On Tue, 22 Feb 2011, David Brown wrote:
On 21/02/2011 22:51, Stan Hoeppner wrote:
Mattias Wadenstein put forth on 2/21/2011 4:25 AM:
On Fri, 18 Feb 2011, Stan Hoeppner wrote:
Mattias Wadenstein put forth on 2/18/2011 7:49 AM:
RAID 5/6 need not apply due the abysmal RMW partial stripe write
penalty, unless of course you're doing almost no writes. But in that
case, how did the data get there in the first place? :)
Actually, that's probably the common case for data analysis load. Lots
of random reads, but only occasional sequential writes when you add a
new file/fileset. So raid 5/6 performance-wise works out pretty much as
a stripe of n-[12] disks.
RAID5/6 have decent single streaming read performance, but sub optimal
random read, less than sub optimal streaming write, and abysmal random
write performance. They exhibit poor random read performance with high
client counts when compared to RAID0 or RAID10. Additionally, with an
analysis "cluster" designed for overall high utilization (no idle
nodes), one node will be uploading data sets while others are doing
analysis. Thus you end up with a mixed simultaneous random read and
streaming write workload on the server. RAID10 will give many times the
throughput in this case compared to RAID5/6, which will bog down rapidly
under such a workload.
I'm a little confused here. It's easy to see why RAID5/6 have very poor
random write performance - you need at least two reads and two writes for a
single write access. It's also easy to see that streaming reads will be
good, as you can read from most of the disks in parallel.
However, I can't see that streaming writes would be so bad - you have to
write slightly more than for a RAID0 write, since you have the parity data
too, but the parity is calculated in advance without the need of any reads,
and all the writes are in parallel. So you get the streamed write
performance of n-[12] disks. Contrast this with RAID10 where you have to
write out all data twice - you get the performance of n/2 disks.
It's fine as long as you have only a few streaming writes, if you go up to
many streams things might start breaking down.
I also cannot see why random reads would be bad - I would expect that to be
of similar speed to a RAID0 setup. The only exception would be if you've got
atime enabled, and each random read was also causing a small write - then it
would be terrible.
Or am I missing something here?
The thing I think you are missing is crappy implementations in several HW
raid controllers. For linux software raid the situation is quite sanely as
you describe in my experience.
/Mattias Wadenstein
--
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
