40Gb NICs may not be used fully and 10Gb can do the > job > well, but if they were available at the begining, why not use them. > > Fernando > > > On 08/08/2017 03:16, Fabrice Bacchella wrote: > > > > > > > > Le 8 août 2017 à 04:08, FERNANDO FREDIANI <fernando.frediani@upx. > > > com> a écrit : > > > Even if you have a Hardware RAID Controller with Writeback cache > > > you will have a significant performance penalty and may not fully > > > use all the resources you mentioned you have. > > > > > Nope again,from my experience with HP Smart Array and write back > > cache, write, that goes in the cache, are even faster that read > > that must goes to the disks. of course if the write are too fast > > and to big, they will over overflow the cache. But on todays > > controller they are multi-gigabyte cache, you must write a lot to > > fill them. And if you can afford 40Gb card, you can afford decent > > controller. > > > > > > > _______________________________________________ > Users mailing list > Users@ovirt.org > http://lists.ovirt.org/mailman/listinfo/users _______________________________________________ Users mailing list Users@ovirt.org http://lists.ovirt.org/mailman/listinfo/users