just one mark on the other thread, decent drive = enterprise level, a home level decent drive is samsung 840 pro (if i'm not wrong), i used ocz vetex2 without problem but i don't know if it's really a good drive, but worked in my workload... 2013/10/9 David Brown <david.brown@xxxxxxxxxxxx>: > On 09/10/13 14:31, Andy Smith wrote: >> Hello, >> >> Due to increasing load of random read IOPS I am considering using 8 >> SSDs and md in my next server, instead of 8 SATA HDDs with >> battery-backed hardware RAID. I am thinking of using Crucial m500s. >> >> Are there any gotchas to be aware of? I haven't much experience with >> SSDs. >> >> If these were normal HDDs then (aside from small partitions for >> /boot) I'd just RAID-10 for the main bulk of the storage. Is there >> any reason not to do that with SSDs currently? >> >> I think I read somewhere that offline TRIM is only supported by md >> for RAID-1, is that correct? If so, should I be finding a way to use >> four pairs of RAID-1s, or does it not matter? >> >> Any insights appreciated. >> >> Cheers, >> Andy > > For two hard disks, raid10 (with either f2 or o2 layout - n2 is almost > identical to normal raid1) can be a lot faster than raid1, because you > get the striping for big data (especially large reads), you get the > faster read throughput because your data is on the fast outer edge of > the disk, and your read latency is better because your head movement is > smaller. Your writes are a bit slower because they are scattered about > the disk. > > But for two SSDs, raid10 (f2, o2) has far fewer benefits because you > have no head movement - it is only large reads that can be faster. If > you need IOPS - and presumably multiple parallel accesses - this is no > help. raid10 has extra complexity and thus extra latency (which will > not be noticeably with HDs, but might be with SSDs), and limitations on > resizing and reshaping. > > Extrapolating to 8 disks, I think therefore 4 sets of raid1 pair is > likely to be faster. As to what you should do with these sets, that > depends on the application. XFS over a linear join might be your best > bet - raid0 will work but you probably want a large chunk size because > you want to avoid striped reads and writes in order to get high IOPs. > > Don't worry too much about TRIM if your SSDs are decent and you have > plenty of overprovisioning, but offline TRIM is worth doing when > supported. (Never use online TRIM.) > > > -- > 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 -- Roberto Spadim -- 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
