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.
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?
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