[Novalug] LVM vs Partitions
seitz at bsd-unix.net
Mon Mar 15 19:13:29 EDT 2010
> Third, MD _does_ very much slow down your disk access. In fact, I'm
> totally chuckling when you say this as you use MD -- with RAID-5 no less!
> Yes, modern CPUs are fast at XOR operations, but they are _not_ I/O
> processors (the whole LOAD-XOR-STOR cycle through the system
> interconnect), or at least the common PC design is not. That's how a sub-1GHz
> I/O microcontroller with in-line XOR ASICs can beat a 3GHz PC system at
> RAID-5. It's fine to do software RAID-5 if your writes are infrequently (like on
> a web server), but if your services are I/O bound, it's much more efficient to
> do the XORs in-line with a dedicated, in-line XOR ASIC sporting microcontroller.
This didn't seem to be the case with my 2.2Ghz Core2Duo and 6 disks in Raid5 :)
In fact, my bonnie++ benchmark beat some hardware raid5 (HP) at work :)
> In other words, when you do Software RAID that requires XORs, you turn
> your I/O into Programmed I/O (PIO) instead of block writes without CPU
> with Direct Memory Access (DMA). That's why software RAID-5 writes
> typically doesn't break 20MiBps on a modern PC (when you remove
> any buffering). The only time I've seen it faster is when people are comparing
> performance to a '90s i960-based or early StrongARM I/O processor.
> But any modern X-Scale of 500MHz-1GHz with storage ASICs will best it.
20MiBps, really??? There are many factors which can play into the performance
of software raid like:
* The IDE/Sata chipset the disks are attached to
* The RPM of the disks, the cache size of the disks, the platter density of the disks
I've definitely seen a lot higher throughput with MD Raid-5, although I don't have any
systems currently using it since FreeBSD started supporting ZFS.
Bryan G. Seitz
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